Use low-tolerance resistors to
achieve high DC CMRR performance. Mismatching of resistors can also lead to
errors in gain and output accuracy.
All resistors and capacitors must
be verified space-grade for this design.
Rg sets the gain of
the input stage. R1a and R1b can be used to set the gain
of the second stage (see Design Steps).
Rf1 and Rf2
are nominally matched in this design. In general, Rf1 and
Rf2 do not need to be matched – it may be desirable in some cases
to have Rf1 and Rf2 unmatched so that the top amplifier
and bottom amplifier in the input stage have different gains. For example, if
Vcm is not at mid-supply but is closer to one of the rails,
Rf1 and Rf2 can be tuned so that neither of the input
stage amplifiers run out of headroom.
Integrated instrumentation
amplifiers normally have a fixed minimum gain. In addition to using an IA in
high-gain configurations, constructing a discrete IA like this affords the
flexibility to achieve any gain less than 1 V/V.
High-value resistors can degrade
the phase margin of the circuit and introduce additional noise in the
circuit.
Add an isolation resistor to the
output stage to drive large capacitive loads.
Linear operation is contingent
upon the input common-mode and the output swing ranges of the discrete op amps
used. For best performance, choose Vcm = (V+ +
V–) / 2 (mid-supply).
C2 along with
R3 || R4 forms a low-pass filter with a corner
frequency of 147.16 Hz.
The Vref pin must be
supplied by a low-impedance reference that can sink and source current, such as
a buffer. Using a high-impedance reference, such as a resistor divider with no
buffer, may result in a mismatch and degradation of CMRR.
Vout_min is chosen as
0.2 V for this design to avoid nonlinearities associated with the output of
LMP7704-SP swinging too close to the rail. If this design is done with a
different op amp, be sure to check the data sheet to determine the minimum and
maximum output values allowed.
The LMP7704-SP supply voltage of 10 V was selected according the
derating specifications provided by the National Aeronautics and Space
Administration (NASA) in document EEE-INST-002 (April 2008) and the
European Cooperation for Space Standardization (ECSS) in document ECSS-Q-ST-30-11C Rev.1 (4 October
2011). The documents specify an 80% and 90% derating of the absolute maximum
supply voltage for linear ICs, respectively.
This design can be implemented
with a single 4-channel LMP7704-SP or a similar device. See Design Alternative Op Amp for a wider supply op amp (36 V). Note
that the listed alternative device meets TID = 50 krad(Si).