50hz spike when using ADuM7440 SPI isolator IC between a micro-controller and DAC - diyAudio
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Old 31st May 2014, 12:52 PM   #1
maxw is offline maxw  United Kingdom
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Default 50hz spike when using ADuM7440 SPI isolator IC between a micro-controller and DAC

I'm wondering if anyone has any idea why I see a 50hz spike when using a ADuM7440 SPI isolator IC between a micro-controller and a DAC. I don't have much experience with isolators but just read the datasheet and designed the PCB myself. I have two 5V PSUs where I intended to use one for the DAC IC and one for the Arduino microcontroller with the ADuM7440 isolating them.

These are my observations:
1x 5V PSU with ADuM7440 bypassed = Good
2x 5V PSU with ADuM7440 for isolation = 50hz spike.

Here is the schematic:
http://www.diyaudio.com/forums/attac...-schematic.jpg

To bypass the ADuM7440 IC I just connected the VDD/5V and GNDs from either side of the IC.

Attached is the layout and two graphs showing the 50hz spike.

Did I make some mistake? Poor layout? It just seems weird that either PSU on its own measure fine but as soon as I use the IC to isolate I get a 50hz spike
Attached Images
File Type: jpg dual-psu.jpg (163.9 KB, 96 views)
File Type: jpg single-psu.jpg (158.2 KB, 93 views)
File Type: jpg MADC-layout.jpg (722.3 KB, 100 views)
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Old 31st May 2014, 03:49 PM   #2
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I would have to guess the problem is with the power supply.
If it was a problem with the board I would expect the spike to
be present all the time.
Try swapping the two supplies and see if the spike goes away.
What power supplies are you using?
Are you able to reverse the phase of the AC input for one of
the supplies?
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Old 31st May 2014, 04:23 PM   #3
maxw is offline maxw  United Kingdom
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Quote:
Originally Posted by Avro Arrow View Post
Try swapping the two supplies and see if the spike goes away.
I tried this. Either power supply on its own doesn't produce the spike. I also tried a battery supplying the Arduino and my 5V PSU powering the DAC but still get the same spike.

Quote:
Originally Posted by Avro Arrow View Post
What power supplies are you using?
Two of these.

Quote:
Originally Posted by Avro Arrow View Post
Are you able to reverse the phase of the AC input for one of
the supplies?
You mean just swapping L and N on the mains side? I just tried this, still a spike!
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Old 31st May 2014, 04:27 PM   #4
maxw is offline maxw  United Kingdom
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I also thought it might just be interference radiating through the air when having 3x power supplies in the same small enclosure so I tried it with a single 5V PSU and the second one still powered up and in the case but not connected, still the spike though.
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Old 31st May 2014, 04:44 PM   #5
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Very interesting, i found exaclty the same problem with an ADUM4160 USB isolator, i have two different boards and one of them has a 50hz spike, the other doesn't. I assumed the issue was due to the 12" of cable between the power supply and isolator module. The board without the issue has an onboard rectifier and gets AC power directly from a wall wart, where as the noisy one has a power supply in a separate enclosure.
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Old 31st May 2014, 06:00 PM   #6
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Hi,
When you see the spikes the micro it is running?
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Old 31st May 2014, 06:09 PM   #7
maxw is offline maxw  United Kingdom
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Quote:
Originally Posted by tauro0221 View Post
Hi,
When you see the spikes the micro it is running?
Yeah. It's on and connected for all measurements. It controls the DAC so is required.
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Old 31st May 2014, 06:32 PM   #8
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Quote:
Originally Posted by mcandmar View Post
i have two different boards and one of them has a 50hz spike, the other doesn't.
I'm guessing I had something to do with one of those...
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Old 31st May 2014, 06:38 PM   #9
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50Hz is the mains frequency in the UK is it not?

The puzzle is why the problem only happens with two power supplies.
Can you power one side with a battery?
At least we can figure out which side to concentrate on.
My bet would be on the DAC side.
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Old 31st May 2014, 06:41 PM   #10
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Hi,
I asked the question because when you send the data from the micro to the dac you have to clock it. You are sending 16 bits to the dac. The clock would radiate noise when clocking the data to the dac. You can do a check to see if it is coming from the communication between the dac and the micro. Connect everything and program the micro to halt and see what happen.
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