AD823 Dual, 16MHz, Rail-to-Rail FET Input Amplifier

AD823–Typical Characteristics801007060V S = +5V314 UNITS = 40µV908070V S = +5V317 UNITS = 0.4pA5060UNITS4030UNITS504020302010100–200 –150 –100 –50 0 50 100 150 200INPUT OFFSET VOLTAGE – µV001 2 3 4 5 6 7 8 9 10INPUT BIAS CURRENT – pAFigure 4. Typical Distribution of Input Offset VoltageFigure 7. Typical Distribution of Input Bias CurrentUNITS22201816141210864V S = +5V–55°C TO +125°C103 UNITSINPUT BIAS CURRENT – pA10k1k100101V S = +5VV CM = 0V20–6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 7INPUT OFFSET VOLTAGE DRIFT – µV/°C0.1025 50 75 100 125TEMPERATURE – °CFigure 5. Typical Distribution of Input Offset Voltage DriftFigure 8. Input Bias Current vs. Temperature31k2V S = +5VV S = ±15VINPUT BIAS CURRENT – pA10–1–2–3INPUT BIAS CURRENT – pA100101–4–5–4 –3 –2 –1 0 1 2 3 4 5COMMON MODE VOLTAGE – Volts0.1–16–12 –8 –4 0 4 8 12 16COMMON MODE VOLTAGE – VoltsFigure 6. Input Bias Current vs. Common-Mode VoltageFigure 9. Input Bias Current vs. Common-Mode Voltage–6–REV. 0

AD823Since the input stage uses n-channel JFETs, input current duringnormal operation is negative; the current flows out from theinput terminals. If the input voltage is driven more positive than+V S – 0.4 V, the input current will reverse direction as internaldevice junctions become forward biased. This is illustrated inFigure 6.A current limiting resistor should be used in series with the inputof the AD823 if there is a possibility of the input voltage exceedingthe positive supply by more than 300 mV, or if an inputvoltage will be applied to the AD823 when ±V S = 0. The amplifierwill be damaged if left in that condition for more than 10seconds. A 1 kΩ resistor allows the amplifier to withstand up to10 volts of continuous overvoltage, and increases the input voltagenoise by a negligible amount.Input voltages less than –V S are a completely different story.The amplifier can safely withstand input voltages 20 volts belowthe minus supply voltage as long as the total voltage from thepositive supply to the input terminal is less than 36 volts. Inaddition, the input stage typically maintains picoamp level inputcurrents across that input voltage range.The AD823 is designed for 16 nV/√Hz wideband input voltagenoise and maintains low noise performance to low frequencies(refer to Figure 15). This noise performance, along with theAD823’s low input current and current noise means that theAD823 contributes negligible noise for applications with sourceresistances greater than 10 kΩ and signal bandwidths greaterthan 1 kHz.OUTPUT CHARACTERISTICSThe AD823’s unique bipolar rail-to-rail output stage swingswithin 25 mV of the supplies with no external resistive load. TheAD823’s approximate output saturation resistance is 25 Ωsourcing and sinking. This can be used to estimate output saturationvoltage when driving heavier current loads. For instance,when driving 5 mA, the saturation voltage to the rails will be approximately125 mV.If the AD823’s output is driven hard against the output saturationvoltage, it will recover within 250 ns of the input returningto the amplifier’s linear operating region.A/D DriverThe rail-to-rail output of the AD823 makes it useful as an A/Ddriver in a single supply system. Because it is a dual op amp, itcan be used to drive both the analog input of the A/D along withits reference input. The high impedance FET input of theAD823 is well suited for minimally loading of high output impedancedevices.Figure 38 shows a schematic of an AD823 being used to driveboth the input and reference input of an AD1672, a 12-bit3 MSPS single supply A/D converter. One amplifier is configuredas a unity gain follower to drive the analog input of theAD1672 which is configured to accept an input voltage thatranges from 0 to 2.5 V.V INV REF(1.25V)1k2356+5VA841k1AD82370.1µF+5VA +5VD +5VD10µF 0.1 0.1µF µF10µF28 19+V CC +V DD20REFOUT21AIN1221549.9ΩAIN2CLOCK2324252627AD1672REFININ COMNCOMP2NCOMP1ACOM16REFDCOM COM19 18131412111098765432110µFOTRBIT1 (MSB)BIT2BIT3BIT4BIT5BIT6BIT7BIT8BIT9BIT10BIT11BIT12 (LSB)0.1µFFigure 38. AD823 Driving Input and Reference of theAD1672, a 12-Bit 3 MSPS A/D ConverterThe other amplifier is configured as a gain of two to drive thereference input from a 1.25 V reference. Although the AD1672has its own internal reference, there are systems that requiregreater accuracy than the internal reference provides. On theother hand, if the AD1672 internal reference is used, the secondAD823 amplifier can be used to buffer the reference voltage fordriving other circuitry while minimally loading the referencesource.The circuit was tested with a 500 kHz sine wave input that washeavily low pass filtered (60 dB) to minimize the harmonic contentat the input to the AD823. The digital output of theAD1672 was analyzed by performing an FFT.During the testing, it was observed that at 500 kHz, the outputof the AD823 cannot go below about 350 mV (operating withnegative supply at ground) without seriously degrading the secondharmonic distortion. Another test was performed with a200 Ω pull-down resistor to ground that allowed the output togo as low as 200 mV without seriously affecting the second harmonicdistortion. There was, however, a slight increase in thethird harmonic term with the resistor added, but it was still lessthan the second harmonic.REV. 0 –13–

