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ADS127L01: ADS127L01 Question regarding separate Analog and Digital Supplies

Christopher Ward
Prodigy 120 points
Part Number: ADS127L01
Other Parts Discussed in Thread: CC3200, CC3100, , REF6025, THS4551

Hello T.I.

We am working on a prototype that is using a CC3200 microcontroller to interface with the ADS127L01 ADC. 

The datasheet for the cc3100 recommends using the same voltage line for all peripherals. Would it be a good idea to use the same regulated 3v3 used for the CC3200 on both the AVDD and the DVDD supply inputs for the ads127L01?

Regarding the asterisk in the picture below, The input for DVDD does not have a trace between the two capacitors unlike AVDD. Is this the correct? In the ADS127L01EVM there appears to be a trace here but it is hard to tell.

 

  • 0
    TI__Guru 66226 points

    Hi Chris,

    Generally speaking, it is not recommended to use the same supply for both analog and digital circuitry in ADCs. Digital circuitry (especially clocks) tends to produce noisy transients on their supplies that can couple onto sensitive analog circuitry and diminish an ADC's overall performance (most notably signal-to-noise ratio or SNR). Sometimes, the noise from digital supplies can be suppressed through additional decoupling capacitors and ferrite beads, but the best practice is to keep the supplies completely separated. You could also consider using a high-PSRR LDO to generate a slightly lower AVDD supply (i.e. 3V) and help suppress the noise from the digital 3.3V supply.

    That said - the ADS127L01 will work just fine if AVDD and DVDD are both 3.3V. The decision to separate them or not purely depends on the performance criteria for your application.

    Regarding the layout image, it looks like the trace between the DVDD capacitors was accidentally deleted. As with all decoupling capacitors, the traces should run from source -> capacitor -> device. I'll make a note to fix this in the next datasheet revision. Good catch! :)

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 120 points

    Ryan, thank you for the advice.

    I have chosen the TPS79601 LDO to generate a 3V AVDD supply. May I ask how supplies are separated in the ADS127L01EVM?

    And another question if you have time. The ADS127L01EVM uses the REF6025 to generate a VREF of 2.5V. The ADS127L01 recommends a common-mode voltage of AVDD/2. Why is it not recommended to have a common-mode of VREF/2 or to have a VREF that is equal to AVDD?


    Thank you,
    Chris

  • 0 in reply to Christopher Ward
    TI__Guru 66226 points

    Hi Chris,

    We use two TPS714901 LDOs to generate the analog and digital supplies on the ADS127L01EVM. You can find more information in the ADS127L01EVM User Guide (sbau261). There is a short section about powering the EVM as well as a full schematic appended to the end of the document.

    The mid-supply input common-mode voltage allows each output pin of the driver amplifier to have the maximum swing between the supply rails without clipping. If instead you used a 1.25V input common-mode voltage, and you wanted to use the full-scale range of the ADC (+/-2.5V), each output pin would try to swing from 0V to 2.5V, which would violate the output voltage spec of the THS4551 since it is not a rail-to-rail output amplifier (see VOL and VOH in the THS4551 datasheet). Some amplifiers can support rail-to-rail output, but generally speaking, the further away you can keep the output voltage from the supply rails, the better performance you will get. Also, remember that the size of the reference voltage only limits the maximum differential input that the ADC can convert.

    I would not recommend inserting a resistor in series with the reference IC output. The REFP/N inputs of the ADS127L01 are not high impedance. The current drawn by the switch-capacitor input stage will produce a voltage drop across this resistor, resulting in a system gain error. Also, although your output data rate is very low, the ADC will be sampling the inputs and the reference much faster at the modulator frequency (i.e. the CLK input for this device). You should consider buffering your reference IC output (see the Reference page of the ADS127L01EVM User Guide for an example). The REF6025 actually includes a built-in buffer, requiring very few external components.

    Best Regards,