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| rev. 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 which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of analog devices. a lc 2 mos high speed, m p-compatible 8-bit adc with track/hold function ad7821 one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 617/329-4700 fax: 617/326-8703 functional block diagram features fast conversion time: 660 ns max 100 khz track-and-hold function 1 mhz sample rate unipolar and bipolar input ranges ratiometric reference inputs no external clock extended temperature range operation skinny 20-pin dlps, soic and 20-terminal surface mount packages general description the ad7821 is a high speed, 8-bit, sampling, analog-to-digital converter that offers improved performance over the popular ad7820. it offers a conversion time of 660 ns (vs. 1.36 m s for the ad7820) and 100 khz signal bandwidth (vs. 6.4 khz). the sampling instant is better defined and occurs on the falling edge of wr or rd . the provision of a v ss pin (pin 19) allows the part to operate from 5 v supplies and to digitize bipolar input signals. alternatively, for unipolar inputs, the v ss pin can be grounded and the ad7821 will operate from a single +5 v sup- ply, like the ad7820. the ad7821 has a built-in track-and-hold function capable of digitizing full-scale signals up to 100 khz max. it also uses a half-flash conversion technique that eliminates the need to gen- erate a clk signal for the adc. the ad7821 is designed with standard microprocessor control signals ( cs , rd , wr , rdy, int ) and latched, three-state data outputs capable of interfacing to high speed data buses. an overflow output ( ofl ) is also provided for cascading devices to achieve higher resolution. the ad7821 is fabricated in linear-compatible cmos (lc 2 mos), an advanced, mixed technology process combining precision bipolar circuits with low power cmos logic. the part features a low power dissipation of 50 mw. product highlights 1. fast conversion time the half-flash conversion technique, coupled with fabrication on analog devices lc 2 mos process, enables a very fast con- version time. the conversion time for the wr-rd mode is 660 ns, with 700 ns for the rd mode. 2. built-in track-and-hold this allows input signals with slew rates up to 1.6 v/ m s to be converted to 8-bits without an external track-and-hold. this corresponds to a 5 v peak-to-peak, 100 khz sine wave signal. 3. total unadjusted error the ad7821 features an excellent total unadjusted error fig- ure of less than 1 lsb over the full operating temperature range. 4. unipolar/bipolar input ranges the ad7821 is specified for single supply (+5 v) operation with a unipolar full-scale range of 0 to +5 v, and for dual sup- ply ( 5 v) operation with a bipolar input range of 2.5 v. typical performance characteristics are given for other input ranges. 5. dynamic specifications for dsp users in addition to the traditional adc specifications, the ad7821 is specified for ac parameters, including signal-to- noise ratio, distortion and slew rate.
rev. a C2C ad7821Cspecifications v dd = +5 v 6 5%, gnd = 0 v. unipolar input range: v ss = gnd, v ref (+) = 5 v, v ref (C) = gnd. bipolar input range: v ss = C5 v 6 5%, v ref (+) = 2.5 v, v ref (C) = C2.5 v. these test conditions apply unless otherwise stated. all specifications t min to t max unless otherwise noted. specifications apply for rd mode (pin 7 = 0 v). parameter k version 1 b, t versions units comments unipolar input range resolution 2 8 8 bits total unadjusted error 3 1 1 lsb max minimum resolution for which no missing codes are guaranteed 8 8 bits bipolar input range resolution 2 8 8 bits zero code error 1 1 lsb max full scale error 1 1 lsb max signal-to-noise ratio (snr) 3 45 45 db min v in = 99.85 khz full-scale sine wave with f sampling = 500 khz total harmonic distortion (thd) 3 C50 C50 db max v in = 99.85 khz full-scale sine wave with f sampling = 500 khz peak harmonic or spurious noise 3 C50 C50 db max v in = 99.85 khz full-scale sine wave with f sampling = 500 khz intermodulation distortion (imd) 3 fa (84.72 khz) and fb (94.97 khz) full-scale sine waves with f sampling = 500 khz C50 C50 db max second order terms C50 C50 db max third order terms slew rate, tracking 3 1.6 1.6 v/ m s max 2.36 2.36 v/ m s typ reference input input resistance 1.0/4.0 1.0/4.0 k w min/k w max v ref (+) input voltage range v ref (C)/v dd v ref (C)/v dd v min/v max v ref (C) input voltage range v ss /v ref (+) v ss /v ref (+) v min/v max analog input input voltage range v ref (C)/v ref (+) v ref (C)/v ref (+) v min/ max input leakage current 3 3 m a max C5 v v in +5 v input capacitance 55 55 pf typ logic inputs cs , wr , rd v inh 2.4 2.4 v min v inl 0.8 0.8 v max i inh ( cs , rd )11 m a max i inh ( wr )33 m a max i inl C1 C1 m a max input capacitance 4 8 8 pf max typically 5 pf mode v inh 3.5 3.5 v min v inl 1.5 1.5 v max i inh 200 200 m a max 50 m a typ i inl C1 C1 m a max input capacitance 4 8 8 pf max typically 5 pf logic outputs db0Cdb7, ofl , int v oh 4.0 4.0 v min i source = 360 m a v ol 0.4 0.4 v max i sink = 1.6 ma i out (db0Cdb7) 3 3 m a max floating state leakage output capacitance 4 (db0Cdb7) 8 8 pf max typically 5 pf rdy v ol 0.4 0.4 v max i sink = 2.6 ma i out 3 3 m a max floating state leakage output capacitance 4 8 8 pf max typically 5 pf power supply i dd 5 15 20 ma max cs = rd = 0 v i ss 100 100 m a max cs = rd = 0 v power dissipation 50 50 mw typ power supply sensitivity 1/4 1/4 lsb max 1/16 lsb typ, v dd = 4.75 v to 5.25 v, (v ref (+) = 4.75 v max for unipolar mode) notes 1 temperature ranges are as follows: k version = C40 c to +85 c; b version = C40 c to +85 c; t version = C55 c to +125 c. 2 1 lsb = 19.53 mv for both the unipolar (0 v to +5 v) and bipolar (C2.5 v to +2.5 v) input ranges. 3 see terminology. 4 sample tested at +25 c to ensure compliance. 5 see typical performance characteristics. specifications subject to change without notice. ad7821 rev. a C3C timing characteristics 1 (v dd = +5 v 5%, v ss = 0 v or C5 v 5%; unipolar or bipolar input range) limit at limit at limit at +25 8 ct min , t max t min , t max parameter (all versions) (k, b versions) (t version) units conditions/comments t css 0 0 0 ns min cs to rd / wr setup time t csh 0 0 0 ns min cs to rd / wr hold time t rdy 2 70 85 100 ns max cs to rdy delay. pull-up resistor 5 k1. t crd 700 875 975 ns max conversion time (rd mode) t acc0 3 data access time (rd mode) t crd + 25 t crd + 30 t crd + 35 ns max c l = 20 pf t crd + 50 t crd + 65 t crd + 75 ns max c l = 100 pf t inth 2 50 C C ns typ rd to int delay (rd mode) 80 85 90 ns max t dh 4 15 15 15 ns min data hold time 60 70 80 ns max t p 350 425 500 ns min delay time between conversions t wr 250 325 400 ns min write pulse width 10 10 10 m s max t rd 250 350 450 ns min delay time between wr and rd pulses t read1 160 205 240 ns min rd pulse width (wr-rd mode, see figure 12b) determined by t acc1 t acc1 3 data access time (wr-rd mode, see figure 12b) 160 205 240 ns max c l = 20 pf 185 235 275 ns max c l = 100 pf t ri 150 185 220 ns max rd to int delay t intl 2 380 C C ns typ wr to int delay 500 610 700 ns max t read2 65 75 85 ns min rd pulse width (wr-rd mode, see figure 12a) determined by t acc2 data access time (wr-rd mode, see figure 12a) t acc2 3 65 75 85 ns max c l = 20 pf 90 110 130 ns max c l = 100 pf t ihwr 2 80 100 120 ns max wr to int delay (stand-alone operation) t id 3 data access time after int (stand-alone operation) 30 35 40 ns max c l = 20 pf 45 60 70 ns max c l = 100 pf notes 1 sample tested at +25 c to ensure compliance. all input control signals are specified with tr = tf = 5 ns (10% to 90% of +5 v) and timed from a voltage level of 1.6 v. 2 c l = 50 pf. 3 measured with load circuits of figure 1 and defined as the time required for an output to cross 0.8 v or 2.4 v. 4 defined as the time required for the data lines to change 0.5 v when loaded with the circuits of figure 2. specifications subject to change without notice. test circuits a. high z to v oh b. high z to v ol figure 1. load circuits for data access time test a. v oh to high z b. v ol to high z figure 2. load circuits for data hold time test ordering guide total temperature unadjusted package model 1 range error (lsb) option 2 ad7821kn C40 c to +85 c 1 max n-20 ad7821kp C40 c to +85 c 1 max p-20a ad7821kr C40 c to +85 c 1 max r-20 ad7821bq C40 c to +85 c 1 max q-20 ad7821tq C55 c to +125 c 1 max q-20 AD7821TE C55 c to +125 c 1 max e-20a notes 1 to order mil-std-883, class b processed parts, add /883b to part number. contact local sales office for military data sheet. 