Monitor tiny variation in voltage

[saomei]

Hi to all, before I am sart I should apologize if my question is trivial since I did not find a similar question, I decided to specifically ask mine.

The situation is following: I have a microscopic membrane in my lab, this piece which we call it NMS has a variation of voltage caused by its operation. Similiar to the action potential in neurons of a nervous system. We are trying to build an electronic measurement system, which could capture the variation of voltage in -70mV to -55mV range. We want to have an accuracy of at least 1mV or possibly less in our measurements. Since the only electronic guy, we have access to is a bachelor student, we shall be able to help him, So I have decided to seek a community help.

The current SoC he knows how to program and feels confident with is ESP32. As I searched its datasheet, it has some cool functionality which we might use here, I add them here as a reference:

http://espressif.com/sites/default/file ... eet_en.pdf

4.1.2 Analog-to-Digital Converter (ADC)
ESP32 integrates 12-bit SAR ADCs and supports measurements on 18 channels (analog-enabled pins). Some of these pins can be used to build a programmable gain amplifier which is used for the measurement of small analog signals. The ULP-coprocessor in ESP32 is also designed to measure the voltages while operating in the sleep mode, which enables low-power consumption. The CPU can be woken up by a threshold setting and/or via other triggers. With the appropriate setting, the ADCs and the amplifier can be configured to measure voltage for a maximum of 18 pins.

4.1.3 Ultra-Low-Noise Analog Pre-Amplifier
ESP32 integrates an ultra-low-noise analog pre-amplifier that amplifies the voltage difference between pins SENSOR_VP and SENSOR_VN and outputs the value to the ADC. The amplification ratio is given by the size of a pair of sampling capacitors that are placed off-chip. By using a larger capacitor, the sampling noise is reduced, but the settling time will be increased. The amplification ratio is also limited by the amplifier, which peaks at about 60 dB gain.

4.1.4 Hall Sensor
ESP32 integrates a Hall sensor based on an N-carrier resistor. When the chip is in the magnetic field, the Hall sensor develops a small voltage laterally on the resistor, which can be directly measured by the ADC, or amplified by the ultra-low-noise analog pre-amplifier and then measured by the ADC.
So, I am basically looking for design suggestions for mentioned measurements with ESP32 chips. We need some advice for building the measurement probe too, specifically, we need a way of connecting our NMS membrane which is sandwiched between a microscope slide and a cover slip to our measurement system.
Regards,
Thank you all advice!

[loboris]

For precise measurement of of very low voltages I would recommend using some external ADC chip with very precise voltage reference.
For example LTC2487 is very good ADC with i2c interface and up to 256x programmable gain. Combined with some precise voltage reference like LTC1790 it can give you the measuring range of less than 8 mV.

[saomei]

For precise measurement of of very low voltages I would recommend using some external ADC chip with very precise voltage reference.
For example LTC2487 is very good ADC with i2c interface and up to 256x programmable gain. Combined with some precise voltage reference like LTC1790 it can give you the measuring range of less than 8 mV.

Thank you, I'll try it.