5
\$\begingroup\$

enter image description here

In this circuit, the gnd1 and gnd2 connected with a cap c27, why?

\$\endgroup\$
5
  • 1
    \$\begingroup\$ Where did you get that? The data sheet doesn't show such a capacitor is needed. ti.com/lit/ds/symlink/iso7141cc.pdf \$\endgroup\$
    – Passerby
    Feb 1, 2016 at 6:08
  • \$\begingroup\$ @Passerby: Its not in datasheet, but I have seen this in most of the isolation based designs, trying to find out the exact reasons, though it is used to couple the ac pulsating noise currents from one ground to another. \$\endgroup\$
    – AKR
    Feb 1, 2016 at 6:12
  • \$\begingroup\$ Have you read the app note, and if they mention it? Probably is a capacitive coupling of two grounds, the voltage differnece between them can't be unlimited, perhaps they use this cap to reference the GND2 potential. \$\endgroup\$ Feb 1, 2016 at 6:14
  • \$\begingroup\$ Oddly enough, TI's white sheets on Mixed Signal Grounding also doesn't show them. ti.com/lit/an/slyt499/slyt499.pdf ti.com/lit/an/slyt512/slyt512.pdf \$\endgroup\$
    – Passerby
    Feb 1, 2016 at 6:14
  • \$\begingroup\$ Check the example on page 2 of the app note. analog.com/media/en/reference-design-documentation/… \$\endgroup\$
    – AKR
    Feb 1, 2016 at 6:19

1 Answer 1

5
\$\begingroup\$

It's not really to do with the isolator- but you'd typically use a DC-DC converter with such an isolator, and the capacitor is usually needed to conduct switching noise from the switchmode power supply back to the input.

Edit: See the document "Cantrell, Mark. Application Note AN-0971, Recommendations for Control of Radiated Emissions with isoPower Devices. Analog Devices." referenced in the back of the document AKR linked to- they use the term 'stitching capacitance' because it joins ('stitches') together two isolated ground planes for RF. Note that the capacitor is a single point failure for the isolation barrier so it must be a safety-rated type such as Y rated if a breach of the isolation barrier could cause a safety issue.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.