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Can LM4140 "directly" drive ADS1255 reference pins?

Sridhar Jonnalagadda
Prodigy 240 points
Other Parts Discussed in Thread: ADS1255, LM4140, REF5025, OPA350

Dear TI team,

We need your help on the following. In our design, are planning to use LM4140 to drive the reference pins for ADS1255 and wanted some help from TI to identify the right approach to do so.

We are wondering - (a) if we could use LM4140 to directly drive the reference

OR

(b) should we consider buffering it (using OPA350) on lines of what is described in the TI EVM schematics and datasheet? TI EVM uses REF5025

On (a), if we use LM4140 to directly driving the ADS1255 reference pins, the output load current on the regulator should be around 135uA (input impedance @ 30KSPS is 18.5KOhm) which LM4140 should be able to source without losing regulation. I have compared REF5025 datasheet to LM4140 and see that REF5025 also "seems" capable to drive the reference pins of ADS1255 directly. But the example shown specifically uses an op-amp (OPA350) to buffer the same and does not connect REF5025 directly to the reference pins.

Could you please explain if an op-amp is required to drive the reference pins or is it safe to drive the ADS1255 reference pins directly with an LM4140? A quick response will be greatly appreciated.

With Best Regards,

Sridhar

  • 0
    TI__Mastermind 44550 points

    Hello Sridhar,

    I looked at this in the lab with the EVM and found that you can drive the ADS1255 reference input directly from the LM4140.

    However, note that noise will limit the ADC's performance. Using the LM4140*, with a 1uF cap as shown in the datasheet, I get ~15 bits of ENOB with a 2.5V DC input @ 30kSPS. *I would consider this a worst case number because I did not have a LM4140-2.5V reference, but instead stacked two LM4140-1.25V on a breadboard to generate the 2.5V reference. This will have increased noise over a single LM4140-2.5V reference with good a PCB layout.

    Next I added a 10 Ohm + 1 uF RC filter to my stacked LM4140 reference output and got the ENOB up to ~17.3 bits ENOB under the same test conditions. Therefore, I highly recommend additional filtering on the reference output. 

    For comparison, the REF5025 + OPA350 circuit on the ADS1256EVM had an ENOB of ~18 bits (with a 2.5V DC input @ 30kSPS).

    Regards,
    Chris

     

  • 0 in reply to Christopher Hall
    Prodigy 240 points

    Hi Chris,

    Thanks for your prompt response. I will do away with buffering the LM4140 output.

    With regard to filtering, i am considering implementing the filter shown on page number 28 of the ADS1255 datasheet (49.9 ohm series resistors on reference lines, with 47uF, 0.1uF, 100pF capacitors in parallel b/w VREFP and VREFN. Do you have any inputs on this?

    With Best Regards,

    Sridhar

  • 0 in reply to Sridhar Jonnalagadda
    TI__Mastermind 44550 points

    Hi Sridhar,

    I think that configuration will work. Some things to look for while building the circuit:

    • Probe the voltage reference output and make sure it is stable and not oscillating. Typically the (1uF) capacitor directly on the reference output is required to make it stable.
    • Probe the voltage on the VREFP pin while the ADC is converting and make sure it is accurate. If the series resistor is too big or the shunt conductor too small you may see that this voltage is not stable and accurate. However, I don't foresee this issue with the values you've picked.

    Regards,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 240 points

    Hi Chris,

    Thanks for your inputs. I was wondering if i could request for a review of our design schematic once it is complete - esp with regard to reference and input filtering. Is this doable? Please advice.

    Best Regards,

    Sridhar

  • 0 in reply to Sridhar Jonnalagadda
    TI__Mastermind 44550 points

    Hi Sridhar,

    I would gladly review your schematic and provide you with my feedback.

    Regards,
    Chris