Hi, I made up a PCB that has 4 ports to control 6v-24v DC devices, this includes an asc712 5amp hall effect current sensor for each port which is read using an ATMega32u4.

Everything works as expected and I'm impressed with the accuracy from the asc712's however there is an issue that I'm not sure how to address and believe it's in the layout.

If nothing is plugged in all asc712's show between 5mA and -5mA which is great. If I plug something in port 1 that draws 2 amps port 2,3 and 4 now show 60mA as they appear to be seeing some of the load as the power needs to pass them to get to port 1.

If I plug something in port 3 port 4 will show the 60mA but now port 2 will remain at 5mA because the load doesn't goes that far, at least that's how it appears. I don't like this because if I plug in a low current device to port 3 that draws 60mA and I have something high powered in port 1 port 3 will now read 120mA which is wrong.

And of course if I plug the high powered device in port 4 all other ports only read 5mA as the power doesn't pass them.

I tried 0.1uf, 0.01uf, 0.001uf and no capacitor on the filter pin but it doesn't seem to make any difference.

Should there be some kind of filtering before +12v goes into the acs712's?

Any ideas would be appreciated, thanks.

 

hi rob,
The single track feeding the ACHS in a series path as per your image is the cause of the problem, the voltage drop along the track is being impressed on the other ACHS inputs.
Route the +V directly to each ACHS input along its own track, from the +v source.
E

BTW I am assuming that ACHS pin '#5, the OV pin is the Ground plane of the PCB.??

 

Thanks Eric, yeah pin 5 is connect to top and bottom ground plane.

When you say power each ACHS from it's own track do you mean run a trace to each from the main input diode (D8 top right corner)?

 

Something like this?

 

hi rob,
That's the way I would lay it out for a 2A current rating for each sensor.
How is pin #5 tracked.? ,the 0V path.

E

 

Hi Eric, thanks, I also updated the image above with a more compact layout.

I'm hoping for each port to handle up to 4amps constant with a max of 8amps for the board. The main input trace is 5mm top and bottom and the lines to each ACHS is 2.2mm. According to this calculator I should be ok.

https://www.4pcb.com/trace-width-calculator.html

For pin 5 I have it named GND and the top/bottom ground planes are also name the same so they connect. Nothing more.

 

In a similar fashion, run separate gnd traces to one point for the outputs as well. Any extra current running in the gnd traces to do with the analog inputs to the ATMega32u4 need to be eliminated. If the trace is small, and you only use one trace for all the Hall sensors, the currents will cause readings to interact.
Usually I have the ground plane of the PCB split in 2 so the digital and analog parts are only connected together at one place, and that is close to the processor AGND pin.

 

Thanks dendad, sounds like I'm not doing something right. I don't have any traces running for any grounds, the whole board is a ground plane so all the grounds are connected together as they share the same name. If I run ground traces after adding the ground planes they disappear.