THS4513-SP: Common mode filter capacitor

Steve Widener

Part Number: THS4513-SP

The THS4513-SP recommends 0.2uF capacitor for the CM pin.

We have a situation where an op-amp is driving the CM voltage but it cannot drive 0.2uF. A series 50 ohm buffer resistor could be used to isolate the amplifier from the capacitive load but that would require an expensive layout / rework.

Would 0.02uF of capacitance be acceptable for the common-mode bypass capacitor for the THS4513-SP? What degradation might be seen?

over 5 years ago

0 Michael Steffes over 5 years ago

Guru 48395 points

Hey Steve, how you doing,

for the FDA CM pin, the cap there sets the corner on the CM output noise - if the model is good, you can simulate the effect of changing cap values. Now that output CM noise only matter to the extent the interstage elements are imbalanced converting CM noise to DM and if the ADC CM rejection is not good.

Are they varying the CM voltage, if not at the internal default, why not just use a resistor divider - usually some variance allowed on output CM for the ADC range.

0 Michael Steffes over 5 years ago in reply to Michael Steffes

Guru 48395 points

And in fact, if you are driving the Vcm pin with an op amp, you may not need the cap at all. Usually we put it there if the default internal midscale is being used as those resistors are really large -

0 Steve Widener over 5 years ago in reply to Michael Steffes

TI__Genius 10445 points

Good to hear from you Michael!

Thank you for the guidance on this.

Steve

0 Christian Yots over 5 years ago in reply to Steve Widener

TI__Genius 14540 points

Thanks for your expert inputs on this Michael!

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THS4513-SP: Common mode filter capacitor