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Design >> Analog Design >> Reference Voltage in a differential ADC
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Message started by jockeymonto on Jan 27^th, 2014, 3:55am

Title: Reference Voltage in a differential ADC
Post by jockeymonto on Jan 27^th, 2014, 3:55am

Hi folks,

In single ended ADC designs, a reference voltage in an ADC is necessary but what about in differential designs? We have two inputs; a comparator can tell us which one is larger but we need some digital conversion logic to know how large, in digital format. If two inputs are driving DAC capacitive arrays in a differential SAR design can we work out their input in digital format without using any reference voltage? Any thoughts?

Title: Re: Reference Voltage in a differential ADC
Post by RobG on Jan 27^th, 2014, 8:32am

I've had stranger ideas than a referenceless ADC, but an ADC output is simply a ratio of an unknown input to a known reference. Thus you need a known reference.

Title: Re: Reference Voltage in a differential ADC
Post by loose-electron on Jan 27^th, 2014, 5:05pm

RobG wrote on Jan 27^th, 2014, 8:32am:

I've had stranger ideas than a referenceless ADC, but an ADC output is simply a ratio of an unknown input to a known reference. Thus you need a known reference.

This gets done all the time when the information you want is relative to the sampled input. sample 1 compared to sample 2 and so forth.

Absolute values are sometimes not needed. Digital feedback control systems are one example.

Title: Re: Reference Voltage in a differential ADC
Post by RobG on Jan 28^th, 2014, 6:34am

loose-electron wrote on Jan 27^th, 2014, 5:05pm:

RobG wrote on Jan 27^th, 2014, 8:32am:

I've had stranger ideas than a referenceless ADC, but an ADC output is simply a ratio of an unknown input to a known reference. Thus you need a known reference.

Well I believe you, but in that case you are using the other signal as the reference. I guess a more precise statement would have been a ratio between two signals.

Title: Re: Reference Voltage in a differential ADC
Post by jockeymonto on Jan 29^th, 2014, 2:05am

Thanks RobG. Well i agree that the output of any ADC is (Vin/Vref)2^N - 1 but there are situations inwhich we can use the differential input voltages to make Vref (like BGR circuit in BJT based temperature sensor) instead of allocating dedicated hardware for Vref. Any thoughts on how ratio-metric ADCs differ from conventional ADCs?

Title: Re: Reference Voltage in a differential ADC
Post by RobG on Jan 29^th, 2014, 2:05pm

jockeymonto wrote on Jan 29^th, 2014, 2:05am:

OK, you seem to be saying doing it without an explicit voltage reference. If you mean injecting a reference charge derived from BJTs (or a single BJT with different currents at different times), then yes I've seen that done in a pipeline (or was it a delta-sigma?). Might be impractical for a SAR since IIRC a practical design required two caps with a ratio on the order of 8:1, the 8x device switching in a PTAT charge and the 1x device switching in the Vbe. If you aren't talking about that then I don't know :).

By ratio-metric I assume you mean the circuit loose-electron talked about - I've never seen it but would be interested in learning more. Seems like a way to divide two signals.

Title: Re: Reference Voltage in a differential ADC
Post by loose-electron on Jan 30^th, 2014, 10:10am

in the situation I described it is generally done to a Vref, but the value of Vref is not well defined. With that Vsample1, Vsample2, etc can be digitally compared.

The Vref scalar is still there but frequently the absolute value is not that important.