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rev. 0 a information furnished by analog devices is believed to be accurate and reliable. however, no responsibility is assumed by analog devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. no license is granted by implication or otherwise under any patent or patent rights of analog devices. one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781/329-4700 www.analog.com fax: 781/326-8703 ? analog devices, inc., 2002 ad7482 3 msps, 12-bit sar adc features fast throughput rate: 3 msps wide input bandwidth: 40 mhz no pipeline delays with sar adc excellent dc accuracy performance two parallel interface modes low power: 90 mw (full power) and 2.5 mw (nap mode) standby mode: 2 a max single 5 v supply operation internal 2.5 v reference full-scale overrange mode (using 13th bit) system offset removal via user access offset register nominal 0 v to 2.5 v input with shifted range capability 14-bit pin compatible upgrade ad7484 available functional block diagram 2.5 v reference nap mode2 buf t/h av dd a gnd c bias dv dd dgnd refsel refout refin vin 12-bit algorithmic sar cs rd mode1 clip stby reset d0 d1 convst d2 d12 d3 d4 control logic and i/o registers ad7482 v drive write busy d11 d10 d9 d8 d7 d6 d5 general description the ad7482 is a 12-bit, high speed, low power, successive- approximation adc. the part features a parallel interface with throughput rates up to 3 msps. the part contains a low noise, wide bandwidth track-and-hold that can handle input fre- quencies in excess of 40 mhz. the conversion process is a proprietary algorithmic successive- approximation technique that results in no pipeline delays. the input signal is sampled, and a conversion is initiated on the falling edge of the convst signal. the conversion process is controlled via an internally trimmed oscillator. interfacing is via standard parallel signal lines, making the part directly compat- ible with microcontrollers and dsps. the ad7482 provides excellent ac and dc performance specifi- cations. factory trimming ensures high dc accuracy resulting in very low inl, offset, and gain errors. the part uses advanced design techniques to achieve very low power dissipation at high throughput rates. power consumption in the normal mode of operation is 90 mw. there are two power- saving modes: a nap mode that keeps the reference circuitry alive for a quick power-up while consuming 2.5 mw, and a standby mode that reduces power consumption to a mere 10 w. the ad7482 features an on-board 2.5 v reference but can also accommodate an externally provided 2.5 v reference source. the nominal analog input range is 0 v to 2.5 v, but an offset shift capability allows this nominal range to be offset by 200 mv. this allows the user considerable flexibility in setting the bottom end reference point of the signal range, a useful feature when using single-supply op amps. the ad7482 also provides the user with an 8% overrange capa bility via a 13th bit. thus, if the analog input range strays outside the nominal by up to 8%, the user can still accurately resolve the signal by using the 13th bit. the ad7482 is powered by a 4.75 v to 5.25 v supply. the part also provides a v drive pin that allows the user to set the voltage levels for the digital interface lines. the range for this v drive pin is 2.7 v to 5.25 v. the part is housed in a 48-lead lqfp package and is specified over a C 40 c to +85 c temperature range.
rev. 0 e2e ad7482especifications 1 (v dd = 5 v 5%, agnd = dgnd = 0 v, v ref = external, f sample = 3 msps; all specifi- cations t min to t max and valid for v drive = 2.7 v to 5.25 v, unless otherwise noted.) parameter specification unit test conditions/comments dynamic performance 2, 3 signal-to-noise + distortion (sinad) 4 71 db min f in = 1 mhz 72 db typ f in = 1 mhz 71 db typ f in = 1 mhz, internal reference total harmonic distortion (thd) 4 ? 86 db max ? 90 db typ ? 88 db typ internal reference peak harmonic or spurious noise (sfdr) 4 ? 87 db max intermodulation distortion (imd) 4 second order terms ? 96 db typ f in1 = 95.053 khz, f in2 = 105.329 khz third order terms ? 94 db typ aperture delay 10 ns typ full-power bandwidth 40 mhz typ @ 3 db 3.5 mhz typ @ 0.1 db dc accuracy resolution 12 bits integral nonlinearity 4 0.5 lsb max b grade 1 lsb max a grade 0.25 lsb typ differential nonlinearity 4 0.5 lsb max guaranteed no missed codes to 12 bits 0.25 lsb typ offset error 4 1.5 lsb max 0.036 %fsr max gain error 4 1.5 lsb max 0.036 %fsr max analog input input voltage ? 200 mv min +2.7 v max dc leakage current 1 a max v in from 0 v to 2.7 v 2 a typ v in = ? 200 mv input capacitance 5 35 pf typ reference input/output v refin input voltage +2.5 v 1% for specified performance v refin input dc leakage current 1 a max v refin input capacitance 5 25 pf typ v refin input current 220 a typ external reference v refout output voltage +2.5 v typ v refout error @ 25 c 50 mv typ v refout error t min to t max 100 mv max v refout output impedance 1 typ logic inputs input high voltage, v inh v drive ? 1v min input low voltage, v inl 0.4 v max input current, i in 1 a max input capacitance, c in 5 10 pf max logic outputs output high voltage, v oh 0.7 v drive v min output low voltage, v ol 0.3 v drive v max floating-state leakage current 10 a max floating-state output capacitance 5 10 pf max output coding straight (natural) binary conversion rate conversion time 300 ns max track-and-hold acquisition time(t acq )7 0 ns max sine wave input 70 ns max full-scale step input throughput rate 2.5 msps max parallel mode 1 3m sps max parallel mode 2
