Reading RTD''s
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Posts: 46,084
Is the frequency of the interference at 60 hertz, or possibly at some
other locally produced frequency? (Here in the SF bay area, we get
huge ground currents radiated from the electric BART rapid transit
tracks, which are nominally DC but the current fluctuates as the
trains move.)
If you lay out a wire on the ground, attached to an oscilloscope
input, you should be able to see the coupling. That kind of
coupling can depend a lot on how wet the ground is.
The AD595 has a limited common mode input range. If the AD595 is
powered from 0 and +5 volts, and the thermocouples common mode
voltage is referenced to 0 volts, then the common mode swing on the
negative side is just a few tenths of a volt. So it does not take
much AC pickup to drive the AD595 beyond the common mode rail and
drive the output whacky.
It sounds like you are going to have to spring for some shielded type
K thermocouple wire!! Or else, put a signal conditioner out at the
measurement point and transmit the signal back to the ADC as a
higher level or 4-20 ma signal.
Have you looked at the MAX667x direct to digital thermocouple conditioners?
-- best regards
Tracy Allen
electronically monitored ecosystems
http://www.emesystems.com
mailto:tracy@e...
>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=OakhGdV859bXCz6wak9_KrSNhrcZtO0vee8nDBj39HElUnPbulPdBdvSYdYzKb5Jv28RdgJMS4CLh2U]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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>
>
>
>
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other locally produced frequency? (Here in the SF bay area, we get
huge ground currents radiated from the electric BART rapid transit
tracks, which are nominally DC but the current fluctuates as the
trains move.)
If you lay out a wire on the ground, attached to an oscilloscope
input, you should be able to see the coupling. That kind of
coupling can depend a lot on how wet the ground is.
The AD595 has a limited common mode input range. If the AD595 is
powered from 0 and +5 volts, and the thermocouples common mode
voltage is referenced to 0 volts, then the common mode swing on the
negative side is just a few tenths of a volt. So it does not take
much AC pickup to drive the AD595 beyond the common mode rail and
drive the output whacky.
It sounds like you are going to have to spring for some shielded type
K thermocouple wire!! Or else, put a signal conditioner out at the
measurement point and transmit the signal back to the ADC as a
higher level or 4-20 ma signal.
Have you looked at the MAX667x direct to digital thermocouple conditioners?
-- best regards
Tracy Allen
electronically monitored ecosystems
http://www.emesystems.com
mailto:tracy@e...
>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=OakhGdV859bXCz6wak9_KrSNhrcZtO0vee8nDBj39HElUnPbulPdBdvSYdYzKb5Jv28RdgJMS4CLh2U]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/
>
>
>
>
>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/