amplifier ina126, single power supply, saturation

Thread Starter

luismanuel

Joined Sep 28, 2018
2
Hello.
I am using an instrumentation amplifier INA126 and I am having some problems with it. The "real circuit" was not working properly so I am simulating the circuit that you can see in the picture using Multisim 14. This amplifier is going to be used to make an ecg circuit, it is a project for a college. But the thing is, we must power the amplifier using a 6V battery. I have used opamps in the past and powered then with a -V and a +V and it was ok, but never with a single supply. As you can see in the picture, the ina126 saturates, and I dont really understand why. You can see more about ina126 in the datasheet prints that I uploaded, such as gains and pin configuration.
Those 1k resistors where not all necessary. I was just trying to have a +5V (connected to pin 7), a 0V (which is the reference point, connected to pin 5) and a -1V (connected to pin 4) to power the amplifier, but it is not working, as it is saturating.
Hope my explanation wasnt very confusing
Thank you for your time
 

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MrChips

Joined Oct 2, 2009
19,929
As with all opamp circuits you need to provide a common reference point.
If you are using a 6V supply, create a 3V common reference point by putting two 1kΩ resistors in series across the power supply.
Better still, use a virtual ground circuit such as TLE2426.
 

MisterBill2

Joined Jan 23, 2018
4,897
In addition, it looks like both inputs are being driven in phase, which is not the way to use an instrument amp. At least in theory all of the in-phase input is canceled as common mode noise. At least if it is functioning correctly that should happen. Thus, that might be your problem.
 

danadak

Joined Mar 10, 2018
3,793
Here is a CM Calculator Tool that predicts voltages vs inputs.

http://www.ti.com/tool/ina-cmv-calc

The need for the ground ref previously mentioned is to establish a bias
current path for the INA inputs back to itself.

MisterBill2, they are in phase but two different freqs.


Regards, Dana.
 
Last edited:

ebp

Joined Feb 8, 2018
2,332
Your circuit, irrespective of other issues, has no path for the input bias current of the amplifier, therefore it will saturate - as will almost all operational or instrumentation amplifiers. Also think about how your oscilloscope is connected given the way you have drawn the schematic.

Pretty much everything you need to know is laid out in the datasheet. Note carefully how pin 5 is dealt with in the various applications circuits shown and what that means for your application.
 
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