rev. 0 ad7482 e3e parameter specification unit test conditions/comments power requirements v dd 5v 5% v drive 2.7 v min 5.25 v max i dd normal mode (static) 12 ma max cs and rd = logic 1 normal mode (operational) 18 ma max nap mode 0.5 ma max standby mode 2 a max 0.5 a typ power dissipation normal mode (operational) 90 mw max nap mode 2.5 mw max standby mode 6 10 w max notes 1 temperature range is as follows: ? 40 c to +85 c. 2 snr and sinad figures quoted include external analog input circuit noise contribution of approximately 1 db. 3 see typical performance characteristics section for analog input circuits used. 4 see terminology section. 5 sample tested @ 25 c to ensure compliance. 6 digital input levels at gnd or v drive . specifications subject to change without notice. (v dd = 5 v 5%, agnd = dgnd = 0 v, v ref = external, f sample = 3 msps; all specifications t min to t max and valid for v drive = 2.7 v to 5.25 v, unless otherwise noted.) specifications (continued) parameter symbol min typ max unit data read conversion time t conv 300 ns quiet time before conversion start t quiet 100 ns convst pulsewidth t 1 5ns convst falling edge to busy falling edge t 2 20 ns cs falling edge to rd falling edge t 3 0ns data access time t 4 25 ns convst falling edge to new data valid t 5 30 ns busy rising edge to new data valid t 6 5ns bus relinquish time t 7 10 ns rd rising edge to cs rising edge t 8 0ns cs pulsewidth t 14 30 ns rd pulsewidth t 15 30 ns data write write pulsewidth t 9 5ns data setup time t 10 2ns data hold time t 11 6ns cs falling edge to write falling edge t 12 5ns write falling edge to cs rising edge t 13 0ns * all timing specifications given above are with a 25 pf load capacitance. with a load capacitance greater than this value, a dig ital buffer or latch must be used. specifications subject to change without notice. timing characteristics * (v dd = 5 v 5%, agnd = dgnd = 0 v, v ref = external; all specifications t min to t max and valid for v drive = 2.7 v to 5.25 v, unless otherwise noted.)
rev. 0 e4e ad7482 absolute maximum ratings * (t a = 25 c, unless otherwise noted.) v dd to gnd . . . . . . . . . . . . . . . . . . . . . . . . . . ? 0.3 v to +7 v v drive to gnd . . . . . . . . . . . . . . . . . . . . . . . . ? 0.3 v to +7 v analog input voltage to gnd . . . . . ? 0.3 v to av dd + 0.3 v digital input voltage to gnd . . . . . ? 0.3 v to v drive + 0.3 v refin to gnd . . . . . . . . . . . . . . . . ? 0.3 v to av dd + 0.3 v input current to any pin except supplies . . . . . . . . . 10 ma operating temperature range commercial . . . . . . . . . . . . . . . . . . . . . . . . ? 40 c to +85 c storage temperature range . . . . . . . . . . . . ? 65 c to +150 c junction temperature . . . . . . . . . . . . . . . . . . . . . . . . . 150 c pin configuration 36 35 34 33 32 31 30 29 28 27 26 25 13 14 15 16 17 18 19 20 21 22 23 24 1 2 3 4 5 6 7 8 9 10 11 12 48 47 46 45 44 39 38 37 43 42 41 40 pin 1 identifier top view (not to scale) d8 d7 d6 d5 v drive dgnd dgnd av dd c bias a gnd a gnd av dd a gnd vin refout refin refsel a gnd dv dd d4 d3 d2 ad7482 a gnd d1 a gnd a gnd av dd clip mode1 mode2 reset convst d12 d11 d10 d9 av dd a gnd a gnd stby nap cs rd write busy r1 r2 d0 ja thermal impedance . . . . . . . . . . . . . . . . . . . . . . 50 c/w jc thermal impedance . . . . . . . . . . . . . . . . . . . . . . 10 c/w lead temperature, soldering vapor phase (60 secs) . . . . . . . . . . . . . . . . . . . . . . . 215 c infrared (15 secs) . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 c esd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 kv * stresses above those listed under absolute maximum ratings may cause perma- nent damage to the device. this is a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. exposure to absolute maximum rating condi- tions for extended periods may affect device reliability. caution esd (electrostatic discharge) sensitive device. electrostatic charges as high as 4000 v readily accumulate on the human body and test equipment and can discharge without detection. although the ad7482 features proprietary esd protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. therefore, proper esd precautions are recommended to avoid performance degradation or loss of functionality. warning! esd sensitive device ordering guide model temperature range integral nonlinearity (inl) package options ad7482ast ? 40 c to +85 c 1 lsb max st-48 (lqfp) AD7482BST ? 40 c to +85 c 0.5 lsb max st-48 (lqfp) eval-ad7482cb 1 evaluation board eval-control brd2 2 controller board notes 1 this can be used as a standalone evaluation board or in conjunction with the eval-control board for evaluation/demonstration pu rposes. 2 this board is a complete unit allowing a pc to control and communicate with all analog devices evaluation boards ending in the cb designators.
