The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> Reference Buffer for below ground voltages.
https://designers-guide.org/forum/YaBB.pl?num=1655344102

Message started by saqibshah on Jun 15^th, 2022, 6:48pm

Title: Reference Buffer for below ground voltages.
Post by saqibshah on Jun 15^th, 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.

Thanks!

Title: Re: Reference Buffer for below ground voltages.
Post by Horror Vacui on Jul 26^th, 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.