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TMS320F2837xS Delfino ADC VREF

MARK PESACH
Intellectual 850 points
Other Parts Discussed in Thread: OPA320, OPA350

hi 

I am designing an schematic for the TMS320F2837xS and i have some questions about placement of capacitor on the Vrefhi pin.

in the datasheet it says: "Place at least a 1-μF capacitor on this pin for the 12-bit mode, or at least a 22-μF capacitor for the 16-bit mode"

i looked at the LAUNCHXL-F28377S  schematic and they use  2.2uf capacitor in series with resistor (the launch pad is designed to use the 12 bit ADC mode).

i will  be very grateful is some one can help me with the following questions:

- what is the resistor roll and how to choose its value?  i never saw this kind of topology.

- if i place a big capacitor >22-uF on the ref pin , is there any disadvantages other the size? 

best regards

Mark

  • 0
    TI__Guru 60865 points
    Hi Mark,

    The series resistance is actually there to ensure the driving op-amp is stable. The larger the capacitance, the more difficult it is to find an op-amp that can safely drive the load without becoming unstable.

    We had the op-amp groups design us a circuit to drive both 12-bit and 16-bit mode. For 12-bit, they used OPA320 + 560m + 2.2uF and for 16-bit they used OPA350 + 100m + 22uF. There is a SPICE simulation which can be run to measure the op-amp stability with a given capacitive load.

    Based on our characterization, you should be fine using OPA320 or OPA350 with a 560m and 2.2uF load, or OPA350 with a 100m and 22uF load. This actually applies to both 12-bit and 16-bit mode; we have found the 560m + 2.2uF circuit is fine in this circuit for 16-bit mode (but I wouldn't recommend using less than the 22uF DS recommendation with anything other than this specific circuit, which we have characterized). We have also found that you can meet DS performance if you share two VREFHI channels for one op-amp driver.

    If you want to try using a different op-amp as the driver, I would recommend heading over to the op-amp e2e forums and asking for some assistance in picking a part and simulating to ensure stability.
  • 0 in reply to Devin Cottier
    Intellectual 850 points
    hi Devin

    thank you very much for your assistance , and for the very detailed answers!

    Mark