2 e = leadless ceramic chip carrier; n = plastic dip; p = plastic leaded chip carrier; q = cerdip; r = soic. ad7821 rev. a C4C absolute maximum ratings* v dd to gnd . . . . . . . . . . . . . . . . . . . . . . . . . . . C0.3 v, + 7 v v ss to gnd . . . . . . . . . . . . . . . . . . . . . . . . . . . . +0.3 v, + 7 v digital input voltage to gnd (pins 6C8, 13) . . . . . . . . . . . . . . . . . . . C0.3 v, v dd + 0.3 v digital output voltage to gnd (pins 2C5, 9, 14C18) . . . . . . . . . . . . . . . C0.3 v, v dd + 0.3 v v ref (+) to gnd . . . . . . . . . . . . . . . v ss C 0.3 v, v dd + 0.3 v v ref (C) to gnd . . . . . . . . . . . . . . . v ss C 0.3 v, v dd + 0.3 v v in to gnd . . . . . . . . . . . . . . . . . . . v ss C 0.3 v, v dd + 0.3 v operating temperature range commercial (k version) . . . . . . . . . . . . . . C40 c to +85 c industrial (b version) . . . . . . . . . . . . . . . . C40 c to +85 c extended (t version) . . . . . . . . . . . . . . . C55 c to +125 c storage temperature range . . . . . . . . . . . . C65 c to +150 c lead temperature (soldering, 10 secs) . . . . . . . . . . . +300 c power dissipation (any package) to +75 c . . . . . . . . 450 mw derates above +75 c by . . . . . . . . . . . . . . . . . . . . . 6 mw/ c *stresses above those listed under absolute maximum ratings may cause permanent damage to the device. this is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. terminology least significant bit (lsb) an adc with 8-bit resolution can resolve one part in 2 8 (1/256 of full scale). for the ad7821 operating in either the unipolar or bipolar input range with 5 v full scale, one lsb is 19.53 mv. total unadjusted error this is a comprehensive specification which includes relative accuracy, offset error and full-scale error. slew rate slew rate is the maximum allowable rate of change of input signal such that the digital sample values are not in error. total harmonic distortion total harmonic distortion is the ratio of the square root of the sum of the squares of the rms value of the harmonics to the rms value of the fundamental. for the ad7821, total harmonic dis- tortion (thd) is defined as 20 log v 2 2 + v 3 2 + v 5 2 + v 6 2 () v 1 ? ? db where v 1 is the rms amplitude of the fundamental and v 2 , v 3 , v 4 , v 5 , v 6 , are the rms amplitudes of the individual harmonics. intermodulation distortion with inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities will create distortion products, of order (m+n), at sum and difference frequencies of mfa+nfb, where m, n = 0, 1, 2, 3,- - - -. intermodulation terms are those for which m or n is not equal to zero. for example, the second order terms include (fa + fb) and (fa C fb), and the third order terms include (2fa + fb), (2fa C fb), (fa + 2fb) and (fa C 2fb). for the ad7821 intermodulation distortion is calcu- lated separately for both the second and third order terms. signal-to-noise ratio signal-to-noise ratio (snr) is measured signal-to-noise at the output of the adc. the signal is the rms magnitude of the fun- damental. noise is the rms sum of all nonfundamental signals (excluding dc) up to half the sampling frequency. snr is de- pendent on the number of quantization levels used in the digiti- zation process. the theoretical snr for a sine wave input is given by: snr = ( 6.02 n + 1.76 ) db where n is the number of bits in the adc. thus, for an ideal 8-bit adc, snr = 50 db. peak harmonic or spurious noise peak harmonic or spurious noise is the rms value of the largest nonfundamental frequency (excluding dc) up to half the sam- pling frequency to the rms value of the fundamental. pin configurations dip and soic lccc plcc warning! esd sensitive device 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 ad7821 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. ad7821 rev. a C5C typical performance curvesC power supply current vs. tempera- ture (not including reference ladder) accuracy vs. t p t intl , internal time delay vs. temperature accuracy vs. t wr accuracy vs. v ref [v ref = v ref (+) C v ref (C)] output current vs. temperature conversion time (rd mode) vs. temperature accuracy vs. t rd effective number of bits vs. input signal ( 2.5 v) frequency ad7821 rev. a C6C pin function description pin mnemonic description 1v in analog input: range v ref (C) v in v ref (+) 2 db0 three-state data output (lsb). 3C5 db1Cdb3 three-state data outputs. 6 wr /rdy write control input/ready status output. see digital interface section. 7 mode mode selection input. it determines whether the device operates in the wr-rd or rd mode. this input is in- ternally pulled low through a 50 m a current source. see digital interface section. 8 rd read input. rd must be low to access data from the part. see digital interface section. 9 int interrupt output. int going low indicates that the conversion is complete. int returns high on the rising edge of cs or rd . see digital interface section. 10 gnd ground. 11 v ref (C) lower limit of reference span. range: v ss v ref (C) v ref (+). 12 v ref (+) upper limit of reference span. range: v ref (C) < v ref (+) v dd . 13 cs chip select input. the device is selected when this input is low. 14C16 db4Cdb6 three-state data outputs. 