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(c) The input voltage noise is reduced to a value of 9 nV/VHz, determined mainly by the collector current and base resistance of the input devices. The internal gain resistors, R1 and R2, are trimmed to an absolute value of 24.7 kW, allowing the gain to be programmed accurately with a single external resistor. The gain equation is then ^ 49.4 kW , G =-+ 1 R, Figure 32. Settling Time Test Circuit G so that Rg = 49.4 kW G - 1 -10- REV. E AD620 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 components and 10x 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 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 0P07 specifications for input voltage offset and noise have been multiplied by V2. This is because a three op amp type in-amp has two op amps at its inputs, both contributing to the overall input error. PRECISION BRIDGE TRANSDUCER AD620A MONOLITHIC INSTRUMENTATION AMPLIFIER, G = 100 SUPPLY CURRENT = 1.3mA MAX “HOMEBREW” IN-AMP, G = 100 *0.02% RESISTOR MATCH, 3PPM/°C TRACKING “DISCRETE 1% RESISTOR, 100PPM/°C TRACKING SUPPLY CURRENT = 15mA MAX Figure 34. Make vs. Buy Table I. Make vs. Buy Error Budget +10V AD620 Circuit “Homebrew” Circuit Error, ppm of Full Scale Error Source Calculation Calculation AD620 Homebrew ABSOLUTE ACCURACY at TA = +25°C Input Offset Voltage, mV 125 mV/20 mV (150 mV x V2)/20 mV 6,250 10,607 Output Offset Voltage, mV 1000 mV/100/20 mV ((150 mV x 2)/100)/20 mV 500 150 Input Offset Current, nA 2 nA x 350 W/20 mV (6 nA x 350 W)/20 mV 18 53 CMR, dB 110 dB^3.16 ppm, x 5 V/20 mV (0.02% Match x 5 V)/20 mV/100 791 500 Total Absolute Error 7,558 11,310 DRIFT TO +85°C Gain Drift, ppm/°C (50 ppm + 10 ppm) x 60°C 100 ppm/°C Track x 60°C 3,600 6,000 Input Offset Voltage Drift, mV/°C 1 mV/°C x 60°C/20 mV (2.5 mV/°C x V2 x 60°C)/20 mV 3,000 10,607 Output Offset Voltage Drift, mV/°C 15 mV/°C x 60°C/100/20 mV (2.5 mV/°C x 2 x 60°C)/100/20 mV 450 150 Total Drift Error 7,050 16,757 RESOLUTION Gain Nonlinearity, ppm of Full Scale 40 ppm 40 ppm 40 40 Typ 0.1 Hz-10 Hz Voltage Noise, mV p-p 0.28 mV p-p/20 mV (0.38 mV p-p x V2)/20 mV 14 27 Total Resolution Error 54 67 Grand Total Error 14,662 28,134 G = 100, Vs = ± 15 V. (All errors are min/max and referred to input.) REV. E -ii- AD620 +5V Figure 35. A Pressure Monitor Circuit which Operates on a +5 V Single Supply Pressure Measurement Although useful in many bridge applications such as weigh scales, the AD620 is especially suitable for higher resistance pressure sensors powered at lower voltages where small size and low power become more significant. Figure 35 shows a 3 kW pressure transducer bridge powered from +5 V. In such a circuit, the bridge consumes only 1.7 mA. Adding the AD620 and a buffered voltage divider allows the signal to be conditioned for only 3.8 mA of total supply current. Small size and low cost make the AD620 especially attractive for voltage output pressure transducers. Since it delivers low noise and drift, it will also serve applications such as diagnostic noninvasive blood pressure measurement. Medical ECG The low current noise of the AD620 allows its use in ECG monitors (Figure 36) where high source resistances of 1 MW or higher are not uncommon. The AD620’s low power, low supply voltage requirements, and space-saving 8-lead mini-DIP and SOIC package offerings make it an excellent choice for battery powered data recorders. Furthermore, the low bias currents and low current noise coupled with the low voltage noise of the AD620 improve the dynamic range for better performance. The value of capacitor C1 is chosen to maintain stability of the right leg drive loop. Proper safeguards, such as isolation, must be added to this circuit to protect the patient from possible harm. OUTPUT 1V/mV Figure 36. A Medical ECG Monitor Circuit +3V -12- REV. E AD620 Precision V-I Converter The AD620, along with another op amp and two resistors, makes a precision current source (Figure 37). The op amp buffers the reference terminal to maintain good CMR. The output voltage VX of the AD620 appears across R1, which converts it to a current. This current le...
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