An external Vref for the p<SoC4 SAR ADC can be as low as 0.01V as the configuration window tells. Buffer gain cannot be used with the current PSoC4 devices. When converting +- values they must be shifted to match your supplied Vref as ground. Challingging will be to deliver a temperature-compensated precise (0.1%) low noise supply independent simple reference Voltage in the range of 25 to 50 mV.
Some thoughts -
1) Input CM range is speced at Vss to Vdda, I think this is an error, and should be
Vssa to Vdda, I will get this clarified.
2) The SAR linearity is speced for Vref >= 1 V, so no data on very low Vref values.
3) The input offset is 2 mV max, but no drift values. At low Vref it becomes a major error
contributor, especially over T.
Some questions -
a. What dynamic range do you want ?
b. What resolution (over T) is needed ?
c. What accuracy (over T) required ?
There is a method of doubling small signal GBW of two matched OpAmps, but requires precision
external R's. There is no characterization howevere of OpAmp matching, so error budget would
be in question.
Attached some low accuracy methods (you have to do the error budget) to level translate
ADinput negative inputs8.zip 87.5 K
There is an approach of removing errors due to T, V, offsets, linearities
during production test. Basically a link is established to DUT, a precision
V generator is commanded by DUT in a cal routine, and measured values
are curve fitted to results, using least squares or power curve fit.
It may seem complicated but actually quite straightforward. But does increase
product test time.
What is the maximum external voltage rating?
Can you be more specific, of what, the part Vdda, Vssa, Vdd, Vss, the SAR inputs....?
Most of that info in datasheets -
Sorry what is the max voltage i can put on vref? I want to 5v on it.
if i can the datasheet says 0x0000 = Vss and 0x7FF =2*Vref. So to get a range from 0v to 5v Vref has to 2.5v. Setting i want are Single and unsigned. Is this statement correct?
If you set the Vdd project rating to 5.5 V in .cydwr tab, then by setting Vdda to 5.5 V
and Vref to Vdda you would achieve the following (that would give you a range of 0 - 5.5 V) -
If you power it off USB then you will get 5 V (not 5.5), from board manual -