Help understanding this sample schematic for an electrochemical cell sensor - Page 1
Forum section: electronics / beginners

author: chogi
author_rank: Newbie
timestamp: January 04, 2025, 08:47:02 pm
content: Im trying to understand this sample schematic for a type of sensor i need to incorporate for a project. I understand the active low pass filter but not the ref for virtual earth or v set supply. Are these just voltage dividers between 5V rail and ground to achieve the desired voltages? I looked a bit online and on this forum and found nothing.Also im brand new to pcb design. Am i going to need an additional power plane layer for this 2.5V virtual ground? Can i split the power plane into 0 and 2.5 V sections if i use the back of the pcb as a power plane? anything else to look out for when designing a schematic like this/ with this kind of sensor? Thanks in advance.

author: inse
author_rank: Super Contributor
timestamp: January 04, 2025, 08:53:08 pm
content: Are you willing to share the datasheet (or link) so everyone will be able to get an understanding of what kind of sensor this is?Or do you prefer a load of guesswork?

author: chogi
author_rank: Newbie
timestamp: January 04, 2025, 09:03:28 pm
content: Data sheethttps://www.sgxsensortech.com/uploads/f_notes/PS1_PS4-Oxygen%20SGX%20Sensortech%20Circuit.pdfSample schematicshttps://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.sgxsensortech.com%2Fcontent%2Fuploads%2F2014%2F08%2FA1A-EC_SENSORS_AN2-Design-of-Electronics-for-EC-Sensors-V4.pdf&psig=AOvVaw0YtcxnywPsIj6W6EQXU4Lg&ust=1736100604731000&source=images&cd=vfe&opi=89978449&ved=0CBcQjhxqFwoTCOj7jfDU3IoDFQAAAAAdAAAAABAE

author: chogi
author_rank: Newbie
timestamp: January 04, 2025, 09:04:47 pm
content: I didn't think this was necessary as the question is about the meaning of the schematic on the left not the sensor itself

author: Analog Kid
author_rank: Super Contributor
timestamp: January 04, 2025, 09:16:31 pm
content: It would help to know exactly what kind of opamp IC1 and IC2 are.My guess™ is that they're using a split-supply opamp as a single-supply device, hence the "virtual ground" (splitting the supply in half).

author: TimFox
author_rank: Super Contributor
timestamp: January 04, 2025, 09:41:54 pm
content: Note that IC2 is an inverting amplifier. When operating from a single supply (+5V here), an intermediate voltage (+2.5V here) for “virtual ground” is mandatory, since as its input goes positive the output goes negative (and vice versa). In the old days, split supplies were used with “real ground” so that the output could go positive or negative.Any op amp will work with a single supply: the question is what range of common-mode input voltage is tolerable and what range of output voltage can happen. Older op amps intended for split supplies often require input voltage (common mode) substantially positive with respect to the negative power terminal. Newer units designed for single supply operation can have input voltage somewhat negative wrt the negative supply, but the output is always positive wrt that terminal (grounded for single supply operation).To forestall quibbling, there are very few op amps with an actual ground terminal (e.g., uA702) and some special ones have internal charge pumps to enable the output to go a bit negative of the power supply.

author: inse
author_rank: Super Contributor
timestamp: January 04, 2025, 09:54:46 pm
content: Everything is pretty much explained in the datasheet from the second link.As you see in the first schematic, the virtual ground is only a voltage divider.For sure you will not need a ground plane for that as it is only a virtual ground without loading and in principle only attached to a single component.

author: chogi
author_rank: Newbie
timestamp: January 05, 2025, 03:46:25 pm
content: I did some reasearch and found some op amps that can operate "rail to rail" I think this solves the issue as I dont think the sensing electrode will output a negative voltage (will it?). i'm just confused on the left side of the circuit how to construct each of the virtual ground and v ref voltage circuits. Also the bias listed for the specific sensor im using is listed as -300 mV. Is that the same as listed here with different notation? im not sure how bias voltage is defined here.What would i need to change to accomplish a bias voltage of -300mV instead of 300mV if needed? would that change anything with the IC2 op amo?

author: inse
author_rank: Super Contributor
timestamp: January 05, 2025, 04:13:10 pm
content: Why not take the MCP6032 as proposed in the first link?

author: Terry Bites
author_rank: Super Contributor
timestamp: January 24, 2025, 07:47:33 pm
content: This is a basic "potentiostat" circuit.The term "virtual earth" is misleading I haven't heard that term used since tubes faded out of use!Its not a ground, no current flows in it and no groundplane is needed.As you've figured the 2.5V reference voltage centers the ouput at mid supply.Vsense is also unhelpful as a signal label. The current from ths terminal is the thing actually being sensed.The RHS amp is an I to V converter in this electrochemical context it will often be refered to as an electrometer. (AKA transimpedance amplifier.)Long leads between the sensor and the circuit can cause problems. Stray capacitance at the current input node can often cause instability and oscillation.You can add simple compensation to bring this under control. See attched note on photodiode amps, the same principal applies here though.You can easily derive the 2.2V from the 2.5V reference. A TL431 reference IC is a good (low cost) starting point. If you can, derive this voltage from your system ADC.Low bias, low offset opamps are needed. eg OPA2192 . Clean your PCB thoroughly after soldering else leakage currents will mess with your measurement.

author: TimFox
author_rank: Super Contributor
timestamp: January 26, 2025, 04:40:37 pm
content: "Virtual ground" lives on as a concept for operational amplifier circuits, and was rarely used in the age of vacuum tubes.For a simple circuit, the voltage at the inverting input will remain damned-near constant, equal to the voltage at the non-inverting input, because of the negative feedback.With split supplies, that voltage might well be "ground", with power rails symmetric about ground, but with a single supply (as is the current fashion), it is often necessary to generate an intermediate voltage, usually with low current drain, to maintain the voltage at the non-inverting input.The term virtual ground implies that the voltage is constant regardless of the current flowing into that node.