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AD708AQ bảng dữ liệu(PDF) 7 Page - Analog Devices |
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AD708AQ bảng dữ liệu(HTML) 7 Page - Analog Devices |
7 / 8 page AD708 REV. B –7– CROSSTALK PERFORMANCE OF THE AD708 The AD708 exhibits very low crosstalk as shown in Figures 24, 25 and 26. Figure 24 shows the offset voltage induced in side B of the AD708 when side A’s output is moving slowly (0.2 Hz) from –10 V to +10 V under no load. This is the least stressful situation to the part since the overall power in the chip does not change; only the location of the power in the output devices changes. Figure 25 shows side B’s input offset voltage change when side A is driving a 2 k Ω load. Here the power is being changed in the chip with the maximum power change occurring at ±7.5 V. Figure 26 shows crosstalk under the most severe conditions. Side A is connected as a follower with 0 V input, and is now forced to sink and source ±5 mA of output current (Power = (30 V) (5 mA) = 150 mW). Even this large change in power causes only an 8 µV (linear) change in side B’s input offset voltage. OPERATION WITH A GAIN OF –100 To show the outstanding dc precision of the AD708 in real application, Table I shows an error budget calculation for a gain of –100 configuration shown in Figure 27. Table I. Maximum Error Contribution AV = 100 (S Grade) Error Sources (Full Scale: VOUT = 10 V, VIN = 100 mV) VOS 30 µV/100 mV = 300 ppm IOS (100 k Ω)(1 nA)/10 V = 10 ppm Gain (2 k Ω load) 10 V/(5*10 6))/100 mV = 20 ppm Noise 0.35 µV/100 mV = 4 ppm VOS Drift (0.3 µV/°C)/100 mV = 3 ppm/ °C = 334 ppm +3 ppm/ °C Total Unadjusted Error @ 25 °C = 334 ppm > 11 Bits –55 °C to +125°C = 634 ppm > 10 Bits With Offset Calibrated Out @ 25 °C = 34 ppm > 14 Bits –55 °C to +125°C = 334 ppm > 11 Bits Figure 27. Gain of –100 Configuration This error budget assumes no error in the resistor ratio and no error from power supply variation (the 120 dB minimum PSRR of the AD708S makes this a good assumption). The external resistors can cause gain error from mismatch and drift over temperature. High Precision Programmable Gain Amplifier The three op amp programmable gain amplifier shown in Figure 28 takes advantage of the outstanding matching characteristics of the AD708 to achieve high dc precision. The gains of the circuit Figure 28. Precision PGA are controlled by the select lines, A0 and A1 of the AD7502 multiplexer, and are 1, 10, 100 and 1000 in this design. The input stage attains very high dc precision due to the 30 µV maximum offset voltage match of the AD708S and the 1 nA maximum input bias current match. The accuracy is maintained over temperature because of the ultralow drift performance of the AD708. The output stage uses an AD707J and well matched resistors configured as a precision subtracter. To achieve 0.1% gain accuracy, along with high common-mode rejection, the circuit should be trimmed as follows: To maximize common-mode rejection: 1. Set the select lines for Gain = 1 and ground VINB. 2. Apply a precision dc voltage to VINA and trim RA until VO = –VINA to the required precision. 3. Next connect VINB to VINA and apply an input voltage equal to the full-scale common-mode expected. 4. Trim RB until VO = 0 V. To minimize gain errors: 1. Select Gain = 10 with the control lines and apply a differential input voltage. 2. Adjust the 100 Ω potentiometer such that V O = 10 VIN (adjust VIN magnitude as necessary). 3. Repeat for Gain = 100 and Gain = 1000, adjusting 1 k Ω and 10 k Ω potentiometers, respectively. The design shown should allow for 0.1% gain accuracy and 0.1 µV/V common-mode rejection when ±1% resistors and ±5% potentiometers are used. BRIDGE SIGNAL CONDITIONER The AD708 can be used in the circuit in Figure 29 to produce an accurate and inexpensive dynamic bridge conditioner. The low offset voltage match and low offset voltage drift match of the AD708 combine to achieve circuit performance better than all but the best instrumentation amplifiers. The AD708’s out- standing specs: open loop gain, input offset currents and low input bias currents, do not limit circuit accuracy. |
Số phần tương tự - AD708AQ |
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Mô tả tương tự - AD708AQ |
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