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AD708BH Datasheet(PDF) 8 Page - Analog Devices |
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AD708BH Datasheet(HTML) 8 Page - Analog Devices |
8 / 8 page AD708 REV. B –8– As configured, the circuit only requires a gain resistor, RG, of suitable accuracy and a stable, accurate voltage reference. The transfer function is: VO = VREF [∆R/(R+∆R)][RG/R] and the only significant errors due to the AD708S are: VOSout = (VOSmatch)(2RG/R) = 30 mV VOSout(T) = (VOSdrift)(2RG/R) = 0.3 mV/ °C To achieve high accuracy, the resistor RG should be 0.1% or better and have a low drift coefficient. Figure 29. Bridge Signal Conditioning Circuit Figure 30. Precision Absolute Value Circuit PRECISION ABSOLUTE VALUE CIRCUIT The AD708 is ideally suited to the precision absolute value circuit shown in Figure 30. The low offset voltage match of the AD708 enables this circuit to accurately resolve the input signal. In addition, the tight offset voltage drift match maintains the resolution of the circuit over the full military temperature range. The AD708’s high dc open loop gain and exceptional gain linearity allows the circuit to perform well at both large and small signal levels. In this circuit, the only significant dc errors are due to the offset voltage of the two ampliliers, the input offset current match of the amplifiers, and the mismatch of the resistors. Errors associ- ated with the AD708S contribute less than 0.001% error over –55 °C to +125°C. Maximum error at 25 °C 30 µV + 10 kΩ () 1nA () 10V = 40 µV/10V = 4 ppm Maximum error at +125 °C or –55°C 50 µV + 2 nA () 10kΩ () 10V = 7 ppm @ +125°C Figure 31 shows VOUT vs. VIN for this circuit with a ±3mV input signal at 0.05 Hz. Note that the circuit exhibits very low offset at the zero crossing. This circuit can also produce VOUT = –|VIN| by reversing the polarity of the two diodes. Figure 31. Absolute Value Circuit Performance (Input Signal = 0.05 Hz) SELECTION OF PASSIVE COMPONENTS To takc full advantagc of thc high precision and low drift characteristics of the AD708, high quality passive components must be used. Discrete resistors and resistor networks with temperature coefficients of less than 10 ppm/ °C are available from Vishay, Caddock, PRP and others. OUTLINE DIMENSIONS Dimensions shown in inches and (mm). TO-99 (H) Package Cerdip (Q) Package Mini-DIP (N) Package |
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