TMS320F28379D: Differential ADC

Hi Alice,

It isn't clear exactly where you are having difficulty.

I'd recommend that you learn about differential inputs from the following training, while noting that F2837x devices have "fully differential" ADC inputs: https://training.ti.com/ti-precision-labs-adcs-sar-adc-input-types 

The input format is also presented in the datasheet in this section:  

You then may also want to note that the common mode input requirement for this ADC is VREFHI / 2 +/- 50mV:  

Hello,

I have watched the tutorial. In my understanding, the full-range differential is from 7FFF (-5V) to 8000 (5V). If I compare my analog input with eg. 4000(HEX), I should  exactly the same outputs from the two methods, am I correct?

Thank you,

Alice

Hi Alice,

The exact range will depend on the particular VREFHI that you are supplying to the chip.  This will typically be 2.5V or 3.0V.  It won't ever be 5V (see "ADC Operating Conditions (16-Bit Differential Mode)" in the device datasheet: https://www.ti.com/lit/ds/symlink/tms320f28379d.pdf)

Applying +5V or -5V to the device pins will likely result in permanent damage.  

Have a look at the TRM sections "Expected Conversion Results" and "Interpreting Conversion Results" sections to understand how digital and analog values relate: https://www.ti.com/lit/ug/spruhm8i/spruhm8i.pdf

Hello Devin,

Thank you so much for your reply. For example, I have a 6Vpk-pk sine as my analog input. My VREFHI will be 3.0V and my effective input voltage will be VREFHI: 3V -VREFHI: -3V. My corresponding digital output will be [0:  2^16-1] . I have another question though, this VREFHI is set internally, right? So this reference cannot be changed by user.

Thank you,

Alice

Sorry I sent two duplicate pictures above.

Hi Alice,

The VREFHI is set externally by the first circuit you specified above.  The VREFHI should be a stable DC value driven by a low-noise and high bandwidth/low impedance source.  

As the 2nd input shows, you can have a 6V peak-to-peak differential signal, but in this case each half of the signal on the pin is within 0V to VREFHI and the common mode needs to be within VREFHI/2 +/- 50mV.  

Hello Devin,

In my understanding, VREFHID is connected to VREFHIB internally. By measuring the voltage across VREFHIB (U11 pin 6) and GND, I got exactly 3V. Doesn't mean the VREFHI is set internally instead?

Thank you,

Best Regards,

Alice

Hi Alice,

The VREFHI pins are inputs supplied by the output of the OPA350 buffer.  If you change the input to the OPA350 the VREFHI voltage will change. 

All 4 VREFHIs (A through D) are separate pins that are not connected internally (but A and C, B and D are connected externally). 

Hi Devin,

I am now a little bit confused on how to setup this VREFHI for my differential input. I got what you said on the shared reference system. Based on my measurements, the output of my OPA350 is 3.0V. So I don't have to provide any additional DC voltage, I get 3V output from buffers. Then this reference voltage will directly sent to ADCD? Or I have to configure some of the settings in ADC block?

Thank you,

Alice

Hi Alice,

You shouldn't have to configure anything in SW for VREFHI for this device; the decisions are made in the design of the PCB.