17 db7 three-state data output (msb). 18 ofl overflow output. if the analog input is higher than (v ref (+) C 1/2 lsb), ofl will be low at the end of conversion. it is a non-three-state output which can be used to cascade 2 or more devices to increase resolution. 19 v ss negative supply voltage. v ss = 0 v; unipolar operation. v ss = C5 v; bipolar operation. 20 v dd positive supply voltage, +5 v. circuit information basic description the ad7821 uses a half flash conversion technique (see func- tional block diagram), whereby two 4-bit flash adcs are used to achieve an 8-bit result. each 4-bit flash adc contains 15 comparators, which compare an unknown input voltage to the reference ladder, to achieve a 4-bit result. the ms (most signifi- cant) flash adc converts an unknown analog input voltage (v in ) to provide the 4 ms data bits. an internal dac, driven by the 4 ms data bits, then recreates an analog approximation of the input voltage. the dac output voltage is subtracted from the analog input, and the difference is converted by the ls (least significant) adc to provide the 4 ls data bits. the ms flash adc also has one additional comparator to detect over- range on the analog input. operating sequence the ad7821 has two operating modes. the rd mode allows a conversion to be started and data to be read with a single, ex- tended, read operation (i.e., cs and rd are taken low). the conversion process is timed out by internal one-shots. the wr- rd mode uses wr to start a conversion and rd to read the data and allows the conversion timing to be externally con- trolled. the operating sequence for the wr-rd mode is shown in figure 3. figure 3. operating sequence (wr-rd mode) a conversion is initiated and the analog input signal (v in ) sampled on the falling edge of wr (falling edge of rd , rd mode). a setup time (t p , delay time between conversions) of 350 ns is required prior to this falling edge. see digital interface section for more details. when wr is low, the internal ms (most significant) adc compares the sampled analog input with the reference ladder to provide the 4 ms data bits. a minimum of 250 ns is required for this comparison. on the rising edge of wr , the ms data result is latched internally and the ls (least significant) conversion begins, to yield the 4 ls data bits. int goes low typically 380 ns after the rising edge of wr . this indi- cates the ls conversion is complete and that both the ls and ms data results are latched into the output buffer. rd going low then enables the output data. if a faster conversion time is required, the rd line can be brought low 250 ns after wr goes high. this latches both the ls and ms data bits and outputs the conversion result on db0Cdb7. reference and input the v ref (C) and v ref (+) reference inputs on the ad7821 are fully differential and define the zero and full-scale input range of the adc. the transfer characteristic of the part is defined by the integer value of the following expression: data ( lsbs ) = 256 v in - v ref ( - ) v ref ( + ) - v ref ( - ) ? ? + 0.5 as a result, the analog input (v in ) of the device can easily be set up to provide both unipolar and bipolar operation. the data output code for unipolar and bipolar operation is natural binary and offset binary, respectively. the span of the analog input voltage can easily be varied. by reducing the reference span, v ref (+) C v ref (C), to less than 5 v the sensitivity of the converter can be increased (i.e., if v ref = 2 v then 1 lsb = 7.8 mv). the reference flexibility also allows the input span for unipolar operation to be offset from zero (v ref (C) > gnd). additionally, the input/reference ar- rangement facilitates ratiometric operation. figures 4 and 5 show some configurations which are possible. for minimum noise a 47 m f capacitor in parallel with a 0.1 m f capacitor should be connected between the reference inputs and gnd. ad7821 rev. a C7C figure 4. power supply as reference. unipolar operation (0 to + 5 v) figure 5. external reference. bipolar operation (C2.5 v to +2.5 v) input current the analog input of the ad7821 behaves somewhat differently to conventional a/d converters. this is due to the adcs sampled data comparators, which take varying amounts of input current depending on the cycle of the converter. the equivalent input circuit of the ad7821 is shown in figure 6. when a conversion ends (e.g., falling edge of int , wr-rd mode, t rd > t intl ) all the input switches are closed and v in is connected to the comparators of the internal ls and ms