If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I am still working on building a precision thermometer (for fun, so no comments that I can buy one).
It uses an RTD probe connected to a LMC6064 opamp for about 3x gain right now. I am working on trying different opamps. The output goes to a 16-bit ADC that ups it another 16x.
I have the probe in an ice bath for hours for a stable temperature. When I turn on the circuit I get a reading - say 23740 out of 32768. Over time, the temperature drifts downward. If I put my warm finger on the opamp the reading starts to drop. So, the chip is sensitive to temperature, and the slight warming from the chip being active is causing an error with my measurements.
Is this the voltage drift I see on the datasheets? I am going to try another chip, like the op275, with a low voltage drift. Other than this, is there any way to prevent this from happening? All of the other components (resistors) have thermal values of 15ppm/C, so I am good here.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
The LMC6064 datasheet gives 1 uV/C input offset drift typical, x3 x16 = 48 uV/C. If your voltage reference is say 3.2768 v with a 16-bit ADC thats about 1 Celcius per bit ... so if the ic heats up by say 30C, the reading should change by a count of 30 When using 16-bit precision normally minor effects become observable ! e.g. the Seebeck/thermoelectric effect
P.S. ... how many wires to your RTD element? two = guaranteed problems three = can compensate quite well four = reliable reading the majority of industrial RTD instruments that we repair use three wires and 10 or 12 bit ADC so for 16-bit I guess four-wire is required.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Please post your full circuit, as it's not clear what you are measuring and with respect to what. For the last little bit of accuracy, the devil really is in the detail.
Raising the total temperature of the opamp could be shifting its voltage offset, or raising one end of it could cause a transient shift while there is a temperature gradient on it, or it could be warming its leads with respect to the wiring on the board or socket which is causing a thermocouple effect. They will all have different mitigations.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I'm modeling my circuit after the 3-wire design in the article above. I'm using 3-wire because that is the type of probe that I have. I don't need anything crazy precise. I've seen a number of different constant-current designs using the NPN transistor in this article vs opamps in others like here
I am not sure which is better, or if they are both acceptable. Certainly, a minor change in the "constant-current" will impart error. My DAC is the ADS1115. However, I do use my own non-inverting gain amplifier to take adavantage of the ADS's full-scale, so I can introduce temperature drift error with my opamp.
Again, I'm doing this project for fun and to learn so I am open to all suggestions for the best components to minimize the issues. RIght now I'm playing with it on a breadboard, which also introduces resistance errors. I wiggle one one of the opamp resistors and the ADC reading changes by 30-100.
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.
changing values of opamp with chip temperature
