INA126 with single supply

[normad]

Hi Guys im trying to interface a 2mV/V load cell output to a PIC adc. Im using INA126 as my amplifier.

Heres my schematic for the input stage:

I have 160ohms as Rg but my output is constantly 0.580V

My best guess is that this has something to do with me grounding the reference. and having V- as the same point as reference.

Given the circumstances it is pretty difficult to convert to dual supply now. So I would like to know what workarounds i have.
will raising the reference a couple of volts work?

The common mode is 2.5V from the load cell bridge.

Any help is greatly appreciated.

Thanks

 

[esp1]

hi,
The INA126 can be operated from a single supply, but take care with the CMMR.

If it becomes a problem, look at the AD623 IA.
E

 

[normad]

But it doesn't work.. I've seen that schematic from the data sheet and the difference I noticed was the 1.25v zener at the reference..
But I thought it might have something to do with the differential ADC. I'm not using a differential ADC. So will raising the reference to 1.25 do the job?
I don't understand what the rc network at the output is for.. Is it just filtering or does it serve a different purpose?

 

[esp1]

hi.
The rc network is a filter.

For a single input ADC, you could add a regular OPA configured to give a single ended output from the differential output single of the INA126.
To see how this is done look at the datasheet for the AD623 and copy the output OPA.

I use the dual OPA MCP6002 rail2 rail, which works OK on 5V and they are not expensive.

E

 

[normad]

Ok thanks.. Any comments on if it would work with the reference at zero? Or do I need to raise it?

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[esp1]

hi,
From the problems other users have had when designing the INA126 for single supply operation, when using a bridge which gives a CMV of Vs/2 it will not work as required.

If you do use an Offset voltage source for Vref, ensure that its a low impedance source.

As I posted earlier I use a MCP6002 R2R after a AD623 [ single supply IA] to provide the low impedance Vref and also to increase the Vout range of the IA from ~ +0.2V +3.5V to 4.9V.

I small Offset in Vo of the IA is required IF your bridge signal can swing slightly negative.
Without some level of Offset you will not show the true Zero output point of the bridge.

What are you weighing.?
E