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Original question:

ADS1278: Reference voltage

ADS1278EVM-PDK: ADS1278EVM-PDK reference circuit

Daniel Maierhafer
Prodigy 120 points
Part Number: ADS1278EVM-PDK
Other Parts Discussed in Thread: ADS1278, REF6025

Tool/software:

Dear TI,

In your ADS1278EVM-PDK Application Note, (SBAU436A, Page 9), I noticed you are using a REF5025AIDGKT reference, followed by a buffer amplifier to provide current and some lowpass filtering into the ADS1278 reference input.

In the ADS1278 datasheet, page 38/52 indicates the REF5025AIDGKT reference is connected directly to the ADS1278 reference input.  My calculations show that the REF5025AIDGKT reference can supply enough current to the ADS1278 reference in all operating conditions with a minimum Zeff = (5.2 kOhm / 8 channels) = 650 ohms.

Can you tell me why you added the buffer amplifier in the ADS1278EVM-PDG Application Note, (SBAU436A, Page 9) please?  I would like to minimize board spins if there is a reason to buffer this reference.

I noted that the TI REF5025 has poor PSRR at typical switching frequencies of a DC-DC converter.  Is that the reason?

Thank you,

-Daniel

  • 0
    TI__Mastermind 47630 points

    Hi Daniel,

    You can use the REF5025 to directly drive the reference input of the ADS1278, but the distortion will be higher than the datasheet specifications for the device.  By how much depends, to some degree, on the layout of your board.  If the REF5025 only needs to drive a single ADS1278 reference input, and it is placed as close as possible to the ADS1278 on the board, then the overall performance should still be very good.

    In general, the ADS1278 is used in multi-channel systems (>8 channels), and it is a good idea to use a buffer in this case, which is another reason we include this buffer on the EVM.

    The REF5025 can easily drive the average input current, which is equivalent to a 650Ohm load (f-CLK=27MHz), but the actual input current is due to a switched capacitor.  The external reference capacitor will supply most of the dynamic current, but not all of it.  The remaining high frequency current needs to come from the reference, in which case a fast response amplifier is needed for optimal performance.

    If you want to minimize board space and component count, the REF6025 would be another option.  This reference includes a fast response amplifier and can directly drive the ADS1278 and meet full device specifications for f-CLK<=27MHz.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 120 points

    Hi Keith,

    Thank you.  Yes, I have designed in one REF5025 / ADS1278, but I did not know the REF5025 could not supply the pulses of current quickly enough.  I am tight on board area, so the REF6025 is an interesting idea, although I see the output is rated for 4mA, which is quite close to the maximum current the ADS1278 requires (2.5V / 650 ohm).  I am reluctant to change from REF5025 to REF6025 because there is virtually no derating factor for current if I use the REF6025, so I would need a current buffer after it anyway.

    Thanks

    -DM

  • 0 in reply to Daniel Maierhafer
    TI__Mastermind 47630 points

    Hi Daniel,

    O.K.  Keep in mind the reference current scales with clock frequency, or more specifically, modulator frequency.  The equivalent 650ohm input resistance is only valid for high-speed or high-resolution mode, f-CLK=27MHz where the modulator operates at f-CLK/4.  If you use a lower clock frequency or a different speed mode resulting in a lower modulator clock frequency, the the input reference current will also proportionally reduce.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 120 points

    OK, I will take your advice and swap the REF5025 for the REF6025.  We are not running at the Max. Frequency.