rev. 0 ad7482 e5e pin function descriptions pin number mnemonic description 1, 5, 13, 46 av dd positive power supply for analog circuitry 2c bias decoupling pin for internal bias voltage. a 1 nf capacitor should be placed between this pin and agnd. 3, 4, 6, 11, 12, agnd power supply ground for analog circuitry 14, 15, 47, 48 7v in analog input. single-ended analog input channel. 8 refout reference output. refout connects to the output of the internal 2.5 v reference buffer. a 470 nf capacitor must be placed between this pin and agnd. 9 refin reference input. a 470 nf capacitor must be placed between this pin and agnd. when using an external voltage reference source, the reference voltage should be applied to this pin. 10 refsel reference decoupling pin. when using the internal reference, a 1 nf capacitor must be connected from this pin to agnd. when using an external reference source, this pin should be connected directly to agnd. 16 stby standby logic input. when this pin is logic high, the device will be placed in standby mode. see power saving section for further details. 17 nap nap logic input. when this pin is logic high, the device will be placed in a very low power mode. see power saving section for further details. 18 cs chip select logic input. this pin is used in conjunction with rd to access the conversion result. the databus is brought out of three-state and the current contents of the output register driven onto the data lines following the falling edge of both cs and rd . cs is also used in conjunction with write to perform a write to the offset register. cs can be hardwired permanently low. 19 rd read logic input. used in conjunction with cs to access the conversion result. 20 write write logic input. used in conjunction with cs to write data to the offset register. when the desired offset word has been placed on the databus, the write line should be pulsed high. it is the falling edge of this pulse that latches the word into the offset register. 21 busy busy logic output. this pin indicates the status of the conversion process. the busy signal goes low after the falling edge of convst and stays low for the duration of the conversion. in parallel mode 1, the busy signal returns high when the conversion result has been latched into the output register. in parallel mode 2, the busy signal returns high as soon as the conversion has been completed, but the conversion result does not get latched into the output register until the falling edge of the next convst pulse. 22, 23 r1, r2 these pins should be pulled to ground via 100 k resistors. 24 ? 28, 33 ? 39 d0 ? d1 1 data i/o bits (d11 is msb). these are three-state pins that are controlled by cs , rd , and write. the operating voltage level for these pins is determined by the v drive input. 29 dv dd positive power supply for digital circuitry 30, 31 dgnd ground reference for digital circuitry 32 v drive logic power supply input. the voltage supplied at this pin will determine at what voltage the interface logic of the device will operate. 40 d12 data output bit for overranging. if the overrange feature is not used, this pin should be pulled to dgnd via a 100 k resistor. 41 convst convert start logic input. a conversion is initiated on the falling edge of the convst signal. t he input track-and-hold amplifier goes from track mode to hold mode and the conversion process com mences. 42 reset reset logic input. a falling edge on this pin resets the internal state machine and terminates a conversion that may be in progress. the contents of the offset register will also be cleared on this edge. holding this pin low keeps the part in a reset state. 43 mode2 operating mode logic input. see table iii for details. 44 mode1 operating mode logic input. see table iii for details. 45 clip logic input. a logic high on this pin enables output clipping. in this mode, any input voltage that is greater than positive full scale or less than negative full scale will be clipped to all ? 1s ? or all ? 0s, ? respectively. further details are given in the offset/overrange section.
