Stanford Research Systems SIM983 Operation And Service Manual page 63

5.1 Circuit Discussion
SIM983 Scaling Amplifier
bias is achieved by mirroring the input current of the second half,
U211B, and injecting it into the input node of U211A. The remaining
input current produces a drift term that is roughly the same as, or
smaller than, the other dominant contributions to the o set stability
of the instrument.
This cancellation scheme increases the contribution of the gain
stage to the overall noise. The noise current of U211 is multiplied
by R(U210B) and by 2 As R(U210B) increases linearly with the gain
for G 1 this terms yields 21 nV Hz, referenced to the input.
The remaining noise contribution is from R(U210A) and R(U210B)
Their Johnson noise at the output of the stage depends on the gain
as
and for large gains is just the noise of the 10 k
referenced to the input. This 13.5 nV Hz term adds in quadrature
with the 22 nV Hz contribution of the earlier three stages, and with
the bias-current contribution, to yield 34 nV Hz ( f
most frequencies f
noise of the SIM983 is independent of the gain to within 2 nV Hz.
The capacitances of the analog switches
resistance MDAC add together at the output of the MDAC. This ca-
pacitance becomes the input capacitance of the inverting amplifier,
and its value places the ultimate limits on the small-signal band-
width achievable in the gain stage and with it, in the whole instru-
ment. The capacitance together with R(U210A) forms an input pole,
so if the gain of the amplifier is not rolled o with a capacitor in
the feedback path, the amplifier will oscillate. The amount of com-
pensation feedback capacitance desired for stability from oscillation
increases with decreasing G The compensation network consists of
PFETs Q205–Q208, funcioning as switches and chosen for ultralow
OFF capacitance, and capacitors C208–C210. One, both, or none
of C209 and C210 are inserted into the feedback path for four ranges
of the gain, resulting in four possible values of the gain-bandwidth
product of the stage (Page vi). With the feedback capacitor selected,
the phase margin of the amplifier improves with increasing G and
with it the overshoot and ringing in the step response decrease.
Output voltage bu er U212 enables the instrument to drive 50
loads. Comparator U215 indicates an overload at the specified output
voltage limits (Section 1.2.4.2).
3
Internal to the MDAC.
e G (1 G )
n
100 Hz, and for G
3
that configure the variable-
5 – 5
resistor R(U210A),
10 kHz). At
1 the input-referenced