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AD622 Datasheet(PDF) 9 Page - Analog Devices |
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AD622 Datasheet(HTML) 9 Page - Analog Devices |
9 / 16 page AD622 Rev. D | Page 9 of 16 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 construction and laser wafer trimming allow the tight matching and tracking of circuit components, thus insuring AD622 performance. Input Transistor Q1 and Input Transistor Q2 provide a single differential-pair bipolar input for high precision (see Figure 16). Feedback through the Q1-A1-R1 loop and the Q2-A2-R2 loop maintains constant collector current of the Q1 and Q2 input devices, thereby impressing the input voltage across External Gain-Setting Resistor RG. This creates a differential gain from the inputs to the A1 and A2 outputs given by G = (R1 + R2)/RG + 1. Unity-Gain Subtracter A3 removes any common-mode signal, yielding a single-ended output referred to the REF pin potential. VB –VS A1 A2 A3 C2 RG R1 R2 GAIN SENSE GAIN SENSE R3 400Ω 10kΩ 10kΩ I2 I1 10kΩ REF 10kΩ +IN –IN 20µA 20µA R4 400Ω OUTPUT C1 Q2 Q1 Figure 16. Simplified Schematic of the AD622 The value of RG also determines the transconductance of the preamp stage. As RG is reduced for larger gains, the trans- conductance increases asymptotically to that of the input transistors. This has the following three important advantages: • Open-loop gain is boosted for increasing programmed gain, thus reducing gain-related errors. • The gain-bandwidth product (determined by C1, C2, and the preamp transconductance) increases with programmed gain, thus optimizing frequency response. • The input voltage noise is reduced to a value of 12 nV/√Hz, 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 25.25 kΩ, allowing the gain to be programmed accurately with a single external resistor. MAKE vs. BUY: A TYPICAL APPLICATION ERROR BUDGET The AD622 offers cost and performance advantages over discrete two op amp instrumentation amplifier designs along with smaller size and fewer components. In a typical application shown in Figure 17, a gain of 10 is required to receive and amplify a 0 to 20 mA signal from the AD694 current transmitter. The current is converted to a voltage in a 50 Ω shunt. In applications where transmission is over long distances, line impedance 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 kΩ resistors. The error budget detailed in Table 4 shows how to calculate the effect of various error sources on circuit accuracy. AD694 0 TO 20mA TRANSMITTER RL2 10Ω RL2 10Ω 0 TO 20mA 50Ω 0 TO 20mA CURRENT LOOP WITH 50Ω SHUNT IMPEDANCE RG 5.62kΩ 1kΩ 1kΩ REF AD622 AD622 MONOLITHIC INSTRUMENTATION AMPLIFIER, G = 9.986 HOMEBREW IN-AMP, G = 10 1kΩ 1kΩ 1/2 LT1013 1/2 LT1013 9kΩ* 1kΩ* 1kΩ* 9kΩ* – + VIN *0.1% RESISTOR MATCH, 50ppm/°C TRACKING Figure 17. Make vs. Buy |
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