Is This Bridge-Like Circuit a Viable Solution for Noisy 2-Wire Voltage Drop Meas - Page 1
Forum section: electronics / projects

author: QWERTYQWQ
author_rank: Contributor
timestamp: July 01, 2025, 02:44:35 pm
content: Hello,I’m here to get your opinion on a design I’m working on. The design consists of a card that includes an instrumentation amplifier, a microcontroller (MCU), and several resistors and capacitors. You can see the circuit schematic below.The purpose of this circuit is to measure the voltage drop across a resistor (potentiometer) located 10 meters away. This resistor will be adjusted by the user, and analog data will be transmitted to the amplifier card through two wires. However, the cable route includes various high- and low-current carrying wires for other devices, which introduces significant common-mode noise onto the signal wires. Therefore, I need to eliminate common-mode noise from the resistor lines.Initially, I considered using Wheatstone bridge circuits. However, the issue is that the Wheatstone bridge requires a 4-wire setup, and the remote resistor is inherently 2-wire — and I cannot modify this. So instead, I’ve been working on a circuit model similar to a Wheatstone bridge but using resistors R1 and R3. This configuration allows me to measure the voltage drop across the remote resistor using an instrumentation amplifier while aiming to reject common-mode noise.I plan to generate V1 using a low-noise regulator, and V2 will be the input voltage of this regulator.I’m writing to ask whether this circuit model can effectively eliminate common-mode noise. Also, I’d appreciate your detailed thoughts on how this design compares to a Wheatstone bridge in terms of common-mode noise rejection and measurement accuracy.Note: I previously asked this question on another forum, but the answers I received were not satisfactory, so I’m now seeking input from users on this forum.Thank you.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 01, 2025, 04:36:57 pm
content: Armchair guesses- the diff amp only helps you if the noise pickup on both wires is the same. What might help more is just lowering the impedance of the pot loop. I don't know what it is now but consider a 100 ohm pot or even lower. Make it a constant current loop like a 4-20 ma loop ( or 0-10 or whatever) and read the voltage. A low impedance circuit isn't going to pick up much noise. You could also do AC excitation with a narrow band pickup, but I bet somebody already thought of that.

author: MariuszD
author_rank: Regular Contributor
timestamp: July 01, 2025, 05:58:31 pm
content: The voltage changes from the potentiometer are very slow. Even without a balanced input, the low pass filter will do its job.

author: mikerj
author_rank: Super Contributor
timestamp: July 01, 2025, 10:38:07 pm
content: Your proposed scheme should give a reasonable level of common mode rejection, remember to also use a twisted pair for the wiring between the resistor and the differential amplifier. You might consider adding protection to the front end of the amp if there is any possibility of voltages higher than 40v being coupled into the resistor wiring.As Conrad mentioned, this kind of problem was solved for industrial sensors a long time ago by sending a current (typically 4-20mA) rather than a voltage. The high impedance of the current loop means that stray coupling from other circuits has little effect on the current. This does require a some circuitry at the resistor end, but you can buy a 4-20mA "potentiometer" of the shelf that has this built in.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 02, 2025, 12:27:50 am
content: Quote from: QWERTYQWQ on July 01, 2025, 02:44:35 pmYou have not said how fast you need to update this reading, or the frequencies of the aggressor signals.You can model common mode noise by coupling a source via equal capacitance, and then also varying that 'equal' to be more real-world.Usually you apply as much passive filtering as your reading rate allows, early in the chain, so spikes cannot overdrive the amplifiers.Is there a power limit or self-heating limit on your remote resistor ?Another way to improve signal to noise is to increase the voltage drop, and current feed allows you to read resistance change with no divider attenuation.

author: QWERTYQWQ
author_rank: Contributor
timestamp: July 02, 2025, 06:04:51 am
content: Quote from: mikerj on July 01, 2025, 10:38:07 pmThank you for your response. The module containing the potentiometer is an original part manufactured by a textile machine producer, so unfortunately, I do not have the right to modify or replace the potentiometer directly. Therefore, although a 4–20 mA current loop would be ideal, this is not an option for me.Regarding the noise issue, yes, it does appear that the circuit is designed to suppress common-mode noise. However, I wasn’t entirely sure and wanted to consult with you. As you confirmed, I will proceed with producing a PCB by adding some protection and filtering components. I believe this circuit should work without major problems. Still, I am curious about your thoughts on whether it can be as effective as a Wheatstone bridge circuit.Quote from: PCB.Wiz on July 02, 2025, 12:27:50 amThe signal doesn’t need to be very fast; high accuracy is what matters most for us. In some of my previous applications, I was reading data directly with the MCU using single-ended measurements through an RC filter. However, this time I felt it needed to be more professional and stable. That’s why I focused on differential measurement circuits. And yes, just like before, I will definitely include a low-pass filter with an RC circuit at the output of the instrumentation amplifier. But unlike last time, I believe I can achieve this with a lower tau value rather than a high one.The potentiometer looks very similar to an ordinary one available on the market. The only difference is that it has a casing I haven’t seen before and am not familiar with. It probably has a power limit, and it's quite restricted, so I’m hesitant to apply high currents.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 02, 2025, 08:42:34 am
content: Quote from: QWERTYQWQ on July 02, 2025, 06:04:51 amNo.early in the chain, meansbeforethe amplifier you are hoping has common mode rejection.Common mode rejection only works if the amplifier inputs are not spiked outside their common mode range, and also not driven with frequencies where common mode is poor.You should also test just how balanced this common mode noise is. A scope can tell you a lot.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 02, 2025, 09:06:26 am
content: Quote from: QWERTYQWQ on July 02, 2025, 06:04:51 amBridge circuits work best with like elements, eg if you know the temperature coefficient of your pot, a resistance with the same tempco (or matching current source) can null that variance.For that reason, pots are often wired 3 terminal or ratio metric but it looks like you are forced to 2 terminal ?

author: QWERTYQWQ
author_rank: Contributor
timestamp: July 02, 2025, 09:32:26 am
content: Quote from: PCB.Wiz on July 02, 2025, 09:06:26 amOnly a 2-wire terminal is provided from within the module.I only know the resistance value of the potentiometer; no other information is available.As for common-mode noise, I plan to connect a TVS diode to the amplifier inputs. This way, the inputs will operate within safe limits.Additionally, I will use matched RC filters on each input to protect the amplifier from sudden voltage spikes.It’s not really possible to connect the oscilloscope without running the device at full capacity,so I will need to estimate the common-mode noise caused by 50Hz motors and some contactors.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 02, 2025, 09:36:57 am
content: Hi,what kind of cable are you using?Is it a propper twisted pair with shielding?Good luck

author: QWERTYQWQ
author_rank: Contributor
timestamp: July 02, 2025, 12:11:31 pm
content: Quote from: Roehrenonkel on July 02, 2025, 09:36:57 amI will most likely use the same cables I use for Modbus. I will ground the shield at a point close to the receiving circuit. And yes, since the cable is for Modbus, it is twisted pair.