From: John Popelish
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Subject: Re: Temperature Probe Interface / Coax cable Issues
Date: Tue, 21 Jan 2003 02:43:03 GMT
NNTP-Posting-Date: Mon, 20 Jan 2003 21:43:03 EST
Kevin Walton wrote:
> Summary - is this just impedence matching problems from using the
> wrong coax?
> I wanted to replicate a temp probe interface that I am using, which I
> have done and it works - ish.
> I didn't try to understand the circuit but it uses an ICL7660 and a
> TS912 IC, 2 variable resistors, a zenier reference, resisters,
> capacitors and 2 1000UH SMD Inductors - one on each input wire.
> The sensing is done by a remote 1K0 3900ppm NTC sensor, connected by
> coax and a bnc plug/socket. The original coax is unmarked, 3.2mm
> external diameter, rubbery coated, 2.5m long.
> In my replacement probe the coax is RG 58 C/U, 5mm dia., 3.5m long.
> I calibrated the temp reading succesfully using the new interface and
> probe with the coax rolled into a say 20cm roll on the table, but then
> unrolling the cable caused significant change in the reading. Placing
> 2 sensor rolls of wire on top of each other caused the readings to go
> None of this occours in the original sensor, it can be rolled,
> unrolled, placed ontop of new sensor cale rolls with no problems.
> Is this an obvious problem that I just don't appretiate, such as
First off, coax impedance is not involved in low frequency signals
(approaching DC), like your thermistor signal. For that type od
signal, all that might matter is DC resistance (not at all likely a
factor at 3.5 meters), shielding effectiveness (if in a very high
noise environment) and possibly, total capacitance, if the circuit is
poorly designed, and this is a factor in an opamp stability. In other
words, about the only way you can get it wrong is of you connect the
outside of the coax as your signal input. If not, and your circuit is
still acting up, you have some other problem that you have created
while changing the coax.