[FAQ] ADC reference in TMS320F28377DZWT

Hi Ashika,

Ashika Ashok11 said:

Can we use reference voltage from single voltage source for both 16 bit and 12 bit ADC?

Yes, why not! One super-stable reference is enough.

Regards,

Gautam

Hi Ashikia,

A Couple points to consider:

Using a single reference IC or reference voltage for all ADCs is recommended. You will definitely want to drive the reference voltage with a low-noise op-amp that can drive a large capacitive load. Ideally, you would drive each VREFHI pin with its own op-amp buffer.

The range for the VREFHI voltage is 2.4V to VDDA. This means that the VREFHI voltage cannot exceed VDDA at any time. Because of this, we typically recommend a 2.5V or 3.0V reference voltage for VREFHI, and not 3.3V; if VDDA and VREFHI are both 3.3V, and VDDA will typically have some drift that may cause it to go below VREFHI - violating the DS spec'.

It is fine to operate some ADCs in 16-bit mode and some in 12-bit mode, or even to switch back and forth on the same ADC between resolutions, but do take a look at the TRM section on synchronous/asynchronous operation and the DS electrical specifications section for the ADC that specifies the performance in each mode. In short, there will be a reduced or non-guaranteed performance if the ADCs are operated in heterogeneous modes or timings if they are operated at the same time (it's no problem if you can guarantee that the conversions don't overlap(.

Hi

Thank you for your input.

Can you let me know what is Iref current for TMS320F28377D ADC.

In datasheet it is mentioned as we need to use de-cap for Vref as 1uF for 12 bit and 22uF for 16 bit. I feel there is big difference in capacitor for 12bit and 16 bit ? Is it correct ?

We spec' the average Iref current in the DS as 130uA for 12-bit mode and 190uA for 16-bit mode. The transient current draw is a little harder to define, but it will definitely be much higher for 16-bit mode. If you want to use both modes, you should use the larger 22uF capacitor.

For 22uF capacitor at Vrefhi(16 bit), what specifications of opamps should we look for while selecting the opamp if we are using the same voltage reference REF3230 for all 4 ADCs (three 16 bit and one 12 bit)

Hi Ashika,

I actually have some characterization data for some specific reference drivers + capacitive loads that we did for one of our characterization boards.  

"Yes"  means that we were able to get datasheet performance with that reference solution.  "Preferred" means we were able to get a little bit better than DS rated performance.  "no" means that we saw significant performance degradation compared to the DS specifications.  Typically the difference between "yes" and "preferred" was sharing the reference between 2 ADCs, or having an independent reference for each ADC.

Most of these references used a series R with the VREFHI capacitor to ensure diver stability (this is detailed in the RC load section).  Note that we found that 22uF was not strictly necessary for 16-bit operation.  The circuits with single op-amps are simply voltage-follower configurations.  For the dual op-amp circuits, you can see the specific circuit in the ADS8866 datasheet.