Stanford Research Systems SR865A, Operation Manual

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Chapter 2 Basics 53
SR865A DSP Lock-in Amplifier
ground loops). The CMRR decreases by about 6 dB/octave (20 dB/decade) starting at
around 1 kHz.
ac vs dc Coupling
The signal input can be either ac or dc coupled. The ac coupling passes signals above 160
mHz (0.16 Hz) and attenuates signals at lower frequencies. ac coupling also degrades the
common mode rejection of differential inputs.
At low signal 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 a PSD output at the reference frequency (sometimes referred to as 1f).
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 or synchronous filter 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.
Current Input (I)
The current input on the SR865A uses a separate input BNC. The current input has a gain
of either 10
6
or 10
8
Volts/Amp
(1 μA or 10 nA ra
nge). The input burden resistance is
either 100 Ω (1 μA range ) or 1 kΩ (10 nA range). Currents from 3 µA down to 1 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 i mpedances, greater than 1 MΩ (1 μA range ) or 100 MΩ ( 10 nA range),
and small currents, use the current input. Its relatively low input 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.
Which current range should you use? The current range 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.
Range Input Noise Bandwidth
1 μA 130 fA/√Hz
400 kHz
10 nA 13 fA/√Hz 2 kHz
The current to voltage preamplifier is always dc coupled.