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I am developing the automated test system depicted in the following picture.

enter image description here

  1. The system receive commands and returns result data via Bluetooth to a remote computer
  2. DK52 is used to fine tune the control software and will be replaced by a custom board based on nRF52832
  3. Control Circuit generates testing signals according to the received commands
  4. ADC measures voltage applied to the DUT and drained current
  5. To respect the ADC specs the two circuits are isolated by ISO7741

Everything works fine.

To simplify the system I replaced the power supply 2 with a isolated DC-DC converter (DPAN02A-05) connected to power supply 1.

enter image description here

After that the current measurements (differential voltage across R1) are completely screwed up. Measured current is definitely lower than the correct value and it is negative instead of positive.

Conversely, voltage measured across DUT is correct (even if a little bit lower than the expected value. Maybe because the supply voltage is noisy since the DC-DC converter).

I measured +5V and +3.3V and they are correct.

I have been bumping my head on the wall for days trying to figure out what's wrong.

Please, give me some help!

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1 Answer 1

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Because you should use a filter and then a LDO and then yet another filter, ferrite bead. This DPAN02A-05 actually has a dual output +/-5v which you dont use it correctly. You should indeed find some low ripple, low noise 3.3V & 5V converter.

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  • \$\begingroup\$ Hi Marko, thank you. 1) I know that the DPAN02A-05 has a dual output and I am using one only of them, but it was ready to hand in the lab. 2) As far as I understand, you are saying that the wrong measures are because the noise coming from the DC-DC converter? 3) Could you please suggest any low noise/ripple isolated DC-DC converter that I can use? \$\endgroup\$
    – Fab
    Aug 31, 2020 at 8:12
  • \$\begingroup\$ @Fab The PSU for your application is somehow undesiderable, since you need 5v for ana part ad 3.3v for digitial part. I would be simpler if you redisign to have 3.3v ana. In such way you could use two separate LDOs. See this recom-power.com/ko/… \$\endgroup\$ Aug 31, 2020 at 18:59

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