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Replacement for OPA350 as Reference Buffer for ADC

Martin Hansel
Intellectual 390 points
Other Parts Discussed in Thread: ADS1255, OPA2333, OPA2350, OPA333, OPA350, OPA2365, OPA2727, ADS1256

Hi @all,

I'm using the ADS1255 with a radiometric interface. The Ref-Interface is like suggested: About 20uF between Ref+/-, driven by an OPA(2)350 as Unity-Gain-Amp.

So far the circuit is working well, Resolution/Noise is excellent!

But: A temperature-Scan is showing a Gain-Drift of about 1..2ppm, which is mainly due to the offset-drift of the Buffer-OP. (4uV/°C == 1.6ppm/°C @2.5V).

When changing the OP to a "zero-drift"-OP like the OPA2333, the Ref-Buffer-Circuit is instable and must be compensated. The drift now is like expected (<1ppm/°C), but I have to change my PCB (because of the compensation components).

So: Do you have a suggestion for a one-to-one replacement of the OPA2350?

 

Regards

Martin

 

 

  • TI__Mastermind 43915 points

    Hi Martin, 

    I moved your post to the amplifiers forum and renamed it accordingly to have the amplifier group add to this reply. 

    From the best of my knowledge, you are going to have problems finding a zero-drift op amp to replace the OPA350 that can drive a 20uF capacitor. Choosing an OPA333 and using a resistor in series with the line will probably not help with isolating the op amp from the capacitor and you will still have stability issues. I am not sure what other zero drift parts we have available that we can offer you as a direct replacement for the OPA350. Compensation components may be the route that you will have to go. 

    I will leave this post open in the amplifier forum to see if they have something to add.

    Regards,

    Tony Calabria 

  • Prodigy 50 points

    Hi, Martin.

    You can try OPA2365 instead. It has 1uV/C(typ) and about 5uV/C(max) Voff drift in comparison with about 15uV/C(max) for OPA350. Why you are using 20uF capacitor at the output of opa350? With output impedance of opa350 and 20uF its seems too low time constant for working with adc sampling 1-2 MHz. Try to remove capacitor or replace it with 100-400pf. Minus is that some more noise will go to the ADC ref input.

    Or OPA2727 with 3uV/C(max) but it is not R-R on input.

     

    Good luck

  • in reply to Oleg Voitovich
    Intellectual 390 points

    Hi Peter,

     

    thanks for your reply and your suggestion.

    You're right, the 365 has a slightly better drift performance, compared to the 350. But, in my application, I have to drive a HUGE Cap (>>1nF), and it seems, that the 365 needs a compensation under this condition. My ADC seems to be very happy with the suggested interface (Huge Cap with a low-noise Buffer).

  • in reply to Martin Hansel
    Prodigy 50 points

    Martin, are you sure you can achieve accuracy of several uV in your schematic? Have you any practical results? (really interesting for whome and for what is the 20-100 uF capacitors at the output of ADC driver in datasheets schematics are given) As i remember i have some troubles with 20Mhz opamp, 67Ohm (in loop) + 1nF and ADS1256 (4Mhz sampling rate on Ref input) - some different results due to different ADC filter settings (input stage have no time to restore to its default value). Ref Input stage as i think must have a few times more bandwidth than 4Mhz. For primer - 50Ohm + 200nF was enough in my application, but im not sure its gives uV accuracy... And you must also take in mind different EMF effects on PCB (junction copper, solder, mc-pin), passive components (R & C) stability over temperature, humidity, vibration and time and so on...............

     

    Best regards