rev. 0 e6e ad7482 terminology integral nonlinearity this is the maximum deviation from a straight line passing through the endpoints of the adc transfer function. the end- points of the transfer function are zero scale, a point 1/2 lsb below the first code transition, and full scale, a point 1/2 lsb above the last code transition. differential nonlinearity this is the difference between the measured and ideal 1 lsb change between any two adjacent codes in the adc. offset error this is the deviation of the first code transition (00 . . . 000) to (00 . . . 001) from the ideal, i.e., agnd + 0.5 lsb. gain error this is the deviation of the last code transition (111 . . . 110) to (111 . . . 111) from the ideal (i.e., v ref ? 1.5 lsb) after the offset error has been adjusted out. track-and-hold acquisition time track-and-hold acquisition time is the time required for the output of the track-and-hold amplifier to reach its final value, within 1/2 lsb, after the end of conversion (the point at which the track-and-hold returns to track mode). signal-to-(noise + distortion) ratio this is the measured ratio of signal-to-(noise + distortion) at the output of the a/d converter. the signal is the rms ampli tude of the fundamental. noise is the sum of all nonfundamental signals up to half the sampling frequency (f s /2), excluding dc. the ratio is dependent on the number of quantization levels in the digitization process; the more levels, the smaller the quanti- zation noise. the theoretical signal-to- ( noise + distortion ) ratio for an ideal n-bit converter with a sine wave input is given by: signal to noise distortion n db ?? + = + () ().. 602 176 thus, for a 12-bit converter this is 74 db . total harmonic distortion total harmonic distortion (thd) is the ratio of the rms sum of the harmonics to the fundamental. for the ad7482, it is defined as: where v 1 is the rms amplitude of the fundamental and v 2 , v 3 , v 4 , v 5 , and v 6 are the rms amplitudes of the second through the sixth harmonics. peak harmonic or spurious noise peak harmonic or spurious noise is defined as the ratio of the rms value of the next largest component in the adc output spec- trum (up to f s /2 and excluding dc) to the rms value of the fundamental. normally, the value of this specification is deter- mined by the largest harmonic in the spectrum, but for adcs where the harmonics are buried in the noise floor, it will be a noise peak. intermodulation distortion with inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities will create distortion products at sum and difference frequencies of mfa nfb, where m and n = 0, 1, 2, 3, and so on. intermodulation distortion terms are those for which neither m nor n are equal to zero. for exam ple, the second order terms include (fa + fb) and (fa ? fb), while the third order terms include (2fa + fb), (2fa ? fb), (fa + 2fb), and (fa ? 2fb). the ad7482 is tested using the ccif standard, where two input frequencies near the top end of the input bandwidth are used. in this case, the second order terms are usually distanced in frequency from the original sine waves, while the third order terms are usually at a frequency close to the input frequencies. as a result, the second and third order terms are specified sepa- rately. the calculation of the intermodulation distortion is as per the thd specification, where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the sum of the fundamentals expressed in dbs. thd db vvvvv v () log = ++++ 20 2 2 3 2 4 2 5 2 6 2 1
rev. 0 t ypical performance characteristicsead7482 e7e adc e code 0.5 0 1024 2048 4096 inl e lsb 0.4 0.1 e0.3 e0.4 e0.5 e0.2 3072 0.3 0 0.2 e0.1 tpc 4. typical inl input frequency e khz 80 75 65 10 10000 100 sinad e db 1000 70 tpc 5. sinad vs. input tone (ad8021 input circuit) input frequency e khz e40 100 1000 thd e db e70 e90 e100 e60 10000 e50 e80 100 10 0 51 200 tpc 6. thd vs. input tone for different input resistances frequency e khz 0 0 200 400 600 800 1400 db e20 e40 e80 e100 e120 e60 1000 1200 f in = 10.7khz snr = +72.97db snr + d = +72.94db thd = e91.5db tpc 1. 64k fft plot with 10khz input tone frequency e khz 0 0 200 400 600 800 1400 db e20 e40 e80 e100 e120 e60 1000 1200 f in = 1.013mhz snr = +72.58db snr + d = +72.57db thd = e94.0db tpc 2. 64k fft plot with 1mhz input tone adc e code 0.5 0 1024 2048 4096 dnl e lsb 0.4 0.1 e0.3 e0.4 e0.5 e0.2 3072 0.3 0 0.2 e0.1 tpc 3. typical dnl
rev. 0 e8e ad7482 220 bias vo ltag e 1 2 3 4 5 6 7 8 ad8021 50 ac signal 220 10pf ev s +v s e + v in 10pf figure 2. analog input circuit used for 1 mhz input tone for higher input bandwidth applications, analog devices ? ad8021 op amp (also available as a dual ad8022) is the recommended choice to drive the ad7482. figure 2 shows the analog input circuit used to obtain the data for the fft plot shown in tpc 2. a bipolar analog signal is applied to the terminal shown and biased up with a stable, low noise dc voltage connected as shown. a 10 pf compensation capacitor is connected between pin 5 of the ad8021 and the negative supply. as w ith the previous circuit, the ad8021 is supplied with +12 v and ? 