in reply to: HART filters posted by Mandar Kale on 02 February 2007 at 15:02:29.
HART filters can serve several purposes. The main ones are:
1. To remove the HART signal from a fast-responding (millisecond sample rates) analogue input to a PLC, which might otherwise respond to instantaneous values including the HART signal, leading to errors in the apparent measurement. Adding a filter here usually needs the PLC input to be voltage-sensitive, rather than current-sensitive.
2. To isolate the output loop from the commonly-used large capacitor on a DCS current output, so that the HART signal (added after the filter) is not reduced by the low impedance of the capacitor at HART frequencies.
Note, these two functions need different circuits for the filter. Both may be included in termination panels designed specifically for HART applications. The second type may be provided as a separate component by the manufacturer of HART valves or positioners.
And note too, that there is usually no need for a filter on DCS analogue inputs, since these typically include a filter with a time constant of a quarter second or more, which effectively removes the HART signal before it reaches the A-to-D converter.