Groundloop always bad? - Page 1
Forum section: electronics / beginners

author: sapien
author_rank: Contributor
timestamp: March 13, 2023, 01:05:33 pm
content: Hi everyone,Currently I'm working on a project in which I want to read out some (analog) sensors (a current sensor: ACS724) which are on a buck converter. The converter has a power- and signal ground plane. Ideally I would use the signal ground plane to power the sensor (it's a ratiometric output), and let the ESP32 (see attached file) use signal ground plane as gnd-reference. However, I also have to (PWM) control a fan, which (I think) needs to be connected to the power ground plane.So: ESP32 ideally would be connected to both power and signal groundplane (with direct via's underneath), however, this would create a ground loop.The main fundamental question: Is this a problem, even though in theory (on average) no current would be flowing in the direction of the dotted line (see attached file) which could create a potential difference in the loop?Furthermore I curious if you have any thoughts on the following:Is there a way to let the sensor use the power ground plane as reference, without being bothered by the potential difference in the power ground plane? (Without having to use expensive sensors)Or is there a way to PWM control the fan (it's a simple N-channel MOSFET setup) and let the noise charging and discharging the gate at 21kHz not couple to the signal ground plane? (if the ESP32 GND is connected to the signal ground plane)

author: Terry Bites
author_rank: Super Contributor
timestamp: March 13, 2023, 03:48:59 pm
content: I'd forget the signal plane if its just for the current sensor.It will probably create more problems than it solves.[you could then double up on groundplanes or add and reduce the ground impedance for the converter.Or you might consider using the unused layer to make a couple of power planes. ]I'd keep the sensor isolated from the converter power and ground.Run separate power supply and signal lines for the ACS724 from the ESP32 board.The sensor will then be refd to the ESP32 gnd and power which gets you a more solid input for the ADC.Tie the converter, PWM ground and at the ESP32 board at a central ground point.

author: sapien
author_rank: Contributor
timestamp: March 13, 2023, 07:14:54 pm
content: Thanks for your reply!My line of thought was pretty similar, as far as isolating the current sensor VCC, GND and Signal from any converter GND/power planes/paths goes.As to where to tie-in of the ESP32 and PWM into the converter its (power) ground plane should be, I'm having some different thoughts.If I'd follow your advice I'm affraid the PWM power, which would have to flow through the same vias that connect the converter's signal and power ground planes. I think the current (up to something like 3A), pulsed at 21kHz, will cause problems in the converter's feedback signals (the frequency is high enough to cause real problems, and low enough to not be filtered out by the IC's internal filters, like the 150kHz switching frequency of the converter). Are these concerns valid, or am I missing something?My thought, see attached figure, is to connect the (power) ground planes of PWM and ESP with the converter at a place on the dotted line close to the tie-in vias, but a bit away from the vias to not have the PWM'ed current of the fan flowing through the converter's signal tie-in vias (see green box in attached figure).Now I'm also manipulating the Vfb of the converter using the ESP32, using a DAC. There is a 2-in-series (second order?) low-pass RC filters (3380 Hz cut-off) close at the converter IC, so I'm not worried for high-frequencies getting through. However I am wondering whether, even though my tie-in on the power ground plane is at the dotted line, a low-frequency/0Hz offset could occur, which can have a large impact on Vfb (the offset signal goes through a resistor network, so the offset would relative to a voltage in the order of 3V, not the actual Vfb. Thus 100mV would be a 10% offset in converter output voltage, not make converter go crazy due to Vfb being overloaded). Could this be a problem, or will it be negligible since it connect at the dotted line?