Low-Frequency AC/DC Voltage Summing Box
This is a transformer-based voltage summing box for low-frequency transport measurements.
It combines a DC bias with a small AC excitation from a lock-in amplifier, allowing the sample to be driven with a well-defined voltage of the form Vout = VAC + VDC.
The device operates reliably in the approximately 17.7–50 Hz range for transport measurements, with load dependence, phase shift, AC linearity, and electrical pickup taken into account.