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TMS320F28377D: Capacitor on VREFHIA pin

Amtech
Genius 4585 points

Part Number: TMS320F28377D
Other Parts Discussed in Thread: OPA320, OPA350

Dear Sir,

As mentioned in the datasheet,"Place at least 1-uF capacitor on this pin for 12-bit mode" (PTP PIN NO. 37 --> VREFHIA)

What will happen if i place 2.2-uF instead of it? What is the advantages and disadvantages of it? Which is better?

Regards,

Maulik Timbadiya

  • 0
    Guru 192875 points

    Hi,

    Amtech said:
    What will happen if i place 2.2-uF instead of it? What is the advantages and disadvantages of it? Which is better?

    2.2uF is better than 1uF. Higher the value better it is (but you need to stop at a certain value). This cap is simply a  decoupling capacitor (or smoothing capacitor) and the main purpose of it to be present at the Vref pin is to maintain the REF stability as a slight deviation from mean can cause ADC value fluctuations.

    Regards,

    Gautam

  • 0 in reply to Gautam Iyer
    TI__Guru 60865 points
    Hi Maulik,

    The capacitor provides charge to the ADC reference, which can draw transient currents that are quite sharp and need to settle well within 1 ADCCLK cycle. A bigger capacitor is better in this respect because it will keep the reference voltage more stable when transient currents are drawn. However, you need to be careful because op-amps can have stability issues when driving large capacitive loads. Therefore you may need to limit the capacitor size and/or provide some small series resistance with the capacitor to keep the driving op-amp stable.

    If you search the C2000 forum you will find that we recommend either using OPA320 + 2.2uF capacitor + 560m resistor or OPA350 + 22uF capacitor + 100m resistor to supply the ADC reference inputs.