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Precisely measuring specifications of differential analog and mixed-signal circuits is a difficult problem for self-test development because the imbalance introduced by the design-for-test circuitry on the differential signaling causes nonlinearity on the test stimulus, resulting in degrading device-under-test (DUT) performance. This problem triggers low test accuracy and serious yield loss. This brief proposes a novel test methodology to accurately predict individual DUT specifications by overcoming the imbalance problem with the imbalance generator, a radio-frequency transformer, and a programmable capacitor array based on a loopback test configuration. The imbalance generator produces spectral loopback responses of different weight. Nonlinear equations are then derived to characterize DUT specifications. Hardware measurement results show that this approach can be used to predict the specifications of a DUT in a production test.