TLK1501EVM: Use of 10nF capacitor connected to ground between the two 50ohm termination resistors on the evaluation board of the TLK1501 (RX lines)

William Vigano

Part Number: TLK1501EVM
Other Parts Discussed in Thread: TLK1501

Hello,

I need a clarification about the use of the 10nF capacitor connected to ground between the two 50ohm termination resistors on the evaluation board of the TLK1501 (RX lines).

On the schematics such capacitor is identified as C26.

On the datasheet there is no capacitor between the termination resistors, and the line is described as "differential terminated with 100ohm", while on the evaluation board there is the 10nF capacitor (C26) which seems transforming the RX lines as two single ended 50ohm lines, because on high frequency C26 behaves as a short to GND.

In other words is C26 introducing a weakness on the receiver lines or not?

Thank you very much in advance for your attention.

Best Regards,

William Vigano'

over 2 years ago

0 Rodrigo Natal over 2 years ago

TI__Mastermind 19135 points

This capacitor is not introducing weakness. It simply serves as bypass capacitor, so higher frequency noise has low impedance path to GND. This capacitor is optional.

Thanks,

Rodrigo Natal

0 William Vigano over 2 years ago

Prodigy 10 points

Hello Rodrigo,

The cut-off frequency for a low pass filter composed by R=50ohm and C=10nF is about 318KHz, so due to the fact the transceivers works with Gbit/s means that such capacitor transforms constantly the line as single ended instead of differential. Does it exist any characterization of the receiver line when configured as differential 100ohm terminated of 50ohm single ended?

Thanks,

William

0 Rodrigo Natal over 2 years ago in reply to William Vigano

TI__Mastermind 19135 points

Hi William,

This capacitor provides decoupling, it does not act as low pass filter. Please see TI application note below, and refer to Section 5.2.2

https://www.ti.com/lit/an/slla120/slla120.pdf

Thanks,

Rodrigo

+1 William Vigano over 2 years ago

Prodigy 10 points

Hi Rodrigo,

Without any test report or theoretical calculation, I cannot take your sentence as an assumption.

But let's drop down the subject here. I will perform tests with and without the capacitor to compare the performance.

Have a good day,

William

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TLK1501EVM: Use of 10nF capacitor connected to ground between the two 50ohm termination resistors on the evaluation board of the TLK1501 (RX lines)