Replacing 10uF tantalum capacitors with X7R in ADM2587E circuit

Question

I'm working on some device with isolated RS485 based on ADM2587E transceiver with integrated high frequency isolated DC/DC converter.

This IC is a bit tricky if you need low EMI, because DC/DC converter makes high frequency noise.

My previous design was based of this evaluation board:

UG-916: 2-Layer EN55022 Class A Radiated Emissions Compliant Evaluation Board for the ADM2582E/ADM2587E 2.5 kV rms Signal and Power Isolated RS-485 Transceivers with ±15 kV ESD Protection

I have used same components and device passed EMC tests.

Since tantalum capacitors are "very bad" (conflict material), like many others I want to get rid of them.

Can I just replace tantalum caps (C2, C10) with good quality X7R and expect similar (maybe even better) results in EMI emission?

I know that X7R MLCC has waaay lower ESR and I guess that these tantalum caps on eval board are tank capacitors (not important at high frequency range).

Answer 1

No, you should not do this. The ESR is too low on the ceramic capacitors and may cause the ADP667ARZ regulator to go unstable, as can be confirmed by reading the datasheet.

If you want to replace them, you can use a ceramic capacitor with a small series resistor of similar ESR to the original tantalum capacitor the designer says you "must" use (probably in the 1-3\$\Omega\$ range).

Also note that ceramic capacitors often have a significantly lower than nameplate capacitance under bias (in extreme cases, they can lose 80% of their rated capacitance) so you must seek out the detailed data from the manufacturer and ensure you are getting something close to 10uF. You might have to use 22uF or more. The smallest (physical) size of capacitors run close to the rated voltage tend to be the worst, but do check the detailed data (often not on the datasheet).