Kottan's Thoughts On:

Lockin Amplifiers:
With some opto-electronic thin film devices, lock-ins show a constant offset of 3e-11 A when measuring with the current input. This has been seen both on SRS and EG&G lock-ins. Presumably this could come from the fact that the device has a very different impedance from the lock-in current input. Always do a baseline check with your data.