AMC7908: Bypass capacitors

Shunsuke Yamamoto

Part Number: AMC7908

Hi team,

Based on the sentence listed on the datasheet, the bypass capacitors between power supply pins and GND should be 3 or 4 times the total capacitance on the respective DAC outputs.

When I see the typical circuit on the datasheet, there are total 30.01uF of the capacitors is connected to DAC outputs. Therefore pver 90uF of the capacitors should be connected to the power supply pins, but it is not. Why?

Best regards,

Shunsuke Yamamoto

10 days ago

0 Paul_Frost 10 days ago

TI__Guru 56923 points

Hi Yamamoto-san,

Very good observation! Our recommendation for 3x-4x bypass capacitance is still valid. In the figure we are just highlighting a smaller value that should still be placed as close to the pin as possible for best supply transient and noise rejection. The larger capacitance can be placed further away from the AMC as it is there to prevent supply collapse during a large output transient, when many of the DACs are entering short-circuit current mode.

Thanks,

Paul

0 Shunsuke Yamamoto 7 days ago in reply to Paul_Frost

TI__Genius 10401 points

Hi Paul-san,

Thank you for the advice. If large output transient doesn't happen in customer's design, The large bypass capacitance isn't needed. Right? My customer would like to minimize the bypass capacitance as much as possible.

Best regards,

Shunsuke Yamamoto

0 Shunsuke Yamamoto 5 days ago in reply to Shunsuke Yamamoto

TI__Genius 10401 points

Hi Paul-san,

This is just a kind reminder. it's really helpful if you could give me reply.

Best regards,

Shunsuke Yamamoto

0 Paul_Frost 5 days ago in reply to Shunsuke Yamamoto

TI__Guru 56923 points

Yes, if you do not expect high-current transient, the bypass cap is not needed.

Thanks,

Paul

0 Shunsuke Yamamoto 5 days ago in reply to Paul_Frost

TI__Genius 10401 points

Hi Paul-san,

Thank you for the answer, and I am sorry for asking a lot. I received additional question. Could you please help me answer?

In customer's use case, 23uF of the capacitor is connected to each DAC, so total capacitance is 23uF x 4ch = 92uF. What is the recommended capacitance of the bypass capacitor in customer's use case?

Best regards,

Shunsuke Yamamoto

+1 Paul_Frost 4 days ago in reply to Shunsuke Yamamoto

TI__Guru 56923 points

Ultimately, this depends on their routing and the supply architecture, but I would estimate that 100uF would be acceptable.

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AMC7908: Bypass capacitors