Re: shift in RTD Reading

DC could be positive or negative.  Depends on which direction the diode
faces that is causing rectification to occur.

Walter.

"Dan Wright" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> One more quick question:
> If this is DC being added to the signal, why, then, is the rtd reading
> dropping?  With the increase in signal level caused by this RF
> Rectification, I would expect the temperature reading to rise, no??
>
> DanW
>
> John Popelish wrote:
>
> > Dan Wright wrote:
> >
> >>So my question then becomes, What can I do to eliminate this DC offset,
> >>provided I 1) am shielding my instrumentation cable properly, 2) keeping
> >>the instrumentation wiring as far away as possible from the VFD power
> >>wiring?  I can deal with pure AC noise, as this averages out to 0.  My
> >>RTD signal is running from the UUT to my NI FieldPoint RTD DAQ module.
> >>Here, I believe the hardware is pulsing a 250 uA signal, and the
> >>'return' voltage is measured across the RTD.
> >>
> >>In the relativly short itme I have been in the instrumentation / control
> >>work force (5 years) this is the first time I am totally stumped.  How
> >>to deal with a DC offset, when you are measuring a DC signal to begin
> >>with?  I honestly value all of this groups input on this matter.
> >>
> >
> >
> > The spiky input gets into your instrument by capacitive (or direct)
> > conduction.  This can happen because the high frequency signal will
> > not equalize out to a single voltage at physically separate points,
> > even if they are all connected together (as building steel is) due ot
> > the propagation speed of light.  So if the RTD is mounted on a piece
> > of (probably grounded) equipment some distance from the transmitter,
> > which is some distance from the signal collection hardware (which is
> > also probably grounded) and all the signal paths are enclosed or near
> > grounded metal, there is still plenty of opportunities for high
> > frequency ground noise to get inside the signal paths.
> >
> > Since it is not usually practical to isolate the entire signal path
> > from ground, except at a single point (and impossible to isolate it
> > from capacitive and radiated coupling), the best you can do is try to
> > reduce the escaping noise at its sources and/or attenuate it at the
> > points where it is being rectified into an offset.
> >
> > I am not familiar with the abbreviation UUT.  But I am willing to bet
> > that the DAC is a node that is subject to this rectification effect.
> > You might try an experiment of putting a common mode choke on the
> > signal lines as close to the DAC input as possible.  This common mode
> > choke is best placed such that the excitation current for the RTD
> > dopes not pass through it, because the wiring resistance compensation
> > will not take its IR drop into account.  I am not familiar with the
> > details of tour digitizing hardware to know if this is possible or
> > not.
> >
> > Do you have a way to detail your hardware setup on a web page
> > (including some photographs and schematics)?  The possibilities are so
> > great it is hard to make useful generalizations.
> >
>