output changes when I touch the enclosure

[hemnath]

Hi,

I have a transmitter board which senses the pressure and outputs 4 to 20mA. The board is mounted in a aluminium enclosure using the Stainless steel standoff. The screws used are Stainless steel.

Normally, the board is working good and provides 4 to 20mA based on the input pressure. But when I touch the aluminium enclosure, the output current value changes to 3.96 mA from 4mA.

Why is this behaving strange?

 

[wwfeldman]

i see two possibilities

by touching the enclosure, you have somehow reduced the pressure and so the current goes down
0.04 mA is likely well within the design margins of the pressure transducer and the output 4 to 20 mA

by touching the enclosure, you change the local distributed parasitic impedance

it is quite possible there is another explanation

please provide a schematic and mechanical layout

 

[dick_freebird]

I would guess that in an industrial environment, with an
ungrounded or poorly grounded (i.e. "earthed") case, you
are adding 50/60Hz hum to the local "ground" and some
rectification / charge pumping (like with input ESD diode
networks) is adding to the real signal.

You might repeat the experiment with the case tied hard
to a known good earth ground, and in a location without
much electrical equipment, fluorescent lighting and so on.
You could put an oscilloscope to the internal input point
and eyeball the waveform as you touch / un-touch the
case, to get quantitative. If you've ever just touched the probe
tip on a 'scope you have an idea of how large human body
line frequency "pickup" can be against a high impedance
sense-point.