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CCS/RM44L520: ADC reference

Susan Yang
Genius 9955 points

Part Number: RM44L520
Other Parts Discussed in Thread: RM42L432

Tool/software: Code Composer Studio

Dear team,

1 What is the purpose of the resistor added (FB3) for the ADC reference voltage? With this resistor added to our own board, the reference voltage cannot reach 3.3V.

2 With the same configuration of the ADC, why is the real sampling rate different with and without this resistor? In theory, this only affects the reference voltage. Why is the sampling rate also affected?

  • 0
    TI__Guru**** 199216 points

    Hello Susan,

    1. Applying ferrite beads correctly can be an effective and inexpensive way to reduce high frequency noise and switching transients. The combination of FB + capacitor forms a LP filter, and can reduce the high frequency power supply noise further. However the FB+capacitor can also produce unwanted resonance in the circuit, so please don't use a high Q capacitor since FB acts as an inductor too. There is a small voltage drop on the FB.

    2. This configuration should not affect the ADC sampling rate. In your test, how is the sampling rate affected: increased or decreased? and how much is the change? Can you check the ECLK to see if the VCLK is affected? 

  • 0 in reply to QJ Wang
    TI__Genius 9955 points

    It is the sch of RM42L432.

    This will cause the voltage to drop when use FB3. So it means the reference voltage is under 3.3V?

  • 0 in reply to Susan Yang
    TI__Guru**** 199216 points

    You are right. The reference is not 3.3V.