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ADC78H90: Analog/Digital Supplies to ADC

Josef Grech
Prodigy 30 points
Part Number: ADC78H90
Other Parts Discussed in Thread: LP2950, LP2950-N

In the typical circuit described in the datasheet, the regulated voltage is conditioned by capacitors before being fed to the ADC, and stating that these capacitors are to be located close to the ADC.

If I am using multiple ADC chips in a single circuitry, should there only be one network of capacitors conditioning the supply voltage, or should each ADC have a network of its own? If only ones capacitor network should be used, where should it be located?

Thanks.

  • 0
    TI__Mastermind 47670 points

    Hello Josef,

    Welcome to TI's e2e community.

    I assume you are talking about Figure 24 in the datasheet.  The 1uF and 0.1uF capacitors should be kept close to the regulator (LP2950)  output.  These are needed for proper operation of the LDO and do not need duplicated for each ADC.  For each ADC78H90, you will need capacitors for each of the DVDD and AVDD pins. These capacitors must be located directly next to the IC pins in order to provide proper power supply bypass.

    Regarding layout, take a look at this blog post as well.

    [FAQ] PCB Layout Guidelines and Grounding Recommendations for High-Resolution ADCs.

    Regards,

    Keith N.
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 30 points

    Hi Keith,

    Many thanks for the warm welcome and thank you for getting back to me.

    Yes, I was in fact talking about figure 24. I had previously built a circuit where I replicated the 0.1 and 1uF capacitances for each of the ADCs and was having some issues.

    So I will now only put one set of the 0.1uF and 1 uF close to the regulator, and then have the 0.1uF and the 680nF capacitors for the Dvdd and Avdd respectively, for each of the ADCs. Is that correct?

    Many thanks for the link to the PCB layout guidelines.

    Regards,

    Josef

  • 0 in reply to Josef Grech
    TI__Mastermind 47670 points
    Hi Josef,

    Yes, your understanding is correct.

    I also want to point out that the 0.1uF and 1uF capacitors near the regulator depend on the regulator requirements, and not the ADC. The LP2950-N requires a minimum of 1uF, but also has an ESR requirement as well. Using a tantalum capacitor will usually meet this requirement; ceramic capacitors can cause regulator instability. Take a look at section 9.2.1.2.1 of the LP2950-n datasheet for more details. If you used several 1uF ceramic capacitors, this could easily result in an unstable output. This usually looks like a low level oscillation and the DC value is nowhere close to the target regulated voltage.

    I hope this is helpful!

    Regards,
    Keith
  • 0 in reply to Keith Nicholas
    Prodigy 30 points
    Many thanks Keith, this was really helpful.

    Regards,
    Josef