I would like to measure output impedance using the J2111A Current Injector, but the 25mA offset is way above my typical operating current. Why is this offset needed, and is there a way around it?

The J2111A includes bias positions of -25mA, 0 and +25mA.  This bias is provided as a convenience for the user.  Since the J2111A can only sink current it is necessary to provide a bias in order to put the device into class A operation.  If we did not do this, only one half of the analyzer signal would be provided, resulting in a severely distorted signal and poor accuracy.

In cases where the 25mA is too much, it is possible to provide an external bias.  The modulation input is 50 Ohms and the transconductance of the J2111A is 10mS.  We routinely use the bias injector along with the J2111A for measuring references.  This combination results in 50uA/V and at the 50V limit of the bias injector the J2111A can produce up to 2.5mA.  The typical offset in the J2111A is 150uA, and it can be as high as 400uA.  It is also possible to use the J2110A in conjunction with the J2111A.