Electronic – Monitor tiny variation in voltage

hall-effectlow-noise-amplifieroperational-amplifier

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

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/files/documentation/esp32_datasheet_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.

Thank you all.

Best Answer

The ESP32 doesn't measure pure voltage (meaning, its input doesn't look like an infinite resistor.) I don't find ohms value for the input pins of ESP32 A/D in their spec sheets. It might be 1Meg, it might be 100K.

What's voltmeter-resistance tolerated by your micropipette setup? If the A/D input needs to look like a 100meg resistor, then you'll definitely need an op-amp to serve as an input buffer. (Also an op-amp can handle both pos and neg inputs over a range of many volts.)

Also this:

  • Many patch clamp amplifiers do not use true voltage clamp circuitry, but instead are differential amplifiers that use the bath electrode to set the zero current (ground) level. This allows a researcher to keep the voltage constant while observing changes in current. To make these recordings, the patch pipette is compared to the ground electrode. Current is then injected into the system to maintain a constant, set voltage. However much current is needed to clamp the voltage is opposite in sign and equal in magnitude to the current through the membrane. See: https://en.wikipedia.org/wiki/Patch_clamp#Recording

For micropipette membrane probes, to attain low enough system noise, usually the voltage is fixed or "commanded," while only the nanoamps are being measured. We use a circuit called a "headstage amp:" an op-amp wired to act as a transimpedance amplifier. This is placed between the pipette electrode and the rest of your circuitry (the ESP32 input.) So, the membrane generates a 10nA pulse, and the amplifier gives a 1.0V output. The amplifier has two inputs (diff amp,) so we can compare potentials between the clamped cell and the fluid environment.

Search words: patch-clamp headstage schematic

I just replaced the fried FETs on one of these last month: an old classic version (not one of the expensive, cutting-edge products being used today.) I sketched out a crude schematic.

This one uses an external dual-JFET to attain extreme low-noise operation, with the FET feeding an inexpensive LF356 op-amp. The 100meg resistor gives 0.1V-per-nanoamp operation.

PS

The fancy and expensive version is Axopatch, Axon corp. Here's one of their headstage amplifiers. Rather than buying dual FETs from Siliconix, they built their own! They mounted them on a peltier cooler, so they can chill it down to -20C, reducing the noise even more. This thing can detect the pulses from a single ion channel opening/closing.

Related Topic