I've never heard about a microcontroller ADC with this (big) problem.There is an issue with the ADC converter of the uC that I’m using. It has a dead band from 0V to 0.15V
No. The Opamp will (or should) never see +/-32V. Only the input resistors will see this big voltage.Input of the op-amp from -32V to +32V
Makes no sense.I should be able to measure with or without the negative input of channel 1 or channel 2 connected to ground.
Why not? Sooner or later you need them. I see no benefit in trying the circuit with random values. It doesn't save time.(resistors are not calculated for the voltages indicated above):
This is no useful option. Connect them to the voltage you wat to refer your voltage measurement.If I have the input open,
Again. OPEN connections are not useful. Especially with your circuit, because the (unconnected) non inverting input tries to pull its voltage away from GND. It will not be zero.immediately I get the value of the ADC with nothing connect the op-amp input, increase from 940 to about 1,115. Obviously that can’t be good.
Definitely YES.Is it possible to measure -32V to +32V or should I stick with 0V to 32V on the inputs?
https://www.esp32.com/viewtopic.php?t=2881Hi,
I've never heard about a microcontroller ADC with this (big) problem.
Why don't you tell us the exact part name and vendor, or post a link to the datasheet?
this should be better and the resistors are with the values I calculated for -32 to +32V inputPicture: are you able to read the part values? I am not.
Agree, what I meant is the voltage applied to the input resistors not to the input of the opampNo. The Opamp will (or should) never see +/-32V. Only the input resistors will see this big voltage.
Makes no sense.
* You build a "differential" amplifier circuit ... but then don't want to use the inverting input.
* every voltage measurement needs to have two lines. One signal input and one signal as reference.
* if you want to "misuse" your circuit by referencing to GND --> you need to connect the inverting input of your circuit to GND. Without additional resistor.
The attached picture is with the values I calculated and used in my tests. I did simulation, but did not see these possible shifts...Why not? Sooner or later you need them. I see no benefit in trying the circuit with random values. It doesn't save time.
--> especially when you are not experienced with analog circuits I recommend to use a (free) simulation software.
I will try some more measurements with inputs connected to V source, not floating.This is no useful option. Connect them to the voltage you wat to refer your voltage measurement.
Again. OPEN connections are not useful. Especially with your circuit, because the (unconnected) non inverting input tries to pull its voltage away from GND. It will not be zero.
Both inputs of your circuits
* don't have equal input impedance
* don't pull to the same voltage
If you want to avoid this, then you need to use a true "instrumentation amplifier (circuit)" (=INA).
But still with an INA: don't leave (unused) inputs floating. At least install a high ohmic resistor to GND. But the result with a "high ohmic" input will be noisy and prone of errors.
Definitely YES.
If you want to use your circuit:
* reverse the inputs:
Use the non inverting circuit input as (GND) reference and the inverting as signal input.
For sure the digital output is reversed, too. Just invert it on the digital side by software.
Klaus
for negative input voltages you need a neg supply for the op-amp unless you level shift the op-amp or the input.
assuming the CM i/p range of the op-amp is nearly 3V, you have to divide down the +32V to just under 3V for the op-amp inputs to be able to handle the signal, lets say 100k in and 5k to gnd ( and f/back R ) this gives a diff-amp gain of 5/100 or 5% or 1/20th, 1/20th of 32V = 1.6 V out of the op-amp. 9k375 as the R to gnd ( & f/back) would give 3V from 32V in.
If you sit the op-amp on +/- 1v5 then you need a f/back R of 9k375/2 to get +/- 1v5 out for +/-32V in - assuming the op-amp is a true rail to rail type ...
The asymmetrical input structure implies unequal input impedance. I prefer a true instrumentation amplifier (either 2 or 3 OP design) for differential voltage divider measurement circuits.
The choice depends on your performance requirements which haven't been specified yet.
Your calculation surely is wrong. Just have a look on "resolution" (overall accuracy will be worse).the requirements are to be able to measure with approx., 5-10uV accuracy. That shouldn't be a problem with real 12 bit ADC
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