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AD625SCHIPS bảng dữ liệu(PDF) 11 Page - Analog Devices |
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AD625SCHIPS bảng dữ liệu(HTML) 11 Page - Analog Devices |
11 / 16 page AD625 REV. D –10– the I × R drops “inside the loop” and virtually eliminating this error source. Typically, IC instrumentation amplifiers are rated for a full ±10 volt output swing into 2 k Ω. In some applications, however, the need exists to drive more current into heavier loads. Figure 29 shows how a high-current booster may be connected “inside the loop” of an instrumentation amplifier. By using an external power boosting circuit, the power dissipated by the AD625 will remain low, thereby, minimizing the errors induced by self- heating. The effects of nonlinearities, offset and gain inaccura- cies of the buffer are reduced by the loop gain of the AD625’s output amplifier. AD625 +VS –VS RF RG RF VIN+ VIN– RI SENSE REFERENCE X1 Figure 29. AD625 /Instrumentation Amplifier with Output Current Booster REFERENCE TERMINAL The reference terminal may be used to offset the output by up to ±10 V. This is useful when the load is “floating” or does not share a ground with the rest of the system. It also provides a direct means of injecting a precise offset. However, it must be remembered that the total output swing is ±10 volts, from ground, to be shared between signal and reference offset. The AD625 reference terminal must be presented with nearly zero impedance. Any significant resistance, including those caused by PC layouts or other connection techniques, will in- crease the gain of the noninverting signal path, thereby, upset- ting the common-mode rejection of the in-amp. Inadvertent thermocouple connections created in the sense and reference lines should also be avoided as they will directly affect the out- put offset voltage and output offset voltage drift. In the AD625 a reference source resistance will unbalance the CMR trim by the ratio of 10 k Ω/R REF. For example, if the refer- ence source impedance is 1 Ω, CMR will be reduced to 80 dB (10 k Ω/1 Ω = 80 dB). An operational amplifier may be used to provide the low impedance reference point as shown in Figure 30. The input offset voltage characteristics of that amplifier will add directly to the output offset voltage performance of the instrumentation amplifier. The circuit of Figure 30 also shows a CMOS DAC operating in the bipolar mode and connected to the reference terminal to provide software controllable offset adjustments. The total offset range is equal to ±(V REF/2 × R5/R4), however, to be symmetri- cal about 0 V R3 = 2 × R4. The offset per bit is equal to the total offset range divided by 2 N, where N = number of bits of the DAC. The range of offset for Figure 30 is ±120 mV, and the offset is incremented in steps of 0.9375 mV/LSB. AD625 +VS –VS VOUT SENSE AD7502 A0 A1 EN GND VDD VSS +IN –IN 1/2 AD712 1/2 AD712 REFERENCE VREF AD589 1.2V VS 39k MSB LSB DATA INPUTS CS WR +VS AD7524 8-BIT DAC RFB C1 OUT 1 OUT 2 +VS R4 10k R3 20k 5k –VS R5 2k 0.01 F Figure 30. Software Controllable Offset An instrumentation amplifier can be turned into a voltage-to- current converter by taking advantage of the sense and reference terminals as shown in Figure 31. AD625 RF RG RF VIN+ VIN– SENSE IL AD711 LOAD +VX– R1 Figure 31. Voltage-to-Current Converter By establishing a reference at the “low” side of a current setting resistor, an output current may be defined as a function of input voltage, gain and the value of that resistor. Since only a small current is demanded at the input of the buffer amplifier A1, the forced current IL will largely flow through the load. Offset and drift specifications of A2 must be added to the output offset and drift specifications of the In-Amp. INPUT AND OUTPUT OFFSET VOLTAGE Offset voltage specifications are often considered a figure of merit for instrumentation amplifiers. While initial offset may be adjusted to zero, shifts in offset voltage due to temperature variations will cause errors. Intelligent systems can often correct for this factor with an autozero cycle, but this requires extra circuitry. |
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