Stanford Research Systems SR810 Manual page 42

Current Input (I)
The current input on the SR810 uses the A input
BNC. The current input has a 1 kΩ input
impedance and a current gain of either 106 or
8
10 Volts/Amp. Currents from 1 µA down to 2 fA
full scale can be measured.
The impedance of the signal source is the most
important factor to consider in deciding between
voltage and current measurements.
For high source impedances, greater than 1 MΩ
6
(10 gain) or 100 MΩ (10
currents, use the current input. Its relatively low
impedance greatly reduces the amplitude and
phase errors caused by the cable capacitance-
source impedance time constant. The cable
capacitance should still be kept small to minimize
the high frequency noise gain of the current
preamplifier.
For moderate to low source impedances, or larger
currents, the voltage input is preferred. A small
value resistor may be used to shunt the signal
current and generate a voltage signal. The lock-in
then measures the voltage across the shunt
resistor. Select the resistor value to keep the
shunt voltage small (so it does not affect the
source current) while providing enough signal for
the lock-in to measure.
8
gain), and small
3-17
SR810 Basics
Which current gain should you use? The current
gain determines the input current noise of the
lock-in as well as its measurement bandwidth.
Signals far above the input bandwidth are
attenuated by 6 dB/oct. The noise and bandwidth
are listed below.
Gain Noise
6
10 130 fA/√Hz
8
10 13 fA/√Hz
AC vs DC Coupling
The signal input can be either AC or DC coupled.
The AC coupling high pass filter passes signals
above 160 mHz (0.16 Hz) and attenuates signals
at lower frequencies. AC coupling should be used
at frequencies above 160 mHz whenever possible.
At lower frequencies, DC coupling is required.
A DC signal, if not removed by the AC coupling
filter, will multiply with the reference sine wave and
produce an output at the reference frequency.
This signal is not normally present and needs to
be removed by the low pass filter. If the DC
component of the signal is large, then this output
will be large and require a long time constant to
remove. AC coupling removes the DC component
of the signal without any sacrifice in signal as long
as the frequency is above 160 mHz.
The current input current to voltage preamplifier is
always DC coupled. AC coupling can be selected
following the current preamplifier to remove any
DC current signal.
Bandwidth
70 kHz
700 Hz