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AD622 Arkusz danych(PDF) 7 Page - Analog Devices |
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AD622 Arkusz danych(HTML) 7 Page - Analog Devices |
7 / 11 page AD622 REV. C –7– THEORY OF OPERATION The AD622 is a monolithic instrumentation amplifier based on a modification of the classic three op-amp approach. Absolute value trimming allows the user to program gain accurately (to 0.5% at G = 100) with only one resistor. Monolithic construc- tion and laser wafer trimming allow the tight matching and tracking of circuit components, thus insuring its performance. The input transistors Q1 and Q2 provide a single differential- pair bipolar input for high precision. Feedback through the Q1-A1-R1 loop and the Q2-A2-R2 loop maintains constant collector current of the input devices Q1, Q2 thereby impressing the input voltage across the external gain-setting resistor RG. This creates a differential gain from the inputs to the A1/A2 outputs given by G = (R1 + R2)/RG + 1. The unity-gain sub- tracter A3 removes any common-mode signal, yielding a single-ended output referred to the REF pin potential. The value of RG also determines the transconductance of the preamp stage. As RG is reduced for larger gains, the transcon- ductance increases asymptotically to that of the input transistors. This has three important advantages: (a) Open-loop gain is boosted for increasing programmed gain, thus reducing gain- related errors. (b) The gain-bandwidth product (determined by C1, C2 and the preamp transconductance) increases with pro- grammed gain, thus optimizing frequency response. (c) The input voltage noise is reduced to a value of 12 nV/ √Hz, deter- mined mainly by the collector current and base resistance of the input devices. The internal gain resistors, R1 and R2, are trimmed to an abso- lute value of 25.25 k Ω, allowing the gain to be programmed accurately with a single external resistor. Make vs. Buy: A Typical Application Error Budget The AD622 offers a cost and performance advantages over discrete “two op-amp” instrumentation amplifier designs along with smaller size and less components. In a typical application shown in Figure 14, a gain of 10 is required to receive and am- plify a 0–20 mA signal from the AD694 current transmitter. The current is converted to a voltage in a 50 Ω shunt. In appli- cations where transmission is over long distances, line imped- ance can be significant so that differential voltage measurement is essential. Where there is no connection between the ground returns of transmitter and receiver, there must be a dc path from each input to ground, implemented in this case using two 1 k Ω resistors. The error budget detailed in Table I shows how to calculate the effect various error sources have on circuit accuracy. The AD622 provides greater accuracy at lower cost. The higher cost of the “homebrew” circuit is dominated in this case by the matched resistor network. One could also realize a “homebrew” design using cheaper discrete resistors which would be either trimmed or hand selected to give high common-mode rejection. This level of common-mode rejection would however degrade significantly over temperature due to the drift mismatch of the discrete resistors. Note that for the homebrew circuit, the LT1013 specification for noise has been multiplied by √2. This is because a “two op- amp” type instrumentation amplifier has two op amps at its inputs, both contributing to the overall noise. “Homebrew” In Amp, G = 10 AD622 Monolithic Instrumentation Amplifier, G = 9.986 0–20 mA Current Loop with 50 Ω Shunt Impedance Figure 14. Make vs. Buy 0–20mA RL2 10 50 RL2 10 AD694 0–20mA TRANSMITTER 1/2 LT1013 1/2 LT1013 VIN *0.1% RESISTOR MATCH, 50ppm / C TRACKING 1k 1k 9k *1k *1k *9k * AD622 REFERENCE 1k RG 5.62k 1k |
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