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REF200: REF200

Part Number: REF200
Other Parts Discussed in Thread: INA126

I made a circuit with INA126 + REF200 (from TI) to generate a 1V to 9 Output voltage for a RTD, Pt1000. This circuit is very similar to what Arthur Kay made, tested and simulated. My circuit does not give me the correct output. I tried to contact Arthur  to see what I am doing wrong. But, I was unable to get his contact info. Could somebody help me solve my problem ? Thanks

  • Hi Nader,

    can you show us a schematic?

    Kai
  • Hi Kai,

    Yes. Pictures of the schematic (layout ) and the actual circuit are attached.

  • I have swapped - by mistake - the resistors for R and RG. But, still I should get an output close to 6 V. I am getting 0.62 V !
  • Hi Nader,

    Can you double check the REF200 pins. Pin 6 should be connected to GND.

    -Marcoo
  • Hi Marcoo,
    I am not using the current mirror at all, so I think we can NC pin 6. On top of that, the voltage difference present at the pin 2 & 3 of the INA126 is what I should get : about 36 mV. which is : [the difference between RTD res @ room temp - 736 Ohms (which is R value in the circuit)] * 0.1 mA. It comes around 36 mV. I think the problem is INA126 ! The output of the INA126 is 170 mV, although, according to my calculations, the Gain should be around 196. The output should come to around 7.00 V, but it comes to 0.170 V !
  • Hi Nader,

    pin 6 of REF200 has nothing to do with the current mirror. It's the substrate, which must be connected to the most negative potential of your circuit!

    Kai
  • Hi Kai,

    In the status quo (i.e. with pin 6 NOT connected to GND) , I am getting correct voltage difference at the pins 2 & 3 of the INA126. The problem seems to be with INA126. The problem is with the RG of 418 Ohms, I should get a G = 200 and so my differential signal, which is 36 mV supposed to be amplified to around 7 V, but INA126 output stays at 0.55V ! Is INA126 output limited to a low output voltage ? Thanks
  • Hi Nader,

    It is difficult for me to tell from your schematic but I suspect you are violating the input common-mode limitations of the INA126. You need a common mode voltage of around 1.5V before you can take advantage of the full output range, otherwise you will be saturating internal nodes and the output might look linear but with the wrong gain. I would suggest incorporating a negative supply or biasing your RTD up to a higher common-mode voltage.

    You can use this calculator to quickly determine the limitations of the input and output for most of our INAs: www.ti.com/.../INA-CMV-CALC
  • Hi Zak,

    yes, you are right. He is violating the common mode input voltage range! 100µA across 736R gives only 73.6mV...

    Kai
  • Hi Zak, Hi Kai,

    You guys are absolutely correct. Last night I just connected the  INA126 to dual supply (+/- 10V and connecting REF pin to GND) and the problem is solved now. Now I am getting the correct output, as expected. The only problem is that my output drifts form 7.00 V to 6.930V and back to 7.20V ! Could it be the self heating of the RTD ? I doubt that, since selfheating will cause the resistance one directionally increase. In my case the output goes down and then up ! Any idea ? Thanks

  • Hi Nader,

    What is the temperature range you are operating over? Pretty much everything in your system is going to drift with temperature, including the offset voltage, bias current, and gain error of the INA126. Since you are operating in a relatively high gain this will make the effect of this drift more pronounced. However, 200mV is a lot of error for a single source. You may be seeing temperature drift that is initially dominated by the INA126 before self heating of the RTD takes over. I'm not sure how you are measuring this but if you haven't already, you may try subjecting the RTD alone to changes in temperature while keeping the board at a constant temperature.
  • Hi Zak,

    Right now, I am testing this in the Room temp. I see this fluctuations in the Room measurements. I am adding input common/differential filter to my circuit. But, I am not sure that the noise is the source of this fluctuations.
  • The INA126 itself should contribute ~600uVrms or 3.53mVpp at the output. With such long cable lengths depending on your environment it is possible you are coupling additional noise at the input. Additionally, it is always a good idea to place bypass capacitors at your supply pins to filter any noise that may be coming in through the supplies or coupling onto the supply lines.
  • You absolutely correct. I will add input filter and then PS bypass caps and then output filter. To measure the effect of each. Thanks
  • Hi Nader,

    if it comes from self heating of PT1000 can easily be checked if you replace the PT1000 by a metal film resistor of same resistance.

    Kai
  • Hi Kai,

    That will work. Thanks. I added LPF for diff and comm noise at the input of INA126. Output is much more stable. Next, I will add bypass caps for PS noise to see even more improvement . I guess my issue is resolved at this point. Thanks Kai, Thanks Zak, Thanks Marcoo.