adcs. therefore, v in is connected to 31 one-pf input capacitors simultaneously . figure 6. ad7821 equivalent input circuit the input capacitors must charge to the input voltage through the on resistance of the analog switches (about 2 k w to 5 k w ). in addition, about 12 pf of input stray capacitance must be charged. the analog input can be modeled as an equivalent rc network as shown in figure 7. as r s (source impedance) increases, the input capacitance takes longer to charge. the comparators track the analog input between conversions. a minimum delay time (t p ) of 350 ns is required between conver- sions to allow for voltage source settling and comparator track- ing time. this allows input time constants of 50 ns without settling time problems. typical total input capacitance values of 55 pf allow r s to be 0.9 k w without lengthening t p to give v in more time to settle. figure 7. rc network model input transients transients on the analog input signal caused by charging current flowing into v in will not normally degrade the adcs performance. in effect, the ad7821 does not look at the in- put when these transients occur. the comparators inputs track v in and are not sampled until the falling edge of wr (wr-rd mode) or rd (rd mode), so at least 350 ns (t p ) is provided to charge the adcs input capacitance. it is, therefore, not neces- sary to filter out these transients with an external capacitor at the v in terminal. inherent track-and-hold a major benefit of the ad7821s input structure is its ability to measure a variety of high-speed signals without the help of an external track-and-hold. any adc which does not have a built- in track-and-hold, regardless of its speed, requires the analog in- put to remain stable to at least 1/2 lsb for the duration of the conversion to maintain full accuracy. this requires the use of a track-and-hold whenever the input is a high-speed signal. the ad7821s sampled-data comparators, by nature of their input switching, inherently accomplish this track-and-hold function. although the conversion time for the ad7821 is 660 ns (wr-rd mode, t wr + t rd + t acc1 ), the time for which v in must be stable to 1/2 lsb is much smaller. the ad7821 tracks v in between conversions only, and its value on the falling edge of wr or rd in the wr-rd or rd modes, respectively, is the measured value. sinusoidal inputs the bandwidth of the built-in track-and-hold is 100 khz max (150 khz typ, 5 v p-p). this is limited by the analog bandwidth of the comparators and timing skew between the comparator switches. this means that the analog input frequency can be up to 100 khz without the aid of an external track-and-hold. the nyquist criterion requires that the sampling rate be at least twice the input frequency (i.e., 3 2 3 100 khz). this requires an ideal antialiasing filter with an infinite roll-off. to ease the prob- ad7821 rev. a C8C lem of antialiasing filter design, the sampling rate is usually set much greater than the nyquist criterion. the maximum sam- pling rate (f max ) for the ad7821 in the wr-rd mode, (t rd < t intl ) can be calculated as follows: f max = 1 t wr + t rd + t ri + t p f max = 1 0.25 e - 6 + 0.25 e - 6 + 0.15 e - 6 + 0.35 e - 6 t wr = write pulse width t rd = delay time between wr and rd pulses t ri = rd to int delay t p = delay time between conversions this permits a maximum sampling rate for the ad7821 of 1 mhz, which is much greater than the nyquist criterion for sampling a 100 khz analog input signal. digital signal processing applications in digital signal processing (dsp) application areas like voice recognition, echo cancellation and adaptive filtering, the dy- namic characteristics (signal-to-noise ratio, harmonic distor- tion, intermodulation distortion) of an adc are critical. since the ad7821 is a very fast adc with a built-in track-and-hold function, it is specified dynamically as well as with standard dc specifications (total unadjusted error, etc.). signal-to-noise ratio and distortion the dynamic performance of the ad7821 is evaluated by apply- ing a very low distortion sine wave signal to the analog input (v in ) which is then sampled at a 512 khz sampling rate. a fast fourier transform (fft) plot is then generated from which signal-to-noise ratio (snr) and harmonic distortion data are obtained. figure 8 shows a 2048 point fft plot of the ad7821 with an input signal of 100.25 khz. the snr is 49.1 db. it should be noted that the harmonics are taken into account when calculat- ing the snr. the theoretical relationship between snr and resolution (n) is expressed by the following