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Part Number: INA126
I uses the INA126 to amplify a high side current sense resistor (0.01ohm).
In my test the Vload is 1.03V
The load is 90ohm
the V(rsense) is 170uV
the gain resistance RG is 34ohm -> gain = 2357
The INA126 output is 33mV -> Gain of ~200
I know the numbers does not add's up. I have some noise and my equipement is not suitable to measure below mV voltage.
Also, if Vout is 0, the INA126 output is below 0.
The INA connects directly to the resistor pads using kelvin connection.
The ina126 is powered with +12V, -5V and the REF pin is connected to GND (as it can be seen on the diagram).
I don't understand what is going on.
Do you have any idea of what is going on and why the gain is so wrong ?
Thank you a lot
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Zak Kaye Precision Amplifiers Applications
In reply to Zak Kaye:
Thank you Zak and Errol, I appreciate your answers.
To reply your comment zak,
1&3. Thank you for your point. I did not see that during the design phase. So, I will try to use a standard 3 opamp instrumentation opamp.
2. I agree with you that there is a problem with the measurements and I cannot understand where is the problem. I think the problem can come from my dmm, it is not precise enough for uV measurements or maybe it is because of the fact : The voltage 1.03V is from a buck regulator (1.5MHz). The voltage ripple is 2mV with spike to 30mV with some LC ringing at the pwm frequency. But, since the opamp is differential, I would have thought the noise would be eliminated. The image show the signal to gnd on the Rsense. I take my measurements in AC mode to remove the DC offset.
In reply to Christian Dufour:
A standard 3 op-amp INA will tend to offer better performance for both common-mode rejection and ADC drive since the bandwidth of the output stage is independent of the gain. However, these still tend to be low bandwidth devices and depending on what you are attempting to drive and the speed of the converter, you may still need a buffer between the converter and the INA. The CMRR of the INA will cut down the amplitude of the oscillation on the output, but for better performance I would suggest including a filter on the input. I have attached an example TINA file showing these changes.
INA126 Output Error.TSC
Thank you a lot Zak,
I will modify my design with your suggestion. Thank you a lot for your help!
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