Why does the DC current bias system indicate incorrect measurement values when the 42841A is turned off?

The 42841A in the system should never be turned off even when DC bias current is not required for inductor measurements. The 42841A feeds DC bias current to DUT from active DC current regulator circuits which have certain output impedances as shown in the figure below:

AC signal leakage from the bridge circuit to the 42841A does not significantly affect measurements since the bias current source output impedances are large enough to reduce the effect of leakage. When the 42841A is turned off, the DC current regulator circuits are deactivated and cannot maintain the required output impedances. As a result, measurement error increases due to the AC signal leakage current flow into the 42841A. When it is needed to accurately measure DUT without using the DC current bias source, perform as follows:

1. Turn the DC bias function of LCR meter to off.
2. Disconnect the bias current test fixture (the 42842A/B) from the 42841A.
3. Disconnect the bias current interface cable from the interface connector on the 42841A rear panel.
4. Perform open/short compensation for this measurement setup.