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Problem on amplifying signal ona differential OpAmp

JotaTR

May 6, 2013
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Hi there

I have bought 5 INA2126 from RS components one month ago. I am trying to use them in order to amplify a signal from thermocouples.
First i am testing the OpAmp and i am getting some results that i dont understand:

I put a resistance od 1400Ohms in order to amplify the signal from a voltage source (65mV). I feed the opAmp with a power suplly imposing 10V. I have confirmed the connections and they are correct (as it says on the datasheet). The output that i get is alway a litle bit less than the supply power (for 10V i get 9.4 instead of 4.04V that i was supposed to get)

Vin- (pin 1) and Vin+ (pin 2) are connected do the 65mV voltage source.
RgA (pin 3) and RgA (pin 4) are connect by the 1.4kOhm resistance.
RefA (pin 5) is connected to the negative supply voltage
SenseA (pin 7) is connected to VoA (pin6).
V- (pin8) and V+ (pin 9) are connected to the negative and positive suply voltages.

What am i doing wrong? Is there any advise that you can give me?

The datasheet of this OpAmp is http://www.datasheetcatalog.org/datasheet/texasinstruments/ina2126.pdf

Thanks
 

Harald Kapp

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Nov 17, 2011
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The INA2126 is not just a differential amplifier, it is an instrumentation amplifier. It operates differently from a normal OpAmp.

RefA (pin 5) is connected to the negative supply voltage
Not a good idea. The REF-pin is the output potential for a 0V input differential. Connetcing it to V- means that the amplifier's output is forced to the negaitve rail. But the INA2126 camn go only to V- + 0.8V. Therefore it is stuck at approx. V-.
I feed the opAmp with a power suplly imposing 10V
That is no good idea either. The INA is meant to be operated from a dual supply, so you need +-10V. You then connect the REF pin to GND, not V-.

If you are stuck with only one +10V supply, connect REF to any voltage in between, e.g. 5V. IN that setup, 5V will be the output voltage for 0V input differential. The output will swing around 5V (virtual ground). You can then use a capacitor to block the DC part of the output voltage and arrive at a pure AC signal.
But using a +-10 V supply really is the better idea.
 

JotaTR

May 6, 2013
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Ok i´m getting it...

what is best way to get the negative power input? Can i use my power source and use the red wire (+) to supply the V+ input, invert the signal using a nomal op-amp and injecting it in the V- pin? Can i use the black wire from the power supply (-) as the reference? Or do i need a different power source?

The alternative that you suggest requires me to get 5V from another source right?
Must the Ref be in the middle of the power suply range? For exemple, if i put 0-10V in V- and V+ the ref must be 5V or it can be 2V?

Thanks
 

Harald Kapp

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what is best way to get the negative power input?
Use a dual supply.

Get a benchtop supply with a dual output (or 2 outputs that can be combined into one dual output) or use 2 batteries.
You cannot use an inverter to generate V- as you think. If you can't find a dual supply, you can generate a negative rail using e.g. a switched capacitor voltage inverter.
 

Laplace

Apr 4, 2010
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Can i use the black wire from the power supply (-) as the reference?
Yes, if the black wire is the GND of a dual power supply. No, otherwise.

Or do i need a different power source?
Recommended.

The alternative that you suggest requires me to get 5V from another source right?
No. It can be derived from a voltage divider using resistors. Note that the REF input load resistance is 40K so two 1K resistors as a voltage divider may not introduce too much loading error. How accurate does your circuit need to be? Use an active buffer if necessary.

Must the Ref be in the middle of the power suply range? For exemple, if i put 0-10V in V- and V+ the ref must be 5V or it can be 2V?
Sure, see Figure 4 on page 9 of the referenced datasheet. The REF input does not need to be at the exact center of the power rails but try to keep it in the middle range.
 

Harald Kapp

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Here's some information on hpw to create a dual supply from a single supply using a virtual ground.
Using the buffered circuit presented on this page is econd best to having a real dual supply.
 
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