Stanford Research Systems SR570 User Manual page 37

CAPACITANCE EFFECTS
Feedback Capacitance
All op amps have some parasitic capacitance
associated with their inputs and output. Together
with the capacitance of the source being
measured and the resistors in the amplifier
circuit, the parasitics affect the frequency
response of the amplifier. Typically, this effect
is manifested as overshoot or "ringing" in the
square wave response of an instrument. One way
to compensate for these unwanted effects is to
put a capacitor in the feedback loop of the op
amp. The value of this capacitor is chosen to
optimize the frequency response of the circuit.
In the SR570, we have implemented a variable
feedback capacitance across the amplifier which
can be controlled from the instrument's front
panel. To get the best frequency response, the
FREQ COMP can be adjusted to give a clean
square wave output with a square wave input. It
is important that the source resistance and
capacitance be the same for the adjustment
procedure as it will be for the actual
measurement to get the best results.
Input Capacitance
One of the most important differences between
using a current amp instead of a voltage amp is
the effect that input capacitance has on the noise
performance of the instrument. In the figure
above, we see that in the traditional
configuration of a current amplifier, with a
APPENDIX C
virtual null at the input and an essentially infinite
input resistance, any voltage noise that appears at the
input to the op amp will also appear at the output
without any gain. On the other hand, if we introduce
some capacitance from the input to ground – say,
coaxial cable capacitance (about 100 pF/m) – then
we have gain for the op amp input voltage noise.
Let's look at the effect that input capacitance would
have on a typical measurement. If we make a
measurement on the 1 nA/V scale, then the feedback
resistor in the low noise mode is 1 GW. The
bandwidth of this sensitivity range, without any
filters in the circuit, is about 20 Hz. If we use one
meter of coax cable at the input, then we have about
100 pF of input capacitance , which at 20 Hz is
about 80 MW to ground. Therefore, the input
voltage noise of the op amp is amplified with a gain
of around 13! Even more important is that this gain
increases with increasing values of input
capacitance.
Incidentally, the FREQ COMP capacitor mentioned
above also has the effect of limiting the ultimate
value of noise gain in the circuit to the ratio of the
input capacitance to the feedback capacitance.
There are a few straightfoward precautions that can
be taken to minimize the effects of input
capacitance:
1) Place the amplifier as close as possible to the
signal being measured and use the shortest cable
length necessary to connect them.
2) Use high quality, low noise coaxial cables.
3) Reduce any stray capacitance to ground at the
output of the circuit being measured.
4) If the amplifier must be placed a large distance
from the circuit under test, then use triaxial cable
with both a guard and a shield to reduce the
capacitance to ground.
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