DS90UB928Q-Q1: Capacitance of CMF pin

Youhei Miyaoka

Part Number: DS90UB928Q-Q1

Hi team,

In the datasheet, a 0.1 μF capacitor must be connected from this pin to GND for additional common-mode filtering of the differential pair. (8.3.5)

My customers want to change this 0.1u to 4.7u in terms of noise. Is this a problem? If there is maximum capacitance, please let me know.

Regards,

Youhei MIYAOKA

over 1 year ago

0 Hamzeh Jaradat over 1 year ago

TI__Guru** 108100 points

Hello Miyaoka-san,

our internal testing and validation showed that our device works best with a 100nF capacitor. If you change this value, you may improve system level performance but worsen device internal performance, hence it is not recommended to change the value of the CMF cap.

0 Roman Rios over 1 year ago in reply to Hamzeh Jaradat

Prodigy 40 points

Hello,

Currently we are having higher emissions levels coming out from the OLDI output pins, this is causing us to fail our radiated emissions.

We tried with 4.7uF and it reduced the e-field at the Tx Outputs and after we tested RE we confirmed that improve the RE spikes observed at the harmonics of Pixel CLOCK.

So, could you help us to confirm if 4.7uF is okay to be used? Or if there is any maximum spec value?

I would like to know why on FPDLINK 2 Deserializer the recommendation was changed from 0.1uF to 4.7uF,

I used 4.7uF based on the following deserializer datasheet.

They previously had 0.1 uF and they increased the capacitance

https://www.ti.com/lit/ds/symlink/ds92lv0422.pdf?ts=1699996549169&ref_url=https%253A%252F%252Fwww.google.com%252F

thanks!

0 Hamzeh Jaradat over 1 year ago in reply to Roman Rios

TI__Guru** 108100 points

Hi,

that is a completely different device with different internal structure. And what works for that device does not need to work exactly for this device.

Let me confirm this with the team and will get back to you shortly.

0 Hamzeh Jaradat over 1 year ago in reply to Hamzeh Jaradat

TI__Guru** 108100 points

Hello Roman,

yes, you can use the 4.7uF capacitor on the CMF pin instead of the 100nF.

0 Roman Rios over 1 year ago in reply to Hamzeh Jaradat

Prodigy 40 points

Hello Hamzeh,

Thank you for the quick reply, I will proceed forward with this new capacitor value on CMF Pin

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DS90UB928Q-Q1: Capacitance of CMF pin