12 v supplies. the supply pins are decoupled as close to the device as possible, with both a 0.1 f and 10 f capacitor connected to each pin. in each case, the 0.1 f capaci- to r should be the closer of the two caps to the device. the ad8021 logic reference pin is tied to analog ground and the disable pi n is tied to the positive supply as shown. detailed information on the ad8021 is available on the analog devices website. frequency e khz e20 10 100 psrr e db e50 e70 e80 e40 1000 e30 e60 e10 0 100mv p-p sine wave on supply pins tpc 7. psrr without decoupling 1 2 3 4 5 6 7 8 ad829 1k 1k 100 ac signal bias vo ltag e 150 220pf ev s +v s e + v in figure 1. analog input circuit used for 10 khz input tone figure 1 shows the analog input circuit used to obtain the data for the fft plot shown in tpc 1. the circuit uses an analog devices ad829 op amp as the input buffer. a bipolar analog signal is applied as shown and biased up with a stable, low noise dc voltage connected to the labeled terminal shown. a 220 pf compensation capacitor is connected be- tween pin 5 and the ad829 and the analog ground plane. the ad829 is supplied with +12 v and ? 12 v supplies. the supply pins are decoupled as close to the device as possible with both a 0.1 f and 10 f capacitor connected to each pin. in each case, 0.1 f capacitor should be the closer of the two caps to the device. more information on the ad829 is available on the analog devices website. temperature e c 0.0004 e55 e25 5 35 95 125 refout e v e0.0004 e0.0008 e0.0012 e0.0016 e0.0020 0 65 tpc 8. reference out error
rev. 0 ad7482 e9e circuit description converter operation the ad7482 is a 12-bit algorithmic successive-approximation analog-to-digital converter based around a capacitive dac. it provides the user with track-and-hold, reference, an a/d con- verter, and versatile interface logic functions on a single chip. the normal analog input signal range that the ad7482 can convert is 0 v to 2.5 v. by using the offset and overrange fea- tures on the adc, the ad7482 can convert analog input signals from ? 200 mv to +2.7 v while operating from a single 5 v supply. the part requires a 2.5 v reference, which can be provided from the part ? s own internal reference or an exter- nal reference source. figure 3 shows a very simplified schematic of the adc. the control logic, sar, and capaci- tive dac are used to add and subtract fixed amounts of charge from the sampling capacitor to bring the comparator back to a balanced condition. capacitive dac switches v in v ref sar control logic control inputs output data 12-bit parallel comparator figure 3. simplified block diagram of ad7482 conversion is initiated on the ad7482 by pulsing the convst input. on the falling edge of convst , the track-and-hold goes from track mode to hold mode and the conversion sequence is started. conversion time for the part is 300 ns. figure 4 shows the adc during conversion. when conversion starts, sw2 will open and sw1 will move to position b, causing the comparator to become unbalanced. the adc then runs through its successive-approximation routine and brings the comparator back into a balanced condition. when the compara- tor is rebalanced, the conversion result is available in the sar register. capacitive dac comparator control logic + sw1 sw2 a gnd v in a b figure 4. adc conversion phase at the end of conversion, the track-and-hold returns to track mode and the acquisition time begins. the track-and-hold acquisition time is 40 ns. figure 5 shows the adc during its acquisition phase. sw2 is closed and sw1 is in position a. the comparator is held in a balanced condition and the sampling capacitor acquires the signal on v in . capacitive dac comparator control logic + sw1 sw2 a gnd v in a b figure 5. adc acquisition phase adc transfer function the output coding of the ad7482 is straight binary. the de signed code transitions occur midway between the successive integer lsb values (i.e., 1/2 lsb, 3/2 lsb, and so on). the lsb size is v ref /4096. the nominal transfer characteristic for the ad 7482 is shown in figure 6. this transfer characteristic may be shifted as detailed in the offset/overrange section. 000...000 0v adc code analog input 111...111 000...001 000...010 111...110 111...000 011...111 0.5lsb +v ref ?1.5lsb 1lsb = v ref /4096 figure 6. ad7482 transfer characteristic power saving the ad7482 uses advanced design techniques to achieve very low power dissipation at high throughput rates. in addition to this, the ad7482 features two power saving modes, nap and standby. these modes are selected by bringing either the nap or stby pin to a logic high, respectively. when operating the ad7482 in normal fully powered mode, the current consumption is 18 ma during conversion and the quies- cent current is 12 ma. operating at a throughput rate of 1 msps, the conversion time of 300 ns contributes 27 mw to the overall power dissipation. 300 1 5 18 27 ns s v ma mw / () () = for the remaining 700 ns of the cycle, the ad7482 dissipates 42 mw of power. 700 1 5 12 42 ns s v ma mw / () () =
rev. 0 C10C ad7482 thus, the power dissipated during each cycle is: 27 42 69 mw mw mw += figure 7 shows the ad7482 conversion sequence operating in normal mode. c onvst busy 1 s 300 ns 700 ns figure 7. normal mode power dissipation in nap mode, almost all the internal circuitry is powered down. in this mode, the power dissipation of the ad7482 is reduced to 2.5 mw. when exiting nap mode, a minimum of 300 ns when using an external reference must be waited before initiat- ing a conversion. this is necessary to allow the internal circuitry to settle after power-up and for the track-and-hold to properly acquire the analog input signal. the internal reference cannot be used in conjunction with the nap mode. if the ad7482 is put into nap mode after each conversion, the average power dissipation will be reduced, but the throughput rate will be limited by the power-up time. using the ad7482 with a throughput rate of 500 ksps while placing the part in nap mode after each conversion would result in average power dissi- pation as follows: the power-up phase contributes: ()( ) 300 2 5 12 ns/ s v ma 9 mw ? = the conversion phase contributes: (/)( ). 