Reading RTD''s
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Posts: 46,084
The Thermocouple side is the long side. The distance between the AD595 and
the ADC is less that 1 inch. I have attempted to isolate and smooth all
power lines. I don't think that is the issue. One interesting thing is I can
coil up 25 feet of cable and I get no noise, but as soon as I start to
uncoil it the noise amplitude goes up. If I lay it close to the ground,
still insolated from the ground. The noise goes off the scale. Also the
thermocouple lines are not shielded. The commercial thermocouple readers do
not show this interference either.
Original Message
From: Tracy Allen [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=OfP5hNynmQI-mdfOqCzUTg1ukNEXhwxwxUfYXsG06M0Mw0FDXqls1-RbC0zeNnj4GnLm3_TDgIUJ7oyQF2o]tracy@e...[/url
Sent: Tuesday, January 28, 2003 12:59 PM
To: basicstamps@yahoogroups.com
Subject: RE: [noparse][[/noparse]basicstamps] Re: Reading RTD's
>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...
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the ADC is less that 1 inch. I have attempted to isolate and smooth all
power lines. I don't think that is the issue. One interesting thing is I can
coil up 25 feet of cable and I get no noise, but as soon as I start to
uncoil it the noise amplitude goes up. If I lay it close to the ground,
still insolated from the ground. The noise goes off the scale. Also the
thermocouple lines are not shielded. The commercial thermocouple readers do
not show this interference either.
Original Message
From: Tracy Allen [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=OfP5hNynmQI-mdfOqCzUTg1ukNEXhwxwxUfYXsG06M0Mw0FDXqls1-RbC0zeNnj4GnLm3_TDgIUJ7oyQF2o]tracy@e...[/url
Sent: Tuesday, January 28, 2003 12:59 PM
To: basicstamps@yahoogroups.com
Subject: RE: [noparse][[/noparse]basicstamps] Re: Reading RTD's
>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...
To UNSUBSCRIBE, just send mail to:
basicstamps-unsubscribe@yahoogroups.com
from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/