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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.
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原文地址：http://www.cnblogs.com/chen-/p/3710433.html
