All SLICE IEPE circuits include a software controlled option for AC or DC coupling. The DC coupling option was implemented for three reasons.
- Support the capability to inject a “Calibration Signal” to check channel gain
- To measure the sensors DC bias voltage
- Adds the versatility to support other constant current sensors that require DC response. Examples include RTDs and heat transfer gages that convert physical measurements into significant resistance changes
With most IEPE sensors, AC coupling works correctly since these sensors do not have DC response. There are reports with certain sensors using AC coupling where the zero level never stabilizes; the specific cause has not been identified. The hypothesis is that certain IEPE sensors have AC characteristics that are (somehow) incompatible with the SLICE IEPE characteristics. In these cases, DC coupling resolved the zero stability issue.
The down side to DC coupling is that thermal drift in the bias voltage of a given sensor will show up unimpeded in the output data. This can be a problem for a long duration test that includes temperature change.
In general, DTS recommends you use AC coupling for an IEPE sensor since any drift in the bias level of the sensor will automatically null to zero. If there is an issue with zero stability, try DC coupling, but be aware that sensors vary and thermal drift of bias voltage could be a problem if your testing requires a stable zero level over time and temperature. As always, test any sensing solution before conducting a real test.
Shouldn't I choose AC since the IEPE is an AC sensor?
Generally yes. However there are some cases where DC coupling in SLICE may allow zero levels to settle faster.
If the output is already DC coupled into SLICE, what is the advantage of AC coupling?
AC coupling adds an additional RC time constant which will cancel thermal drift in the bias voltage of many IEPE sensors.
The AC coupling adds a capacitor and resistor in the signal path that increases the time constant (charge and zeroing timing), is there a condition when that is desirable?
Yes. The bias voltage (zero point) of many IEPE sensors will change significantly over temperature. AC coupling will help cancel any long term thermal drift in the bias voltage.