AD620 芯片

93 dB min Common-Mode Rejection Ratio (G = 10)

0.28 mV p-p Noise (0.1 Hz to 10 Hz)

THEORY OF OPERATION

The AD620 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

0.15% at G = 100) with only one resistor. Monolithic construction

and laser wafer trimming allow the tight matching and

tracking of circuit components, thus insuring the high level of

performance inherent in this circuit.

The input transistors Q1 and Q2 provide a single differentialpair

bipolar input for high precision (Figure 31), yet offer 10′

lower Input Bias Current thanks to Superbeta processing. 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 subtracter 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 transconductance

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 gainrelated

errors. (b) The gain-bandwidth product (determined by

C1, C2 and the preamp transconductance) increases with programmed

gain, thus optimizing frequency response. (c) The

input voltage noise is reduced to a value of 9 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 24.7 kW, allowing the gain to be programmed accurately

with a single external resistor.