I am using a pressure sensor with my Arduino Uno. Below is the amplification circuit for the sensor.

The recommended reference voltage is 1V DC, but I had been using the 3.3 V of Arduino as reference and been getting 206 ADC values at 1 volt dc with rise of 2 ADC value for 1 mmHg rise.

To increase the sensitivity of the system I first desired to bring the base value of the ADC down to 0, so I added resistances in the path of the reference voltage 3.3 V but after a point no matter how high the resistance the base value wouldn't drop below 54 ADC. I am currently holding the reference voltage at close to 1V DC. Now with my ADC at 54, I should be able to amplify my signal by 1024/54 = 18 times, but to do that I used a LM324N and the outcome was not ideal. To estimate the gain in each op-amp I used the below calculation which was for a LM358 but which I thought might be the same here.

So I placed a 56K in R2 of first Op amp of the four in LM324N, and I placed a 10K in the R2 of the second Op-amp. Result - a base ADC of 700 (with noise fluctuation) But no shift in ADC values with change in pressure. In fact I notice that even if I change the R2 of first op-amp to 10K, then there is a shift in ADC with pressure rise but not as much as needed. If I use the third op-amp the signal goes haywire and AC like.

I need to know which amplifier to use so that noise is minimal possible, drift is minimal possible, reaction to pressure change is instantaneous, outcome stays linear.

If you want more gain from the circuit, don't add another amplifier - just adjust the value of R6. A lower value there will give you more gain from the 3-opamp instrumentation amplifier circuit.
The 3rd opamp in that part of the circuit is a standard differential amplifier, so you can also adjust the gain by changing the ratio of R4/R9 and R8/10 - but make sure to keep R9 = R10 and R4 = R8.

This leads on to adjusting the Ref input. You say that you've been adding resistors between Ref and the voltage source - this is a bad idea, because you're effectively increasing the value of R10 (by putting other resistors in series with it) without similarly increasing R9 by the same amount - this complicates the otherwise reasonably simple gain equations for that part of the circuit.
If you want to adjust the Ref input, make sure that you use a low-impedance voltage source to drive it, for example you could use another opamp as a non-inverting buffer and feed the input of that buffer from a voltage divider.

I'm assuming that you are powering the LM324 from +5 volts and ground/0 volts.

An LM324 powered from 5 volts will not be able to provide the circa 4.718 volts after amplifying by 18. An LM324 does not have a rail-to-rail output voltage. It can get close to 0 volts at the output but, the upper limit will be around 3.5 volts on a 5 volt positive power rail.

• How would a lmv324 fare? That's rail to rail I believe Dec 2, 2022 at 13:17
• electronics.stackexchange.com/help/someone-answers plus, you should supply a data sheet link to the device. @DribbleNibble Dec 2, 2022 at 13:19
1. It looks like you need a reasonable accuracy, but trying to use a very inaccurate and noisy voltage source (3.3V from Arduino) as reference. I would suggest a dedicated voltage reference for that - it will be not noisy, it will be temperature stable.
2. Using Arduino's ADC - that is far from the best choice indeed. It is way to simple primitive (resolution, linearity, reference voltage stability) for good measurements.

To have a better answer from an engineer I would recommend to mention overall "accuracy budget" in principle.

Performing a preliminary calculation of it - which part of signal path brings which errors would provide understanding of whether built-in ADC is "good enough" or not.

• The thing about accuracy is I cannot be sure. The reason I am doing all this amplification exercise is that I am trying to pick a signal in pressure change but I don't know how strong or weak that signal is going to be. I was hoping that if I can amplify the base signal by at least ten times I might detect it. I understand arduino's reference 3.3V is noisy, would it be a good idea to set a 1V reference with a buck convertor and a independent power source? Dec 2, 2022 at 13:43
• Buck converter = tons of switching noise. Research dedicated voltage references; these devices do exist. Dec 2, 2022 at 13:54