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Amplifier, Noise & Thermocouple

Hello,

I have a thermocouple that I need to use as sort of a critical temperature sensor. I don't need any 'real' resolution -- I figured so long as I'm in the ballpark of say: 30-40 degrees, I'm fine. However, I need temperatures that are greater than a semiconductor can give me, and I needed something heavier duty than a thermistor.

So, I figured: A simple non-inverting amplifier with a cap across the feedback path to act as a low pass filter.

I've got it set to a gain of about 200, and honestly, I'm having no luck. The noise is horrible. I've put a scope on it, and I'm getting noise going nearly half a volt on the output.

When I put my DMM on it (A fluke 87V), the output is perfect. It doesn't even wobble by so much as a single mV. When I try to do an ADC on the line (I'm just using a PIC with a 10bit ADC), I get numbers all over the place. I've added another passive low pass filter to the line between the amplifier and the PIC, I've also tried putting a filter cap on the thermocouple's ungrounded line, and a giant electrolytic on the output, but I can't seem to get rid of the problem.

I've also tried implementing some digital filtering, and while it (obviously) stabilizes the readings, I'm not convinced of their accuracy anymore.

How does my DMM get such a nice reading? Is there anything I can do to try and get the same kind of result?

Thanks, Dan

Reply to
Mio
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Find someplace to post a schematic - there are websites that will host pix for you, or you could even sign up for one of those free web page things.

Just off the cuff, I'd say you've got way too much gain in the first stage. If the noise is coming in from the outside, it shouldn't be there because usually thermocouple wires are twisted pairs. If it is coming from the outside (i.e., RFI/EMI), then there's something wrong with the setup - it should be fairly immune, being a twisted pair and all. What are the odds that the thermocouple itself is faulty? Do you have cold joints at your cold junction? ;-D

Good Luck! Rich

Reply to
Rich Grise

Is the sensor grounded or floating wrt ground?

Is your power supply floating wrt ground? What kind of power supply?

What kind of "noise"? (viz, what exactly does it look like on the 'scope?) Is it mains frequency mostly, for example?

The DMM runs off batteries which makes things easier. Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

What happens if you read the thing with a scope and a DMM at the same time? Does it calm down what you see on the scope, or mess up the DMM?

The DMM may be averaging things, or as Sphero pointed out it may just not be messing things up by virtue of being battery powered.

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Tim Wescott
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Reply to
Tim Wescott

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Upon further inspection, I actually think this scope might be broken, so I'm going to ignore it.

The DMM plugged in offers no change on the reading. Since I don't think I can trust this scope, I'm not sure where my problem lies. When I do the ADC, my readings can vary as much as half a volt as I sample over a period of a few seconds. If I put something else on the ADC pin, it converts fine, so I don't _think_ it's the chip.

I have two amplifiers at my disposal on this board. What's the best way to set this up? A two-stage amplifier with caps on both feedbacks to make two LPFs? How should the gains be? Does it even matter (is it best to split them evenly?)

The junction is grounded, but it's isolated from the contact surface by a teflon tube thing. My power supply is a switching power supply, operating at around 90KHz, I think, but I'm not 100% sure of the frequency. I'll try powering the circuit by USB only and seeing if that helps. Also, the thermocouple is fine. I plugged it into a Fluke type-k thermocouple meter, and it gives me the right readings.

Reply to
Mio

Dan, I suggest you try an Analog Devices AD595 Thermocouple Amplifier. It has everything you need. It has a differential input with its inherently good common mode noise rejection. It has built-in cold junction compensation. Its Cheap. It's accurate. Regards, Kral

Reply to
jd_lark

I was thinking that, but I don't have one on hand, and if I can do this with just a dual opamp, that'd be ideal for right now.

Reply to
Mio

From the thread it would seem you just have pickup. If you have a spool of thermocouple wire try various lengths and moving your hand close and away. If it's pickup you need to shield the twisted pair going to the thermocouple; probably grounding the shield close to the amplifier. You are providing a bias path for the amplifier; and know how much the bias current affects your reading? In addition you might bypass the leads when they first enter the board; Analog Devices has app-notes and suggestions along that line; for a start try 1nF.

RayR

Ray

Reply to
RRogers

There are plenty of reference designs for thermocouples on the web, almost all the major chip makers have an application note on their sites. Noise is seldom a problem because of the low impedence and low bandwidth. Drift is much more of a problem. Have you made sure your software is working correctly? Use a voltage from a pot instead of the amplifier into the PIC. If you want a good answer you need to include things like temp range, thermocouple type, and avoid phrases like "numbers all over the place", "giant electrolitic".

Reply to
cbarn24050

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What are you doing for cold junction compensation? Low pass filters should be below 1 Hz. Can you try averaging say 20 readings?

Reply to
JosephKK

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