equation: snr = ( 6.02 n + 1.76 ) db . . . . . . . . . . . . . . . . . . . . . (1) figure 8. ad7821 fft plot effective number of bits by working backwards from equation (1) it is possible to get a measure of adc performance expressed in effective number of bits (n). a plot of the effective number of bits versus input fre- quency is given in the typical performance characteristics sec- tion. the effective number of bits typically falls between 7.7 and 7.9, corresponding to snr figures of 48.1 and 49.7 db. intermodulation distortion for intermodulation distortion (imd), an fft plot consisting of very low distortion sine waves at two frequencies is generated by sampling an analog input applied to the adc. figure 9 shows a 2048 point plot for imd. figure 9. fft plot for imd histogram plot when a sine wave of specified frequency is applied to the v in in- put of the ad7821 and several thousand samples are taken, it is possible to plot a histogram showing the frequency of occur- rence of each of the 256 adc codes. a perfect adc produces a probability density function described by the equation: p ( v ) = 1 p ( a 2 - v 2 ) 1/ 2 where a is the peak amplitude of the sine wave and p ( v ) is the probability of occurrence at a voltage v. if a particular step is wider than the ideal 1 lsb width, then the code associated with that step will accumulate more counts than for the code for an ideal step. likewise, a step narrower than the ideal width will have fewer counts. missing codes are easily seen because a missing code means zero counts for a particular code. the absence of large spikes in the plot indicates small differen- tial nonlinearity. figure 10 shows a histogram plot for the ad7821, which corre- sponds very well with the ideal shape. the plot indicates very small differential nonlinearity and no missing codes for an input frequency of 100.25 khz. figure 10. ad7821 histogram plot ad7821 rev. a C9C in digital signal processing applications, where the ad7821 is used to sample ac signals, it is essential that the signal sampling occurs at exactly equal intervals. this minimizes errors due to sampling uncertainty or jitter. a precise timer or clock source, to start the adc conversion process, is the best method of gen- erating equidistant sampling intervals. the two modes of operation given in the data sheet are suitable for dsp applications because the sampling instant of the ad7821 is well defined. v in is sampled on the falling edge of wr or rd in the wr-rd or rd modes, respectively. digital interface the ad7821 has two basic interface modes which are deter- mined by the status of the mode pin. when this pin is low, the converter is in the rd mode, with this pin high, the ad7821 is set up for the wr-rd mode. the rd mode is designed for microprocessors that can be driven into a wait state. a read operation (i.e., cs and rd are taken low) starts a conversion and data is read when the conversion is complete. the wr-rd mode does not require mi- croprocessor wait states. a write operation (i.e., cs and wr are taken low) initiates a conversion, and a read opera- tion reads the result when the conversion is complete. rd mode (mode = 0) the timing diagram for the rd mode is shown in figure 11. this mode is intended for use with microprocessors which have a wait state facility, whereby a read instruction cycle can be extended to accommodate slow memory devices. a conversion is started by taking cs and rd low (read operation). both cs and rd are then kept low until output data appears. figure 11. rd mode in this mode, pin 6 of the ad7821 is configured as a status out- put, rdy. this rdy output can be used to drive the processor ready or wait input. it is an open drain output (no internal pull-up device) which goes low after the falling edge of cs and goes high impedance at the end of conversion. an int line is also provided which goes low when a conversion is complete. int returns high on the rising edge of cs or rd . wr-rd mode (mode = 1) in the wr-rd mode, pin 6 is configured as a write ( wr ) in- put for the ad7821. with cs low, conversion is initiated on the falling edge of wr . two options exist for reading data from the converter. in the first of these options the processor waits for the int sta- tus line to go low before reading the data (see figure 12a). int typically goes low within 380 ns after the rising edge of wr . it indicates that conversion is complete and that the data result is in the output latch. with cs low, the data outputs (db0Cdb7) are activated when rd goes low. int is reset by the rising edge of rd or cs . figure 12a. wr-rd