Verifying Specifications - Stanford Research Systems SR570 User Manual

Verifying Specifications

To verify the specifications given for the
SR570 current amplifier, a few
straightforward procedures should be
followed. First, the unit must be warmed up
for about 60 minutes. Second, for best
performance, the input current should produce
an output voltage of about 1 V or less. This
eliminates problems with slew rate limiting in
the various amplifier stages. Finally, care must
be taken in selection of a current source for
any measurement. Since an ideal current
source has infinite impedance, any source
used for measurements should have an
impedance greater than the inverse of the
sensitivity in ohms. Most specifications listed
above were measured with an input
capacitance of 100 pF. Higher input
capacitance will lead to a decrease in
performance.
Lets look at a simple example to illustrate
some of these principles. To test the gain and
frequency response of the instrument at 1
nA/V sensitivity, we might use a 1 V RMS
sine wave across a 1 GΩ resistor and through
1 meter of coax cable into the amplifier front-
end. The cable itself has about 100 pF of input
capacitance to ground. Any other sources of
capacitance will only increase this value, and
degrade the noise performance of the
instrument. The 1 GΩ resistor, while a good
current source at DC, will be less accurate at
higher frequencies due to capacitance of the
resistor. A typical resistor will have about 0.1
pF capacitance, which will provide a parallel
impedance of 1 GΩ at about 1.6 kHz. Since
this effect provides an alternate path for
current, the actual current to the amplifier will
be increased and may be misinterpreted as a
peaking in the frequency response of the
amplifier near 1 kHz. These are only a few
examples of what can go wrong when making
a measurement. It is very important that the
current source be completely characterized
before performing specification verification.
Keep in mind the following items when trying
to verify specifications or when making
sensitive measurements:
1. Make sure the source impedance is greater
than the inverse of the sensitivity (e.g.
with a sensitivity of 1 nA/V use a source
impedance greater than 1 GΩ).
2. If using a voltage source and a big resistor
to source the current, use several smaller
resistors in series instead of one larger
value to reduce the shunting capacitance.
3. Adjust the FREQ COMP pot on the front
panel to optimize frequency response for
the source character- istics and for the
sensitivity selected.
4. Use short lengths of high quality coaxial
cable to connect to the amplifier input.
5. Keep the amplifier output below 1 VRMS
to avoid slew rate limiting at high
frequencies.
6. Ground the chassis (green connector on
back) but do not connect the chassis to the
amplifier ground (white connector).
7. For low level measurements, disconnect
the power cord and use the internal
batteries.
ix
Specifications