Capacitors on digital signal out Hall IC - Page 1
Forum section: electronics / projects

author: SirrCharles
author_rank: Newbie
timestamp: February 09, 2018, 04:55:29 pm
content: I'm designing a fairly simple quadrature rotary encoder in combination with a magnetic multipole disk.This encoder is featuring 2 omnipolar hall sensors of the Type:Diodes Incorporated AH1806-W-7Datasheet:https://www.diodes.com/assets/Datasheets/AH1806.pdfThe sensors are put on a specific angle so 1 hall sensor will produce a a 90 degree phase shifted signal.This encoder will be mounted on a DC motor similar to the one on this pictureMy Question:I'm trained as a mechanical engineer and on school we only learned about 3/4 phase high voltage AC systems.I do get the concept of bypass and decoupling capacitors near the IC somewhat.I have seen some Chinese encoder designs featuring acapacitoron the digital signal line of the the hall sensor on the encoder side.In my case the wires of40cmwill run from the Encoder PCB to the main PCB.Is there any good reason to putcapacitors(C1 & C2) on the signal out lines of the hall sensor, before the signal has to cross 40cm to the main pcb to the MCU?Could somebody elaborate.

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 09, 2018, 05:11:20 pm
content: The sensor is an open collector device. Loading the output with a capacitor will slow down the waveform's rising edge, and create a brief current spike in the device's internal transistor on the falling edge.It seems they added this capacitor to satisfy EMI requirements, but in my opinion, it is a brute force approach.Rather use a 10X smaller cap and have a 200 to 1000 ohm resistor in series with the sensor output.Your board appears to have sufficient space to add a pair of 0201 resistors. Perhaps even 0402s.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: February 09, 2018, 10:41:12 pm
content: Quote from: schmitt trigger on February 09, 2018, 05:11:20 pm

author: John Heath
author_rank: Frequent Contributor
timestamp: February 10, 2018, 12:59:32 am
content: I can read between the lines. Been there done that. My suggestion for a solution is to buy an off the shelf motor assembly and finish the project. When the project is finished and only when the project is finished do you give yourself a well deserved treat and experiment with the finer details of a Hall effect sensor.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: February 10, 2018, 09:58:11 am
content: 40cm is significant, depending on motor power and quality it may or may not generate a lot of problems.Without knowing any details, in general, you can greatly improve noise resilience of this circuit by:1) minimising Hall circuit output impedance or in other words keeping pull up resistor values as low as practically possible. In your case it is somewhere close to 1-3k depending on Vdd voltage. Much better - stick a buffer with low impedance right after the Hall sensor - line driver, low side MOSFET driver or just a 74 gate. Any of them are better than open drain with 10k pullup.2) using suppresion caps across DC motor terminals3) placing two or three ferrite cores over the cable: one around all hall sensor wires and second/third around motor wires - together or separately (experiment)4) keeping motor wires on nearby pins, not on opposite sides of the connector. While you are at it, move Halls signals to be on both sides of GND pin. This will minimise their inductive loop areas.With your current design and cable layout my wild guess is that biggest problem is commutation noise coupling into Hall signal over 40cm of wiring. It is, essentially, a nicely coupled flat transformer.Leo