AD7794 sampling capacitor

Hi,

I would like to design an antialiasing filter for the AD7794 channel and reference input. I have read in the FAQ that wrong RC combinations may starve the sampling capacitor and cause gain errors. Can you please specify the value of the sampling capacitor and internal resistance for the channel and reference input, so I can do the calculations? I assume that switching frequency is 64 kHz?

Regards,

Valentin

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  • Thanks Mary,

    The circuit you have posted is a resistance bridge configuration, while I am using a simple serial connection of the unknown and reference resistor, supplied with the same 1 mA measurement current. This enables a ratiometric resistance measurement. The input channel is buffered, while the reference channel is not. The problem is that when I place capacitors towards ground on the input channel, I get a large change in measured resistance. I have made a simulation in Multisim, which indicates that in fact the capacitors on the input channel are interfering with the sampling capacitor on the reference channel. Using the simulation I can also find suitable RC values, but obviously I need the correct values values of the AD7794 internal sampling capacitor and input resistor.

    I was not able to find these values in the datasheet. I have figured out that if the average input current is 400nA/V (stated in the datasheet) at 64 kHz, this would result in a sampling capacitor of 6.25 pF. Can you please confirm that this value is correct and provide me also the value of input resistor?

    I have seen that some of the othe ADCs by Analog devices have these two parameters stated in the datasheet, for example AD7713 has a sampling capacitance 20 pF and input impedance 30 kohm.

    Regards,

    Valentin

Reply
  • Thanks Mary,

    The circuit you have posted is a resistance bridge configuration, while I am using a simple serial connection of the unknown and reference resistor, supplied with the same 1 mA measurement current. This enables a ratiometric resistance measurement. The input channel is buffered, while the reference channel is not. The problem is that when I place capacitors towards ground on the input channel, I get a large change in measured resistance. I have made a simulation in Multisim, which indicates that in fact the capacitors on the input channel are interfering with the sampling capacitor on the reference channel. Using the simulation I can also find suitable RC values, but obviously I need the correct values values of the AD7794 internal sampling capacitor and input resistor.

    I was not able to find these values in the datasheet. I have figured out that if the average input current is 400nA/V (stated in the datasheet) at 64 kHz, this would result in a sampling capacitor of 6.25 pF. Can you please confirm that this value is correct and provide me also the value of input resistor?

    I have seen that some of the othe ADCs by Analog devices have these two parameters stated in the datasheet, for example AD7713 has a sampling capacitance 20 pF and input impedance 30 kohm.

    Regards,

    Valentin

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