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The function of a cap in a filter for ADC testing circuit

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youngwx

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Hi all,
Recently I am working on a SAR ADC testing PCB board. When I refer to a 18-bit 250kS/s SAR ADC (LTC2376-18) testing board, it gives the following circuit. The LPF2 is composed of two RC LPF(20 Ω+6800pF) and a 3300pF capacitor. I calculate the cut-off frequency of the filter is
f=1/[2*pi*2*20Ω*(3300pF+6800pF/2)]=594K Hz. The question I want to ask is what is the function of the 3300pF Cap between the IN+ and IN-? Without it the LPF2 can also work correctly.Thanks. : )

pre-filter.jpg
 

I don`t know if the system without the 3.3nF capacitor works "correctly" or not.
But it is clear that this capacitor adds to the other filter capacitors and, thus, determines the cut-off frequency.
The formula reveals that this parallel capacitor Cp adds with a factor of two. This can be verified easily because both parts of the system are (a) symmetrical and (b) in anti-phase to each other.
Thus the voltage across the capacitor Cp is doubled - which means, if referenced to each part of the system that the value of this capacitor enters the cut-off frequency with a value of 2*Cp.

This can be confirmed by a simple re-arrangement of the formula:
f=1/[2*pi*2*20Ω*(3300pF+6800pF/2)]=1/[2*pi*20Ω*(2*3300pF+6800pF)].
 
Related to differential mode filter characteristic, the 3.3 nF capacitor could be replaced by doubled common mode capacitors. The common mode capacitors are essential, because they also absorb common mode switch charge sourced by the ADC input which would otherwise overload the buffer output.

A practical reason why a separate differential mode capacitor is used can be capacitor ESL and respective self resonance frequency of the input filter. The latter will be shifted upwards by the additional capacitor. It may be also the case that the buffer doesn't well tolerate the larger common mode capacitive load.
 
Thanks for your reply. Can I just replace Cp by adding two capacitors with the same value of Cp on IN+ and IN-? After replacing Cp, does the circuit work the same as the previous one?

- - - Updated - - -

FvM said:
Related to differential mode filter characteristic, the 3.3 nF capacitor could be replaced by doubled common mode capacitors. The common mode capacitors are essential, because they also absorb common mode switch charge sourced by the ADC input which would otherwise overload the buffer output.

A practical reason why a separate differential mode capacitor is used can be capacitor ESL and respective self resonance frequency of the input filter. The latter will be shifted upwards by the additional capacitor. It may be also the case that the buffer doesn't well tolerate the larger common mode capacitive load.
Click to expand...

Thanks very much! I think I learned a lot from it. : )
 

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