300 2 5 18 13 5 ns s v ma ma ? = while in nap mode for the rest of the cycle, the ad7482 dissipates only 1.75 mw of power. ()(.). 1400 2 5 0 5 1 75 ns/ s v ma mw ? = thus, the power dissipated during each cycle is: 9135175 2425 mw + . mw + . mw = mw . figure 8 shows the ad7482 conversion sequence if putting the part into nap mode after each conversion. c onvst 600ns nap busy 1400ns 300ns 2 s figure 8. nap mode power dissipation figures 9 and 10 show a typical graphical representation of power versus throughput for the ad7482 when in normal and nap modes, respectively. throughput ?ksps 90 0 3000 power ?mw 500 1500 2000 2500 85 80 75 70 65 60 1000 figure 9. normal mode, power vs. throughput throughput ?ksps 0 0 250 power ?mw 500 750 1000 1250 1500 1750 2000 10 20 30 40 50 60 70 80 90 figure 10. nap mode, power vs. throughput in standby mode, all the internal circuitry is powered down and the power consumption of the ad7482 is reduced to 10 w. the power-up time necessary before a conversion can be initiated is longer because more of the internal circuitry has been powered down. in using the internal reference of the ad7482, the adc must be brought out of standby mode 500 ms before a conver- sion is initiated. initiating a conversion before the required power-up time has elapsed will result in incorrect conversion data. if an external reference source is used and kept powered up while the ad7482 is in standby mode, the power-up time required will be reduced to 80 s.
rev. 0 ad7482 e11e offset/overrange the ad7482 provides a 8% overrange capability as well as a programmable offset register. the overrange capability is achieved by the use of a 13th bit (d12) and the clip input. if the clip input is at logic high and the contents of the offset register are zero, then the ad7482 operates as a normal 12-bit adc. if the input voltage is greater than the full-scale voltage, the data output from the adc will be all ? 1s. ? similarly, if the input voltage is lower than the zero-scale voltage, the data output from the adc will be all ? 0s. ? in this case, d12 acts as an overrange indica tor. it is set to ? 1 ? if the analog input voltage is outside the nominal 0 v to 2.5 v range. if the offset register contains any value other than ? 0, ? the con tents of the register are added to the sar result at the end of conversion. this has the effect of shifting the transfer func tion of the adc as shown in figure 11 and figure 12. however, it should be noted that with the clip input set to logic high, the maximum and minimum codes that the ad7482 will output will be 0xfff and 0x000, respectively. further details are given in table i and table ii. figure 11 shows the effect of writing a positive value to the offset register. if, for example, the contents of the offset register contained the value 256, then the value of the analog input voltage for which the adc would transition from reading all ? 0s ? to 000...001 (the bottom reference point) would be: 05 256 155 944 . ?? . lsb lsb mv () = the analog input voltage for which the adc would read full- scale (0xfff) in this example would be: 25 15 256 2 3428 . ? . ? . v lsb lsb v () = analog input 0v 1lsb = v ref /4096 0.5lsb eoffset 000...000 adc code 111...111 000...001 000...010 111...110 111...000 011...111 +v ref e 1.5lsb eoffset figure 11. transfer characteristic with positive offset the effect of writing a negative value to the offset register is shown in figure 12. if a value of ? 128 was written to the offset register, the bottom end reference point would now occur at: 05 128 78 43 . ?? . lsb lsb mv () = following this, the analog input voltage needed to produce a full-scale (0xfff) result from the adc would now be: 25 15 12 8257 72 . ? . ?? . v lsb lsb v () = 000...000 0v adc code analog input 111...111 000...001 000...010 111...110 111...000 011...111 0.5lsb eoffset +v ref e 1.5lsb eoffset 1lsb = v ref /4096 figure 12. transfer characteristic with negative offset table i shows the expected adc result for a given analog input voltage with different offset values and with clip tied to logic high. the combined advantages of the offset and overrange features of the ad7482 are shown clearly in table ii. it shows the same range of analog input and offset values as table i but with the clipping feature disabled. table i. clipping enabled (clip = 1) offset e128 0 +256 v in adc data, d[0:11] d12 ? 200 mv 0 0 0 1 1 1 ? 155.94 mv 0 0 0 1 1 0 0 v 0 0 256 1 0 0 +78.43 mv 0 128 384 0 0 0 +2.3428 v 3710 3838 4095 0 0 0 +2.5 v 3967 4095 4095 0 0 1 +2.5772 v 4095 4095 4095 0 1 1 +2.7 v 4095 4095 4095 1 1 1 table ii. clipping disabled (clip = 0) offset e128 0 +256 v in adc data, d[0:12] ? 200 mv ? 456 ? 328 ? 72 ? 155.94 mv ? 384 ? 256 0 0 v ? 128 0 256 +78.43 mv 0 128 384 +2.3428 v 3710 3838 4094 +2.5 v 3968 4096 4352 +2.5772 v 4095 4223 4479 +2.7 v 4552 4680 4936 values from ? 327 to +327 may be written to the offset register. these values correspond to an offset of 200 mv. a write to the offset register is performed by writing a 13-bit word to the part as detailed in the parallel interface section. the 10 lsbs of the 13- bit word contain the offset value, while the 3 msbs must be set to ? 0. ? failure to write zeros to the 3 msbs may result in the incorrect operation of the device.
