Hi,
I'm planning to use ADS7828 for an application where I remotely sense the supply pin of a DUT to ascertain the supply levels at that point. I'm curious about the input impedance specification during acquisition and conversion phases of this part. I don't find an explicit specification for the input impedance for the ADC channels, but based on the 1uA input leakage current specification noted and based on the understanding that this holds even for the max allowed input level of 2.5V, back calculating from these can I assume that the input impedance will be at the very least 2.5 Megaohms worst case? Conversion rate planned is 8kHz.
Also, does this mean even during the acquisition or sampling phase where the ADC input is connected to the internal sample and hold capacitor (I assume this ADC is not internally buffered) the impedance will not dip below 2.5 Megaohms? Or will there be a non-negligible dip in the instantaneous looking in impedance during this phase? If yes, what will be the minimum expected instantaneous impedance during the sampling phase? Or more importantly can I assume that the instantaneous current spike drawn by the ~25pF sample and hold capacitor will still be <=1uA or noticeably higher, and if higher how high? I'm assuming during the conversion phase it will be at least 2.5 Megaohms or more anyway.
Since this is mostly for a supply sensing application (albeit a remote sensing case and not directly from the supply output, hence with a slightly higher output impedance, but still must be <1 ohms), the driving source output impedance should be pretty low and hence I was hoping to avoid any additional buffering external to the ADC to simplify my design, but still it would be great if the details requested above are shared.
Thanks,
Anoop
