TMS320F28379D: ADC and DAC

Ganeshapandi K,

Have you observed the ADC results directly to see if they are also saturated?  It would help to isolate the issue to either the ADC or DAC.

I suspect that the ADC is saturating because the input signal is exceeding VREFHI.  Try to reduce the amplitude of your input.

-Tommy

Verify me whether the Reference Voltage of ADC is 3V or 3.3V. and also for DAC 3V or 3.3V?

In control card there is no 3V reference voltage to ADC I connected 3.3V to 12th pin of Control Card

Using the above configuration, When I give 3V input supply for ADC input pin, ADCRESULT REGISTER SHOWS 4095. for 3.3V also it showed 4096 Does this mean the reference should not be 3.3V?

Ganeshapandi K said:

Verify me whether the Reference Voltage of ADC is 3V or 3.3V.

The reference voltage is supplied externally to the ADC VREFHI pin. The ADC full-scale range is determined by VREFHI.

Ganeshapandi K said:

and also for DAC 3V or 3.3V?

The reference voltage for the Buffered DAC is software configurable between VREFHI or VDAC.  The full-scale output level is determined by the reference voltage.

Ganeshapandi K said:

In control card there is no 3V reference voltage to ADC

I do not understand this remark.  Are you saying that you have removed the on-board reference circuitry?

Ganeshapandi K said:

Using the above configuration, When I give 3V input supply for ADC input pin, ADCRESULT REGISTER SHOWS 4095.

This would suggest that the effective VREFHI is at or below 3V.

Ganeshapandi K said:

for 3.3V also it showed 4096 Does this mean the reference should not be 3.3V?

I assume you mean 4095.  The valid ADC VREFHI range and conversion behavior is specified in the device datasheet and TRM:

Datasheet said:

TRM said:

Thank you dear tlea. In datasheet they have given Vrefhi(Max) = Vdda(3.3V).

As per the datasheet I set VREFHI=VDDA(3.3V). then I gave input to ADC from a waveform generator which generates exact 3V. Then I checked the ADCRESULT register It showed 4095. But as per my level of understanding I expected ADCRESULT REGISTER=4095((3-0)/(3.3-0))=3723. Am I missing anything? (waveform generator gives exact 3V).

Ganeshapandi K said:

As per the datasheet I set VREFHI=VDDA(3.3V)

Can you elaborate on how you have done this?  We need to be sure that this is taking effect as expected.

As an experiment, I recommend setting the DAC to a static output voltage while VREFHI is selected as its reference, and sampling this DAC output using the ADC channel that is assigned to the same pin.  This would help to eliminate most external sources of error.

Sorry for my mistake. I didn't mean that VREFHI is set by me but the external circuitry in the control card supplies to ADC. Here my doubt is that whether ADC works with reverence 3.3V or 3V?

Hi, based on the schematic snippet Tommy provided, it appears VREFHI on the control card is supplied by the output of a 3.0v regulator.  I would measure the voltage on the VREFHI pin to confirm, but if that is the case then the full scale input range of the ADC will be limited to 3.0V and any voltage over 3.0v will saturate with a conversion of 4095.

The VREFHI is capable of going as high as VDDA which would extend the ADC input range further but the control card is not configured that way.

Please note Tommy and many other subject matter experts are out of the office due to US holiday and wont be back until next week.

Regards,

Joe