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Re: HART analog data frequency limits

Message posted by Romilly Bowden ( : on 27 October 2001 at 20:10:27.

in reply to: HART analog data frequency limits posted by Chase Sebor on 22 October 2001 at 20:09:57.

A tricky one, this! It comes down to the (obvious?) fact that you can't transmit both high-speed analogue and HART on the same wires without interference between them. So what can you do?

Well, simplifying a bit, the HART signal is between 950 and 2500 Hz, and the HART data receiver will typically accept signals between about 500 Hz and 2500 Hz. So the analogue transmitter must not generate significant signal amplitudes in this frequency range. To ensure this, the HART spec defines maximum allowable step change and slew rate for the analogue signal. The result is that the maximum allowed signal amplitude is reduced from a full scale 16 mA peak-to-peak at low frequencies, by 40 db/decade above 25 Hz. This is usually achieved partly by a software filter and partly by a low-pass filter in the analogue output circuit to smooth between digital output updates. (In principle, small higher-frequency signals are still allowed, but I'd be surprised to see a field device which was that clever.)

And, conversely, the analogue receiver must filter out the HART signal frequencies before reaching the A-to-D converter (otherwise aliassing will produce large errors in the analogue measurement). Typically, the analogue receiver might have a single-pole 5 Hz cutoff, which would reduce the HART signal to less than 0.025% of span. If you wanted more analogue bandwidth, a more-complex filter here could, in principle, allow it, up to the 25 Hz limit of the transmitter. But I doubt if this has ever been done in practice.

In summary, I'd say you would be lucky to get more than about 10 Hz analogue signal through the system, using off-the-shelf instruments and host systems. Of course, for normal temperature and (most) pressure and other process measurements, this is ample.

What is the physical measurement in this case? Sounds like maybe your client shouldn't be trying to use HART, unless it is on a separate pair of wires from the analogue signal.


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