mode (t rd > t intl ) the alternative option can be used to shorten the conversion time. this is a method for bypassing the internal time-out circuit. the int line is ignored and rd can be brought low 250 ns after the rising edge of wr . in this case rd going low transfers the data result into the output latch and activates the data output (db0Cdb7). int is driven low on the falling edge of rd and is reset on the rising edge of rd or cs . the timing for this interface is shown in figure 12b. figure 12b. wr-rd mode (t rd < t intl ) the ad7821 can also be used in stand-alone operation in the wr-rd mode. cs and rd are tied low, and a conversion is ini- tiated by bringing wr low. output data is valid 530 ns (t intl + t id ) after the rising edge of wr . the timing diagram for this mode is shown in figure 13. figure 13. wr-rd mode stand-alone operation, cs = rd = 0 ad7821 rev. a C10C microprocessor interfacing the ad7821 is designed for easy interfacing to microprocessors as a memory mapped peripheral or an i/o device. this reduces to a minimum the amount of external logic required for interfacing. ad7821 C 68008 interface figure 14 shows an ad7821 interface to the 68008 micropro- cessor. the adc is configured for the rd interface mode. this means that one read instruction starts a conversion and reads the result when the conversion is completed. the read cycle is stretched out over the entire conversion period by taking the int line back to the dtack input of the 68008. starting a conversion and reading the relevant data consists of a ad7821 rev. a C11C applying the ad7821 the ad7821 is specified for a unipolar input range of 0 to +5 v and a bipolar input range of C2.5 v to +2.5 v. the v ref (C) and v ref (+) voltages required for these input ranges are outlined be- low. see the typical performance characteristics section for op- eration with unspecified input voltage ranges. unipolar operation figure 18 gives the configuration and reference voltages re- quired for 0 v to +5 v operation. the nominal transfer charac- teristic for this input range is shown in figure 19. the output code is natural binary with 1 lsb = (5/256) v = 19.5 mv. figure 18. ad7821 unipolar/bipolar operation figure 19. nominal transfer characteristic for unipolar (0 v to +5 v) operation bipolar operation figure 18 gives the configuration and reference voltages required for C2.5 v to +2.5 v operation. the nominal transfer characteristic for this input range is shown in figure 20. the output code is offset binary with 1 lsb = ([+2.5 C (C2.5)]/256) v = 19.5 mv. figure 20. nominal transfer characteristic for bipolar (C2.5 v to +2.5 v) operation 16-channel telecom a/d converter the fast sampling rate (1 mhz) and bipolar operation of the ad7821 makes it useful in telecom applications for sampling a number of input channels using a multiplexer. figure 21 shows a circuit for such an application. the maximum signal frequency required for acceptable quality in telecom applications is 3 khz. the circuit given in figure 21 permits each of the 16-input channels to be sampled at a rate of 16 khz maximum. the sampling rate takes account of such multiplexer parameters as t on , settling time etc. the circuit also eases the problem of the antialiasing filter design by sampling at a rate much greater than that required by the nyquist criterion. figure 21. 16-channel telecom a/d converter system ad7821 rev. a C12C outline dimensions dimensions shown in inches and (mm). 20-pin plastic dip (n-20) 20-pin cerdip (q-20) 20-terminal plastic leaded chip carrier (p-20a) 20-terminal leadless ceramic chip carrier (e-20a) c1186C10C4/88 printed in u.s.a. simultaneous sampling a/d converters the ad7821s inherent track-and-hold and well-defined sam- pling instant makes it useful, in such applications as sonar, where a number of input channels are required to be sampled simultaneously. figure 22 shows a circuit for such an applica- tion. figure 22. simultaneous sampling a/d converters the actual sampling instant, which is the instant at which v in is measured, occurs approximately 50 ns after the falling edge of wr or rd in the wr-rd or rd modes, respectively, due to in- ternal logic delays. however, the internal logic delay and, there- fore, the sampling instant can vary from device to device, but is typically within 5 ns. this means that a maximum common in- put sine wave of 2.5 v at 32 khz, applied to any number of ad7821s in the circuit of figure 22, will yield a maximum dif- ference between the converter outputs of typically 1/4 lsb. |
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