rev. 0 C12C ad7482 the data lines d0 to d12 leave their high im pedance state when both the cs and rd are logic low. therefore, cs may be perma- nently tied logic low if required, and the rd signal may be used to access the conversion result. figure 15 shows a timing specification called t quiet. this is the amount of time that should be left after any databus activity before the next conversion is initiated. writing to the ad7482 the ad7482 features a user-accessible offset register. this allows the bottom of the transfer function to be shifted by 200 mv. this feature is explained in more detail in the offset/overrange section. to write to the offset register, a 13-bit word is written to the ad7482 with the 10 lsbs containing the offset value in two s complement format. the 3 msbs must be set to 0. th e offset value must be within the range C 327 to +327, correspond ing to an offset from C 200 mv to +200 mv. the value written to the offset register is stored and used until power is removed from the device, or the device is reset. the value stored may be updated at any time between conversions by another write to the device. table iv shows some examples of offset register values and their effective offset voltage. figure 16 shows a timing diagram for writing to the ad7482. table iv. offset register examples d9?0 (two? offset code (dec) d12?10 complement) (mv) C 327 000 1010111001 C 200 C 128 000 1110000000 C 78.12 +64 000 0001000000 +39.06 +327 000 0101000111 +200 driving the convst pin to achieve the specified performance from the ad7482, the convst pin must be driven from a low jitter source. since the falling edge on the convst pin determines the sampling in stant, any jitter that may exist on this edge will appear as noise when the analog input signal contains high frequency components. the relationship between the analog input frequency ( f in ), timing jitter ( t j ), and resulting snr is given by the equation: snr db ft jitter in j () () = 10 1 2 2 log as an example, if the desired snr due to jitter was 100 db with a maximum full-scale analog input frequency of 1.5 mhz, ignor- ing all other noise sources, the result is an allowable jitter on the convst falling edge of 1.06 ps. for a 12-bit converter (ideal snr = 74 db), the allowable jitter will be greater than the figure given above, but due consideration must be given to the design of the convst circuitry to achieve 12-bit performance with large analog input frequencies. parallel interface the ad7482 features two parallel interfacing modes. these modes are selected by the mode pins as detailed in table iii. table iii. operating modes mode 2 mode 1 do not use 0 0 parallel mode 1 0 1 parallel mode 2 1 0 do not use 1 1 in parallel mode 1, the data in the output register is updated on the rising edge of busy at the end of a conversion and is avail- able for reading almost immediately afterward. using this mode, throughput rates of up to 2.5 msps can be achieved. this mode should be used if the conversion data is required immedi- ately after the conversion has completed. an example where this may be of use is if the ad7482 was operating at much lower throughput rates in conjunction with the nap mode (for power-saving reasons), and the input signal was being compared with set limits within the dsp or other controller. if the limits were exceeded, the adc would then be brought immediately into full power operation and commence sampling at full speed. figure 17 shows a timing diagram for the ad7482 operating in parallel mode 1 with both cs and rd tied low. in parallel mode 2, the data in the output register is not updated until the next falling edge of convst . this mode could be used where a single sample delay is not vital to the system operation and conversion speeds of greater than 2.5 msps are desired. this m ay occur, for example, in a system where a large amount of samples are taken at high speed before a fast fourier trans- form is performed for frequency analysis of the input signal. figure 18 shows a timing diagram for the ad7482 operating in parallel mode 2 with both cs and rd tied low. data must not be read from the ad7482 while a conversion is taking place. for this reason, if operating the ad7482 at throughput speeds greater than 2.5 msps, it will be necessary to tie both cs a nd rd pi ns on the ad7482 low and use a buffer on the data lines. this situation may also arise in the case where a read operation cannot be completed in the time after the end of one conversion and the start of the quiet period before the next conversion. the maximum slew rate at the input of the adc should be limited to 500 v/ s while busy is low to avoid corrupting the ongoing conversion. in any multiplexed application where the channel is switched during conversion, this should happen as early as possible after the busy falling edge. reading data from the ad7482 data is read from the part via a 13-bit parallel databus with the standard cs and rd signals. the cs and rd signals are inter- nally gated to enable the conversion result onto the databus.
