When using IEPE sensors at very low frequencies (<5 Hz) or for quasi static (close to DC) applications, the method of decoupling the sensor signal from the approximate +10 volt DC bias of the sensor amplifier, can be a limiting factor in system low frequency response.

The conventional method of decoupling used in most Dytran power units is by means of a 10 µF capacitor. This capacitor, working into the input impedance of the readout instrument, acts as a high pass filter, effectively limiting the low frequency response of the measurement system.

The model 4115B power unit provides an alternative DC coupling mode. In this mode, the output signal from the sensor is recatalogd to a zero volts bias level by use of a DC–coupled, level shifting circuit, actually, a summing amplifier. An adjustable negative DC voltage is summed with the sensor DC bias (approx. 10 Volts), nulling the output voltage to zero volts.

Other features of the 4115B include an AC/DC coupling mode selector switch for situations where DC coupling is not advisable, such as in thermally unstable environments. A "monitor" switch allows the monitoring of the sensor bas voltage or the output voltage level of the 4115B with a front panel mounted voltmeter.

An "output zero" knob on the front panel controls a 10 turn potentiometer which allows the user to very precisely zero the output voltage when in the "DC" coupling mode. This voltage may be monitored by the front panel meter by switching to "output" metering mode.