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AD883B Datasheet(PDF) 11 Page - Analog Devices |
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AD883B Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page AD620 REV. E –11– Make vs. Buy: A Typical Bridge Application Error Budget The AD620 offers improved performance over “homebrew” three op amp IA designs, along with smaller size, fewer compo- nents and 10 × lower supply current. In the typical application, shown in Figure 34, a gain of 100 is required to amplify a bridge output of 20 mV full scale over the industrial temperature range of –40 °C to +85°C. The error budget table below shows how to calculate the effect various error sources have on circuit accuracy. Regardless of the system in which it is being used, the AD620 provides greater accuracy, and at low power and price. In simple R = 350 +10V PRECISION BRIDGE TRANSDUCER AD620A MONOLITHIC INSTRUMENTATION AMPLIFIER, G = 100 “HOMEBREW” IN-AMP, G = 100 *0.02% RESISTOR MATCH, 3PPM/ C TRACKING **DISCRETE 1% RESISTOR, 100PPM/ C TRACKING SUPPLY CURRENT = 15mA MAX 100 ** 10k * 10k ** 10k * 10k * 10k ** 10k * SUPPLY CURRENT = 1.3mA MAX OP07D OP07D OP07D AD620A RG 499 REFERENCE R = 350 R = 350 R = 350 Figure 34. Make vs. Buy Table I. Make vs. Buy Error Budget AD620 Circuit “Homebrew” Circuit Error, ppm of Full Scale Error Source Calculation Calculation AD620 Homebrew ABSOLUTE ACCURACY at TA = +25°C Input Offset Voltage, µV 125 µV/20 mV (150 µV × √2)/20 mV 16,250 10,607 Output Offset Voltage, µV 1000 µV/100/20 mV ((150 µV × 2)/100)/20 mV 14,500 10,150 Input Offset Current, nA 2 nA × 350 Ω/20 mV (6 nA × 350 Ω)/20 mV 14,118 14,153 CMR, dB 110 dB →3.16 ppm, × 5 V/20 mV (0.02% Match × 5 V)/20 mV/100 14,791 10,500 Total Absolute Error 17,558 11,310 DRIFT TO +85 °C Gain Drift, ppm/ °C (50 ppm + 10 ppm) × 60°C 100 ppm/ °C Track × 60°C 13,600 16,000 Input Offset Voltage Drift, µV/°C1 µV/°C × 60°C/20 mV (2.5 µV/°C × √2 × 60°C)/20 mV 13,000 10,607 Output Offset Voltage Drift, µV/°C 15 µV/°C × 60°C/100/20 mV (2.5 µV/°C × 2 × 60°C)/100/20 mV 14,450 10,150 Total Drift Error 17,050 16,757 RESOLUTION Gain Nonlinearity, ppm of Full Scale 40 ppm 40 ppm 14,140 10,140 Typ 0.1 Hz–10 Hz Voltage Noise, µV p-p 0.28 µV p-p/20 mV (0.38 µV p-p × √2)/20 mV 141,14 13,127 Total Resolution Error 14,154 101,67 Grand Total Error 14,662 28,134 G = 100, VS = ± 15 V. (All errors are min/max and referred to input.) systems, absolute accuracy and drift errors are by far the most significant contributors to error. In more complex systems with an intelligent processor, an autogain/autozero cycle will remove all absolute accuracy and drift errors leaving only the resolution errors of gain nonlinearity and noise, thus allowing full 14-bit accuracy. Note that for the homebrew circuit, the OP07 specifications for input voltage offset and noise have been multiplied by √2. This is because a three op amp type in-amp has two op amps at its inputs, both contributing to the overall input error. |
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