ADS7924: Input impedance

Hello Julian,

This is a SAR ADC, this type of architecture samples the input using a switch capacitor filter. This means that the input impedance is dynamic, and thus not specified in the datasheet.

If you are trying to design the input circuit to the ADC, this will be dependent on the nature of the input (DC vs AC) and the desired sampling rate of the device. 

We can help with this if you provide what your input source is and the aspects mentioned above. 

Regards

Cynthia 

Hi Cynthia,

Thanks for the reply. So, the inputs are not buffered. Given that we may be sampling more than one kind of source, I think it may be best to add a unity buffer stage to the input. Would you agree?

best rgds

Julian

..And I should also have said that the input will be a dc voltage. I'm looking at the TLV9004 as a possible buffer, which has an open-loop output impedance of 1200R.

Hello,

Since you mentioned that there will be different type of inputs, a buffer at the ADC pins MUXOUTto ADCIN would be a good implementation

This would help drive all inputs, as it will be common to all channels, this would allow for the use a single channel op amp. The TLV9061 would be a good choice.

I also did a quick parametric search for general purpose op amps, narrowed it by bandwidth and and price, and came up with 19 results. Which may seem like a lot, but you can continue to narrow it down given your liking.

Regards

Cynthia