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Mar 26th, 2023, 11:57pm
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Reference Buffer for below ground voltages. (Read 196 times)
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Reference Buffer for below ground voltages.
Jun 15th, 2022, 6:48pm
I am currently working on the design of a reference buffer for a Sigma Delta.
I came across the TI ADS1248(datasheet). It seems that the chip can accept negative voltage references below ground (AVSS-100mV), as well as postive voltage references above AVDD (AVDD+100mV).
Doing this with a normal unity gain OPMAP would seem impossible (cannot drive the outputs completely to either rail). Does anyone have some sort of idea, of how such a thing can be accomplished? Seems a swtiched cap front might be able to do it, albeit at the cost of an input current.

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Horror Vacui
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Posts: 125
Dresden, Germany
Re: Reference Buffer for below ground voltages.
Reply #1 - Jul 26th, 2022, 4:52am
#1 you can create another VDD/VSS supply domains and run the opamp from them. Though beware of potentially forward biased parasitic pn junctions in the chip (if you are designing an IC)
#2 if you do not need to be in continuous mode: add a series capacitor with a predefined voltage to generate a level shift. The real cap will be leaky, so you have to recharge it, or periodically swap it with a recharged one. --> here comes the discontinuity. Note that due to the switching you might have other problems.
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