Stanford Research Systems SR865A Operation Manual page 67

Chapter 2 Basics 49
In the SR865A the only real gain is in the input amplifier, which amplifies the signal
before it reaches the A/D converter. After this point, the signal is processed digitally so
there's no further gain. Therefore, the only question is how to set the gain of the input
amplifier.
In principle, the gain only needs to be high enough such that the input noise of the signal
and the amplifier is greater than the input noise of the A/D converter. Increasing the
analog gain beyond this point will not reduce the output noise.
Input Range
The Input Range sets the analog gain in the SR865A. The settings reflect the largest
signal at the input before the amplifier overloads. Setting the Input Range to a smaller
value increases the analog gain. The signal strength LEDs on the front panel indicate how
much of the A/D range is being used. The general rule is to decrease the Input Range as
much as possible without overloading the amplifier. This increases the A/D range used
and optimizes the output noise performance of the lock-in. At an Input Range of 10 mV
no additional gain is available since noise performance will not improve with more gain.
Remember, we have defined dynamic reserve as a property of the input signal generated
by the experiment. The ratio of noise to signal at the BNC is determined by factors
outside the lock-in. The Input Range must be set to accommodate the largest signal
present at the input, whether it is at f or is just noise. Once the Input Range is set, use
ref
the Time Constant filters to achieve the best results. Then set the Sensitivity to optimize
the bar graphs and numeric displays as well as the X, Y and R outputs.
SR865A DSP Lock-in Amplifier