Source of DC offset in circuit
simulate this circuit &ndash; Schematic created using CircuitLab

_Figure 1a. A uni-polar squarewave will have an average DC value of 50% of peak. Figure 1(b). A bi-polar squarewave signal will have a DC value of 0 V.

It sounds as though you are using a uni-polar signal. This will have a DC offset of half of peak voltage.

To fix this you need to redesign the current source to alternate the current direction as shown in Figure 1b.