Well 100M is larger than 1M, so there's no problem with the impedance.
The amp's inputs are rail-to-rail so that's fine too - don't need any negative bias, just feed a knownvoltage to the REF input that the outputs can handle, that's the whole point of the REF input, its thereference for the output voltage.
You have fairly high impedances so noise-pickup will be something to consider. Careful attention to goundingwill be essential with these low voltages, especially as the ADC is single ended.
What measurement bandwidth are you looking for?
Do you mean the impedances between INA and ADC? If so, where did you find these two specific values in the datasheet?
Output impedance I can't find but there are some charts at p.18 of the data sheet showing how the output current swing is affected by the current drawn. Much stronger effect than I expected. Nonetheless no problem when the output goes to an Arduino ADC which has in input impedance in the GOhm range.
Thanks a bunch for your reply Mark! Do you mean the impedances between INA and ADC? If so, where did you find these two specific values in the datasheet?
Can not find the output impedance value fo the INA and im not sure about the ADC input impedance either (Is it the value for 'Common Mode input Impedance' = 25MOhm for my single ended measurement?).Even this is a rail to rail INA its output is only expected to swing max. V_gnd + 0.05V appart from the rail. To swing completly to the bottom rail as my signal needs a true zero I chose the negative bias. Would that work though or do you see potential problems using that method?
Also If I understand the use of the reference pin properly I should feed a voltage of at least 0.05V into it to circumvent my INA to saturate before the bottom rail is fully reached. By applying a ref voltage of at least 0.05V the proposed INA could always handle the output, also if my sensor outputs a 0V signal, did I understand that correctly?
How would I generate this kind of low reference votlage in that case? (by using a resistive voltage divider I had to add a buffer stage as well as most of the ref inputs only accept low input impedances).
And by adding a higher ref voltage, I would loose too much of my ADC range (ADC has a ref voltage of 2.048V).
Would really appreciate it if you could help me to improve my understanding here. But all in all there should be no issue regarding the negative bias and this idea should work?
So my signal chain Sensor->INA->ADC should be able to detect a true zero signal right?
I will provide a ground return path = providing a path trough a 10MOhm resistor to ground for both of the sensor inputs (before going into the INA).
I will bypass every supply of every device and trying to add a filter after the INA (before going in to the ADC).Or did you mean smth. else? I would be very happy to learn from you .I will need to take one measurement with this circuit every second. The sensor signal changes very very slowly so no fast sampling rates are needed at all. Or do you need more exact info on this?Thank you so much!
No, the sensor and the INA's input. The output impedance of an opamp or instrumentation amp is very very low.
The REF is an input pin with a very high input impedance, so its not at all critical, use a divider certainly.
Yes, but its completely not necessary and will cause issues due to the input offset voltageshowing up as the output being able to go a bit below the REF voltage (a problem if REF = 0V)
You always have to calibrate for the input offset voltage, unless you use a CAZ or similarinstrumentation amp with zero offset
There shouldn't be any high value resistors, I'm talking about ensuring no ground loops or voltage injection due to bad grounding layout.Your idea of 10M resistors would completely break the requirement that the sensor only see a load impedance of 100M or greater.