Part Number:INA128
Hello,
Let me preface my question with the fact that I am a circuit design novice, so please forgive me for any glaringly obvious mistakes or omissions.
I am working on building an amplifier circuit for a Wheatstone bridge arrangement sensor. My input signal range is from 100 Microvolts to about 1 volt. Since it seemed relevant I measured the input resistance and it varies from 140 ohms to 31k ohms depending on bridge output. My desired maximum output would be 10VDC.
I am using an INA128 amp and here is a picture of the amplifier circuit. Currently I am powering it with a ±12VDC power supply, but will be getting a ±15VDC power supply which I believe is more ideal for this Amplifier? I am using a 5k ohm resistor yielding a gain of 11±.05. Generally the gain seems to be just a little off at very low input voltages. ±.05 or so. But i think this is acceptable for me in general. This circuit was mostly taken from the application notes.
Other important notes are i am only concerned with stability, this will be used to amplify a DC signal. There is some noise present in the input signal from the bridge that i would like to eliminate, but putting a small cap (.01microF) did not seem to do much. I also tried a 2.2microF and had slightly reduced noise. I have read but not tried referencing my inputs to ground with a small cap, maybe this would improve my noise issue?
Generally I am happy with the results I am seeing with the circuit shown, but I would like to get feedback on improvements from others.
Also I have a question regarding why grounding the -input terminal makes this circuit work correctly, but if that is omitted, i do not get the expected results. I have a feeling it may have some thing to do with my input circuitry. As ab example of what i mean about it not working correctly, with the -in terminal grounded, if my input applied is 0.506V i get 5.554V out Gain=10.94, if i remove the reference to ground i get 3.06 V out unless i am remembering incorrectly and it was 3.06mV. Either way, i am not sure what is causing me to need to reference my input to ground.
Edit: I tried capacitively referencing my input to ground and that worked as well. I am just stumped as to why this is required when i do not see it on the application notes. Also are there any downsides to using all these caps?
Also the black/white wires in the upper area of the picture are where i am measuring the output. The input signal wiring is not shown, but is across terminals 2 and 3(14 and 15 on the breadboard) if that is not obvious.
I really just want to create the most linear and quietest amplification circuit that i can, any help is appreciated.
Thank you,
Justin![]()
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