Stanford Research Systems SR865A Operation Manual page 117

Chapter 3 Operation 99
BNC cables to connect the A output from the preamp to the A input of the SR865A and
the B output from the preamp (preamp ground) to the B input of the SR865A. Use A−B
as the input configuration. Be sure to twist the A and B cables so that there is no
differential noise pickup between the cables.
The signal coupling and input impedance is now determined by the preamp.
The SR865A does NOT compensate for the gain of the preamp. The preamps will default
to their maximum gain. Measurements made by the SR865A with a preamp need to be
divided by the gain of the preamp.
SR865A DSP Lock-in Amplifier