The Excelitas LHI 878 motion sensor is a dual element detector, serial opposed format, two elements based on pyroceramic technology.
There are are a couple derivatives of the LHI 878.
3901 is the standard version
3902 has a 1kohm Drain resistor that slightly reduces electromagnetic interferences.
As the LHI878 will eventually be phased-out, we recommend going to the PYD 1388 for new designs.