Could you tell me the input capacitors value of AD7768?
I would like to find suitable external input buffer.
The value of the input capacitor is approximately 4 pF. It can be calculated as follows:
Conditions: Fast power mode, MCLK = 32.768MHz, MCLK_div = 4, analog input precharge buffer off
Formula: I = CVF
The analog input current has two components, differential and common mode, so total current for this example is equal to 65uA/V (from the Datasheet)
It is important to remember that the modulator switches at twice the Fmod frequency. The Fmod frequency is MCLK/MCLK_div = 8.192MHz. Switching at twice this is = 16.384MHz
I = CVF and so:
C = I / (F*V) as the current is in amps per volts the voltages cancel to give:
C = 65uA / 16.384MHz = 3.967pF
The following application note can be used as a starting point to select a suitable driver amplifier: Modular Data Acquisition (DAQ) Footprint Using the AD7768/AD7768-4
Thank you for your help !
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