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ads8881x input RC filter(common mode and differential mode)

Robert Luo
Prodigy 50 points

Hi all,

    I just study the SLAU513B about ads8881x ADC.

   At the page of 12,it describes as below:

The selected value of capacitance for each input of the ADC is set slightly higher than the calculated value
to 4.4 nF if the filter is designed in a common-mode configuration as shown in Figure 8. However, this
design uses a differential configuration for the RC-filter, so a 2.2 nF capacitor is connected across the
differential inputs of the ADC.

My question is why the differential mode use the value of capacitor  half of that in common mode?4.4 nf -->2.2 nf?

Also how about the resistor difference between common and differential mode?

Thanks

Robert 

  • 0
    TI__Guru** 100755 points

    Robert,


    The two filters are equivalent in cutoff frequency.

    For the common mode diagram, the cutoff frequency for each input is 1/(2*pi*R*C). This becomes 1/(2*3.14*22*4.4nF).

    For the differential RC filter, the input sees double the resistance. This becomes 1/(2*3.14*(22+22)*2.2nF).

    Both results come to about 1.64MHz,


    Joseph Wu