AD823Figure 39 is an FFT plot of the results of driving the AD1672with the AD823 with no pull-down resistor. The input amplitudewas 2.15 V p-p and the lower voltage excursion was350 mV. The input frequency was 490 kHz, which was chosento spread the location of the harmonics.The distortion analysis is important for systems requiring goodfrequency domain performance. Other systems may requiregood time domain performance. The noise and settling timeperformance of the AD823 will provide the necessary informationfor its applicability for these systems.15dB/DIV5 6149 7V IN = 2.15Vp-pG = +1F I = 490kHzFigure 39. FFT of AD1672 Output Driven by AD8233 Volt, Single Supply Stereo Headphone DriverThe AD823 exhibits good current drive and THD+N performance,even at 3 V single supplies. At 20 kHz, total harmonicdistortion plus noise (THD+N) equals –62 dB (0.079%) for a300 mV p-p output signal. This is comparable to other singlesupply op amps which consume more power and cannot run on3 V power supplies.In Figure 40, each channel’s input signal is coupled via a 1 µFMylar capacitor. Resistor dividers set the dc voltage at the noninvertinginputs so that the output voltage is midway betweenthe power supplies (+1.5 V). The gain is 1.5. Each half of theAD823 can then be used to drive a headphone channel. A 5 Hzhigh-pass filter is realized by the 500 µF capacitors and theheadphones, which can be modeled as 32 ohm load resistors toground. This ensures that all signals in the audio frequencyrange (20 Hz–20 kHz) are delivered to the headphones.23 8CHANNEL 1CHANNEL 295.3kΩ1µFMYLAR95.3k1µFMYLAR+3V95.3k47.5k3 81/2AD823210k10k14.99k4.99k647.5k 1/2AD823 75 40.1µF+500µFHEADPHONES32Ω IMPEDANCE500µF+0.1µFFigure 40. 3 Volt Single Supply Stereo Headphone DriverSecond Order Low-Pass FilterFigure 41 depicts the AD823 configured as a second orderButterworth low-pass filter. With the values as shown, the cornerfrequency will be 200 kHz. The equations for componentselection are shown below:R1 = R2 = user selected (typical values: 10 kΩ to 100 kΩ).V INC1( farads ) =R120kR220kC128pF1.4142 πf cutoff R1 ; C2 = 0.7072 πf cutoff R1C256pF+5VC30.1µF1/2AD823–5VC40.1µF50pFFigure 41. Second Order Low-Pass FilterLRV OUTA plot of the filter is shown below; better than 50 dB of high frequencyrejection is provided.–14–REV. 0

AD823HIGH FREQUENCY REJECTION – dB0–10–20–30–40–50–601kV DB – V OUT10k 100k 1M 10M 100MFREQUENCY – HzFigure 42. Frequency Response of FilterSingle-Supply Half-Wave and Full-Wave RectifiersAn AD823 configured as a unity gain follower and operatedwith a single supply can be used as a simple half-wave rectifier.The AD823’s inputs maintain picoamp level input currents evenwhen driven well below the minus supply. The rectifier puts thatbehavior to good use, maintaining an input impedance of over10 11 Ω for input voltages from 1 volt from the positive supply to20 volts below the negative supply.The full- and half-wave rectifier shown in Figure 43 operates asfollows: when V IN is above ground, R1 is bootstrapped throughthe unity gain follower A1 and the loop of amplifier A2. Thisforces the inputs of A2 to be equal, thus no current flowsthrough R1 or R2, and the circuit output tracks the input. WhenV IN is below ground, the output of A1 is forced to ground. Thenoninverting input of amplifier A2 sees the ground level outputof A1, therefore, A2 operates as a unity gain inverter. The outputat node C is then a full-wave rectified version of the input.Node B is a buffered half-wave rectified version of the input.Input voltage supply to ±18 volts can be rectified, depending onthe voltage supply used.+V S0.01µFA831V IN A121/24AD823ABC10090100%R1100kΩ2V 2V200µs2V65A2R2100kΩ71/2AD823FULL-WAVERECTIFIED OUTPUTBHALF-WAVERECTIFIED OUTPUTFigure 43. Single Supply Half- and Full-Wave RectifierCREV. 0 –15–

AD823OUTLINE DIMENSIONSDimensions shown in inches and (mm).8-Lead Plastic DIP(N-8)PIN 10.165±0.01(4.19±0.25)85140.39 (9.91) MAX0.25(6.35)0.31(7.87)0.035±0.01(0.89±0.25)0.30 (7.62)REFC2035–7.5–5/950.125(3.18)MIN0.018±0.003(0.46±0.08)0.10(2.54)BSC0.033(0.84)NOM0.18±0.03(4.57±0.76)SEATINGPLANE0.011±0.003(0.28±0.08)15°0°8-Lead Plastic SOIC(SO-8)PIN 181540.1574 (4.00)0.1497 (3.80)0.2440 (6.20)0.2284 (5.80)0.0098 (0.25)0.0040 (0.10)0.1968 (5.00)0.1890 (4.80)0.0500(1.27)BSC0.0192 (0.49)0.0138 (0.35)0.0688 (1.75)0.0532 (1.35)0.0098 (0.25)0.0075 (0.19)8°0°0.0196 (0.50)x 45°0.0099 (0.25)0.0500 (1.27)0.0160 (0.41)PRINTED IN U.S.A.–16–REV. 0