rev. 0 ad7482 C13C typical connection figure 13 shows a typical connection diagram for the ad7482 operating in parallel mode 1. conversion is initiated by a falling edge on convst . once convst goes low, the busy signal goes low, and at the end of conversion, the rising edge of busy is used to activate an interrupt service routine. the cs and rd lines are then activated to read the 12 data bits (13 bits if using the overrange feature). in figure 13, the v drive pin is tied to dv dd , which results in logic output levels being either 0 v or dv dd . the voltage applied to v drive controls the voltage value of the output logic signals. for example, if dv dd is supplied by a 5 v supply and v drive by a 3 v supply, the logic output levels would be either 0 v or 3 v. this feature allows the ad7482 to interface to 3 v devices, w hile still enabling the adc to process signals at a 5 v supply. c/ p reset p arallel interface mode1 mode2 write clip nap stby d0?12 cs convst rd busy c bias refsel refin refout vin ad7482 adm809 v drive dv dd av dd 0.1 f digital supply 4.75v?.25v 10 f 1nf + 0.1 f 0.1 f + 47 f analog supply 4.75v?.25v 0v to 2.5v 1nf 0.47 f 0.47 f ad780 2.5v reference figure 13. typical connection diagram board layout and grounding to obtain optimum performance from the ad7482, it is recom- mended that a printed circuit board with a minimum of three layers be used. one of these layers, preferably the middle layer, should be as complete a ground plane as possible to give the best shielding. the board should be designed in such a way that the analog and digital circuitry is separated and confined to certain areas of the board. this practice, along with avoiding running digital and analog lines close together, should help to avoid coupling digital noise onto analog lines. the power supply lines to the ad7482 should be approxi- mately 3 mm wide to provide low impedance paths and reduce the effects of glitches on the power supply lines. it is vital that good decoupling also be present. a combination of ferrites and decoupling capacitors should be used as shown in figure 13. the decoupling capacitors should be as close to the supply pins as possible. this is made easier by the use of multi- layer boards. the signal traces from the ad7482 pins can be run on the top layer, while the decoupling capacitors and ferrites can be mounted on the bottom layer where the power traces exist. the ground plane between the top and bottom planes provide excellent shielding. figures 14a to 14e show a sample layout of the board area immediately surrounding the ad7482. pin 1 is the bottom left corner of the device. figure 14a shows the top layer where the ad7482 is mounted with vias to the bottom routing layer high- lighted. figure 14b shows the bottom layer where the power routing is with the same vias highlighted. figure 14c shows the bottom layer silkscreen where the decoupling components are soldered directly beneath the device. figure 14d shows the silkscreen overlaid on the solder pads for the decoupling compo- nents, and figure 14e shows the top and bottom routing layers overlaid. the black area in each figure indicates the ground plane present on the middle layer. figure 14e figure 14a figure 14c figure 14b figure 14d c1?: 100 nf, c7?: 470 nf, c9: 1 nf l1?: meggit-sigma chip ferrite beads (bmb2a0600rs2)
rev. 0 e14e ad7482 busy write rd c onvst d[12:0] t 1 t 2 data valid t 3 t q uiet t conv t acq t 8 t 14 t 15 t 7 t 4 figure 15. parallel mode read cycle convst cs rd d[12:0] offset data t 12 t 13 t 9 t 10 t 11 write figure 16. parallel mode write cycle
rev. 0 ad7482 e15e busy c onvst d[12:0] t 6 data ne1 data n t conv n+1 n t 1 t 2 figure 17. parallel mode 1 read cycle busy c onvst d[12:0] t 2 t 5 data ne1 data n n n+1 t conv t 1 figure 18. parallel mode 2 read cycle
c02638C0C8/02(0) printed in u.s.a. C16C ad7482 outline dimensions 48-lead plastic quad flatpack [lqfp] (st-48) dimensions shown in millimeters top view (pins down) 1 12 13 25 24 36 37 48 0.27 0.22 0.17 0.50 bsc 7.00 bsc sq seating plane 1.60 max 0.75 0.60 0.45 view a 7 3.5 0 0.20 0.09 1.45 1.40 1.35 0.15 0.05 0.08 max coplanarity view a rotated 90 ccw pin 1 indicator 9.00 bsc sq compliant to jedec standards ms-026bbc

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