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I tried to design an inamp for a K-type thermocouple. In theory the gain of this amp should be 250 (close enough to 10 mV/K), and if my understanding of instrumentation amplifiers is correct the output voltage should be the difference in voltage multiplied by the gain of the amplifier(s). My problem is that no matter how I connect the tcl, or if I leave the input floating, the output is stuck high (Vsupply = +5 V, Vtcl = 7 mV (doesn't really matter since Vout does not change), Vout = 4.2 V pinned). Could it be because I'm not supplying my op with +-Vsupply, or is my multimeter not a big enough load and the output is floating or is my design outright flawed, if it is where did I go wrong and how can I fix my issue?

UPDATE 1: Tried with a +-12V supply. TCL hooked up with the correct polarity, the output is ~11V the other way the output is ~-9V. The Non inverting amplifier at the end works fine, but the inamp seems to be stuck at 0.42V on the output. I'll get a precision(or more precise) op and try again.

Instrumentation amplifier for K-Type tcl

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    \$\begingroup\$ There are few worse choices for a thermocouple front end amplifier than a TL074. Ignoring your common mode range issues (covered in the answer below) the TCVos is not guaranteed and is typical 18uV/K which means you're typically getting almost 1°C error for each 2°C change in room temperature (and it could be double that). The input offset error can lead to as much as +/-3.75V error in the output with your gain. Don't forget the cold junction compensator in your circuit. \$\endgroup\$ Aug 31 '18 at 13:22
  • \$\begingroup\$ In response to your update, make sure your resistors are well matched. Your better option is to buy an instrumentation amp, instead of trying to build one. \$\endgroup\$ Aug 31 '18 at 16:16
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From the datasheet, the maximum output swing with a +/-15V supply for the TL074 is about +/-12V. The common mode input range for that power arrangement is -12 to 15V.

While they don't supply the numbers for 0V and 5V rails, it's a fairly safe bet that, without a negative supply, the input range does not include ground, plus you don't have much headroom on the output.

All in all, the TL074 is a poor choice for a 5V single supply. You should look for an op amp that is rail to rail on the inputs. For TI, I believe you should look for a part that begins with TLV, but please verify (those won't be the only rail to rail parts, and I can't guarantee that every TLV part is rail to rail). The part may or may not be pin-compatible with the TL074, but the pin layout of the TLV074 is very common.

There is also the possibility that with a gain of 250, you might simply be saturating something because of a simple offset somewhere.

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  • \$\begingroup\$ I used a TL084 in my prototype, because it's all I had at hand, I have a suspicion that it might be just too imprecise. The Inamp it's self has a gain of 25 and after that non inverting amp has a gain of 10, that has no problem but the inamp isn't working like it should. \$\endgroup\$
    – R4MP4G3RXD
    Aug 31 '18 at 16:04
  • \$\begingroup\$ @R4MP4G3RXD same holds, more or less, for the 084. It's not imprecision, you're just using the amp in a way it wasn't designed for. An offset at the input gets multiplied by 250, whether it's all at once, or half at a time. Your last stage might be saturating. More likely is the choice of op amp, though -- I just included the saturation part for completeness. \$\endgroup\$ Aug 31 '18 at 16:10
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The TLo7x and TL08x opamps have a problem called Phase Inversion. If an input voltage gets within a few volts of the negative supply voltage then the output voltage goes as high as it can. Your inputs ARE DIRECTLY AT the negative supply voltage.

The datasheet says its input common mode voltage range is 3V away from the negative supply. Its minimum recommended supply is 7V.

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  • \$\begingroup\$ Hmm.. I wonder what cheap op I can use for this.. Because precision op's cost a whole lot, and at that point it's probably better just to get an integrated inamp like an INA317. \$\endgroup\$
    – R4MP4G3RXD
    Aug 31 '18 at 16:01
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  • The TL074's minimum recommended supply voltage is -5 volts and +5 volts (or, if a single-ended supply is used, 0 volts and +10 volts). Your circuit fails in that respect.
  • The input common mode voltage range (how close you can use the inputs near the supply rails without causing problems) is -Vcc + 4 volts to +Vcc - 4 volts or, with a single supply of 0 volts and +10 volts, that's +4 volts to +6 volts. Your circuit fails to meet this criteria significantly irrespective of the power voltage problem above.
  • The output is not guaranteed to swing to within 3 volts of either power rail.

Regarding the common mode input range, let's be clear by what I mean: -

enter image description here

The TL074 guarantees that it will work within an input range of +/- 11 volts when the supply is +/- 15 volts. In other words, get any closer than 4 volts to either supply rail and you are operating the device outside the guarnteed limit.

Sure you could take a risk and use typical values but, do you want to take that risk?

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  • \$\begingroup\$ The common mode input range is Vcc- + 4 to Vcc+ (all the way to the rail) \$\endgroup\$ Aug 31 '18 at 15:45
  • \$\begingroup\$ @ScottSeidman No, it's not guaranteed to work within 4 volts of the positive rail. See my add. \$\endgroup\$
    – Andy aka
    Aug 31 '18 at 15:55
  • \$\begingroup\$ Tried with +-12V, see the update on my post, will switch to a more precise op tomorrow, and try again. \$\endgroup\$
    – R4MP4G3RXD
    Aug 31 '18 at 15:57
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    \$\begingroup\$ You're right! Serves me right, I was looking at typical numbers, wich is a bad thing. \$\endgroup\$ Aug 31 '18 at 16:08
  • \$\begingroup\$ @R4MP4G3RXD did you connect R19 to the midpoint between the two voltage supplies? \$\endgroup\$
    – Andy aka
    Aug 31 '18 at 16:09

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