
91-16 Signal Processing
R
1
and R
2
must be matched and R
G
should be comparable to R
2
. Noise in instrumentation ampliers
built from discrete op amps may exceed that from integrated IAs because noise from separate op amps
will be uncorrelated.
91.5.2 Composite Instrumentation Ampliers
Instrumentation ampliers have a gain limited to about 10,000, at low frequencies, and a bandwidth
around 4 MHz for a gain of 10. Moreover, their inputs must be either dc-coupled or, if ac-coupled with
input series capacitors, there must be a path for bias currents; if that path is a resistor from each input
to ground, then the common-mode input impedance Z
c
de