Reading RTD''s
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>The problem with interference arouse from using Analog Devices AD595. This
>is supposed to read the thermocouple, Condition it, linearize, and
>Compensate for the cold junction, then output 10mv per degree Celsius. The
>whole thing works great in my workshop, but I take it onsite and I get a odd
>spiking signal. It shows fluctuations up and down of 500 or more degrees. It
>almost looks like it has a heart beat... The only thing that is different is
>the location. I am running it a max length of 75 feet. With experimentation
>I have found the interference occurs when I extend the length beyond 10
>feet. I am also using K type thermocouples with K type thermocouple
>extensions. I have also tried tyroid coils, capacitors, and resistors. The
>capacitors and resisters helped, but changed the linearity of the output
>from the chip. It need to be accurate to about 1 degree Celcius or less, as
>stipulated be the government!
Hi Phillip,
Is the long run of cable on the thermocouple side, or on the output
(pins 8 & 9, 10mV/degree C) side? If there is any length of cable is
on the output, I would look for instability there, as the AD595
output stage is not good at driving capacitive (cable) loads. That
would need to be isolated with a resistor or a snubber, and a good
bypass capacitor is necessary across the power supply at the chip. I
would be surprised to find oscillations caused by a long cable run at
the input side. Are the thermocouples shielded?
I have a thermocouple circuit using the AD595 posted at
http://www.emesys.com/OL2therm.htm#THC4
-- best regards
Tracy Allen
electronically monitored ecosystems
http://www.emesystems.com
mailto:tracy@e...
>is supposed to read the thermocouple, Condition it, linearize, and
>Compensate for the cold junction, then output 10mv per degree Celsius. The
>whole thing works great in my workshop, but I take it onsite and I get a odd
>spiking signal. It shows fluctuations up and down of 500 or more degrees. It
>almost looks like it has a heart beat... The only thing that is different is
>the location. I am running it a max length of 75 feet. With experimentation
>I have found the interference occurs when I extend the length beyond 10
>feet. I am also using K type thermocouples with K type thermocouple
>extensions. I have also tried tyroid coils, capacitors, and resistors. The
>capacitors and resisters helped, but changed the linearity of the output
>from the chip. It need to be accurate to about 1 degree Celcius or less, as
>stipulated be the government!
Hi Phillip,
Is the long run of cable on the thermocouple side, or on the output
(pins 8 & 9, 10mV/degree C) side? If there is any length of cable is
on the output, I would look for instability there, as the AD595
output stage is not good at driving capacitive (cable) loads. That
would need to be isolated with a resistor or a snubber, and a good
bypass capacitor is necessary across the power supply at the chip. I
would be surprised to find oscillations caused by a long cable run at
the input side. Are the thermocouples shielded?
I have a thermocouple circuit using the AD595 posted at
http://www.emesys.com/OL2therm.htm#THC4
-- best regards
Tracy Allen
electronically monitored ecosystems
http://www.emesystems.com
mailto:tracy@e...