Stanford Research Systems SIM983 Operation And Service Manual page 61

5.1 Circuit Discussion
5.1.2.2 Offset voltage generator
5.1.2.3 Summing amplifier
SIM983 Scaling Amplifier
bandwidth) composite operational amplifier, running at gain 1. Cas-
coded radiofrequency FET Q201 provides the slew rate and the band-
width, whereas U201, a precision JFET operational amplifier (op-
amp), disciplines Q201 to a maximum of 900 V of o set voltage,
12 V C maximum o set drift, and 8.5 nV Hz typical noise.
output of the voltage bu er is monitored for overload by compara-
tor U213, which trips at the voltage limits specified in Section 1.2.4.1.
The o set voltage is provided by U204, a 16-bit digital-to-analog con-
verter (DAC). Because of the ultralow drift of the DAC and precision
resistor network R215, the temperature stability of the generated
o set is largely determined by the 5 ppm C typical performance
of scaling resistor R217. The integral nonlinearity of U204 puts
a 200 ppm limit on the overall accuracy of the instrument's o set.
A second, 12-bit DAC U206 allows for fine tuning of the generated
o set, cancelling the contribution of the input o set voltage of U201
and o sets in subsequent stages in order to achieve the specified ac-
curacy. The o set voltage is filtered by the 6.3 k
of U204 in combination with C206, with f
The voltages at the outputs of the input bu er and the two o set-
generating DACs are combined by a summing amplifier built
around U208A. This low-noise, high-speed op-amp is disciplined
by one half of precision op-amp U207, so its input o set contributes
negligibly to the overall o set error and the o set drift. However,
the input bias current of the op-amp does contribute to the error, and
this contribution is partially cancelled by a constant current injected
into the summing node through R219. The remaining contribution
is calibrated out via U204 and U206.
At this stage, three major contributions to the overall noise of
the SIM983 come into play; these contributions are comparable in
magnitude, and add in quadrature. The 1.5 nV Hz noise (at 10 kHz)
of U208 faces a noise gain of 6 from R216 and R217. The input
bu er contributes another 9.5 nV Hz. Finally, the Johnson noise
of the 2.5 k resistors of R216 and the 604
gain, yields 16.5 nV Hz, referenced to the input. Therefore, the
total noise at the output of this stage is typically 21 nV Hz. The
input bias current of U208, passing through the feedback portion
1
The 9.4 nV Hz noise at the output of the input voltage bu er includes the con-
tribution of the 1 k input protection resistor R203.
5 – 3
1
The
output resistance
11 Hz.
3 dB
R217, times the noise