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TMS320F28379D: TMS320F28379S External REF Circuitry..

Newman
Expert 6015 points

Part Number: TMS320F28379D
Other Parts Discussed in Thread: OPA320, OPA350

Hello Team C2000,

I have a question WRT external REF (like for VREFHIA) circuitry spec'd in the datasheets and the TRMs for these devices..

The datasheet (http://www.ti.com/lit/ds/symlink/tms320f28379d.pdf ) Table 4-1 says for pins like (VREFHIA).. "ADC-A high reference. This voltage must be driven into the pin from external circuitry. 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. This capacitor should be placed as close to the device as possible between the VREFHIA and VREFLOA pins. NOTE: Do not load this pin externally." w/ no mention of a series resistor to be included.

The TRM (http://www.ti.com/lit/ug/spruhm8g/spruhm8g.pdf ) section 10.3.4 says the following.. "A capacitor between the high and low reference pins should be placed on the PCB as close to the pins as practical to help absorb high frequency currents. A series resistor (typically <1Ω ) in series with this capacitor may be necessary to ensure op-amp".  Two issues w/ this..

1. The TRM recommends a 1Ohm resistor in series w/ the cap on the REF pins while the datasheet does not.

2. The referenced quote in the TRM is an incomplete sentence.. there is nothing after "to ensure op-amp"... what is the rest of the sentence?

What is the overall recommend.. a 1Ohm resistor or not?  

Thanks, Merril

  • 0
    TI__Guru 60865 points

    Hi Merril,

    The series resistance is to ensure op-amp stability while driving a large capacitive load.

    What you should do is a phase margin simulation in SPICE using your particular op-amp driver + capacitance value. If the op-amp is not stable, you can add some series resistance (in series with the capacitor, not the op-amp output) to make the circuit stable. You don't want to add too much capacitance resistance [edited after posting] because this reduces the BW of the capacitor and thus reduces its ability to absorb high-frequency transient current drawn from the VREFHI pin.

    You can potentially reduce or eliminate the resistance value based on including in the simulation the resistance extracted from the PCB traces and/or the parasitic resistance of the capacitor.

    Some possible choices of circuits we've already tested would be OPA320 + 2.2uF + 560mOhm (see F28377S launchpad) or OPA350 + 22uF + 100mOhm (see F28379D controlCard)

  • 0 in reply to Devin Cottier
    TI__Mastermind 40540 points
    Hi Merril,

    Just checking to see if you still have questions on this topic. For your point in item #2, the rest of the sentence should read " ... to ensure op-amp output is stable"

    Regards,
    Joseph
  • 0 in reply to Joseph Casuga
    TI__Mastermind 40540 points
    Hi Merril,

    Have not heard from you on this topic, so I am assuming that your issue has been clarified, hence closing this thread. If you still have concerns on this topic, please post it in the forum.

    Thanks and regards,
    Joseph