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TMS320F28075: Questions about the ADC

Tom Lightning
Intellectual 305 points
Part Number: TMS320F28075
Other Parts Discussed in Thread: OPA320, OPA350

Hey guys,

I'm working on a project with the TMS320F28075 and i got a few questions about the inner structure of the ADCs.

The datasheet of the F28075 describes that the PTP version has 19 pinouts for the ADC:

ADCIN14 & ADCIN15

ADCINA0 - ADCINA5

ADCINB0 - ADCINB5

ADCIND0 - ADCIND4

So the F28075 has a amount of 3 Independent ADCs with a 16 channel multiplexer. But i don't unterstand why every ADC has got a 16 channel multiplexer.

For example ADC A:

-ADCIN14 & ADCIN15 are connected to every ADC module

-ADCINA0 - ADCINA5

This is a sum of 8 channel which can be multiplexed to the ADC A, right? Is my comprehension about the ADC module right or wrong?

I have to convert 16 different analog signals, which pins should i use for it?

Kind regards,

Tom

  • 0
    TI__Guru 60865 points
    Hi Tom,

    Yeah, your understanding is correct.

    These ADCs have more MUX positions than physical pins to allow compatible pin positions between the F2837xD, F2837xS, and F2807x device families. The additional MUX positions also connect to internal-only channels like VREFLO or temperature sensor.

    You can distribute your 16 signals however you want among the 3 ADCs. Pins like A0 must go to ADCA but pins for inputs IN14 and IN15 can be sampled by whichever ADC you want. The 3 ADCs can operate in parallel, but you will want to make sure they are sampling in lock-step for best performance.
  • 0 in reply to Devin Cottier
    Intellectual 305 points
    Hi Devin,

    tanks for your quick support. This has answered my question perfectly.

    Do i tie REFHI to VDDA and REFLO to VSS (without any capacitors) if i don't use external references?
  • 0 in reply to Tom Lightning
    TI__Guru 60865 points
    You can tie VREFHI to VDDA, but the accuracy and noise performance will definitely suffer.

    If you really want to push the cost lower, get a cheap 3.0V shunt reference and a single OPA320 or OPA350 to drive all the VREFHI pins.

    VREFLO should indeed be tied to VSSA, as close to the device pin as possible. The capacitor on the VREFHI pin should also be grounded as close as possible to the point where VREFLO and VSSA are tied together.