I would like to know how the cutoff frequency for the differential amplifier for the AD7764 ADC is calculated. The passive values for the differential amplifier is given in Table 7 on AD7764 datasheet.
Kindly revert at the earliest.
Are you looking for this with regard to setting an anti-alias pole in your signal chain? or is it for noise analysis?
If it is related to the anti-alias pole then the main pole around the amplifier is created (differnetially) by the RFB and CFB values.
So our characterisation showed that using values of 3.01k Ohm and 47pF gave the best noise and distortion from the Amp and ADC combination.
This pole is in compound with the poles created by RIN and Cs, and Rm and Cm. the Rm and Cm pole is at a high frequency though so does not impact on anti-aliasing protection too much.
The corner of 3.01k and 47pF is 1.125Mhz rolling off at 20dB/decade.
The corner of the RIN/Cs pole: 4.75k and 8.2pF (half this (i.e. 4.1pF)to get the capacitance related to one side of the diff input) is ~8MHz, 8.17Mhz to be precise.
The corner of the RM/CM is set by the 43 Ohms and 33pF combination. This is a really wide bandwidth to allow the kick back from the ADC switch cap input to settle back through the amplifier and is not really related to anti-alias.
Let me know if this goes some way to answering your questions or not?
We can continue discussion around the Bode plot of this versus what will fold down as the ADC itself has some interesting functionality which helps the first frequencies which would alias down (as an inherent function).
The cutoff frequency as given in the datasheet is 19.6MHz @ 25dB.
If I calculate the cutoff frequency then it comes to around 1.12MHz @ 0dB for the main pole.
For the poles created at higher frequency the resistor value is given as 43Ohm and the frequency comes to about 56.1MHz, but due to non availability of the 43Ohm resistor we wanted to go for 50Ohm resistor. The frequency of which comes to about 48.25MHz.
I wanted to know if there will be any concerns in the stability if I am reducing the higher cutoff frequency.
Since this discussion is about the AD7764 ADC, I have moved it to the Precision ADCs community. Please continue the discussion here.
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Can you please let me know the answer for my previous question?
Apologies, I have been out of the office during our winter break.
Our characterisation of this resistor as shown in the datasheeet suggests that you stay within the 36 Ohms to 47 ohms range to enable the best THD and noise performance. Is there any availability of either a 39 Ohm or 47 Ohm resistor which you could use?
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