Subject: Re: Temperature Probe Interface / Coax cable Issues
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Date: Mon, 20 Jan 2003 23:51:47 -0500
NNTP-Posting-Date: Mon, 20 Jan 2003 23:51:52 EST
Organization: Bell Sympatico
I have a thermometer that has this IN and OUT temperatures reading.
I extended the length of the OUTsensor by about 15 feet and the
reading changed by about 2 degrees C.
Connected the X10 probe to the input connector of the meter and
observe two things :
a) The signal is measured once a minute by a short pulse ( may be to
reduce the heating effect on the sensing element). and probably using
a sample and hold circuit.
b) The waveform of the pulse lost some of the high frequency
components when the longer cable was used, probably caused by the
increased capacitance of the cable. Rounded top corners.
In this case the most importante caracteristic of the cable his it's
On Tue, 21 Jan 2003 02:43:03 GMT, John Popelish
>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.