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Stable power supply from USB for the ADS1247

Sjordi Van Damme
Prodigy 20 points
Other Parts Discussed in Thread: ADS1247, REF5050, OPA350, REF5045, LP38798

Hello,

As an electrical engineering student i need to develop an accurate temperature measurement board which gets its power supply from USB. Now i know the USB voltage can be anyware between 4.75V and 5.25V and it can be very noisy. Since i am using a 24 bit ADC (ADS1247) i need a very stable power supply.

After looking around on this forum a bit i found a post :(http://e2e.ti.com/support/data_converters/precision_data_converters/f/73/t/153420.aspx)
which uses a REF5050 combined with an OPA350 to generate the power supply.

From this idea i made the following circuit but i have some doubts/questions about it:

My main doubt is the supply power on the OPA350, since this is the noisy power coming from the USB i fear it will influence the output voltage and make noisy agian. Is this correct ?

Are there some design aspects i am forgetting about ? and is this even viable at all ?

I really hope to get some feedback on this, if you have any other solutions i would be happy to hear those aswell.

Regards, Sjordi

  • 0
    TI__Guru* 94005 points

    Sjordi

    This post should probably be in the Precision Amplifiers forum or the Amplifiers & Other Linear, but I'll take a stab at it anyway.

    The schematic should work ok, but I did have a couple of comments and concerns.

    First add a supply decoupling cap for the OPA350. It may help with some of the supply noise. I'd also be worried that the USB won't always be high enough in voltage. The REF5045 needs a minimum of 4.7V to operate at this voltage, and I'm not sure about the minimum USB voltage output (I believe that it might go lower than 4.7V). I'd also be worried that the ground itself is noisy coming from a USB port. The OPA350 us pretty high in bandwidth, adding some low pass filtering from the REF5045 might be a good idea.

    To be honest, I've never played with or tried this solution, so there may be other issues with this application that I hadn't considered.


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru* 94005 points

    Another possibility would be to use a low-dropout regulator to supply voltage. I know that the LP38798 typically has a low dropout voltage, low noise, and good PSRR. I'm sure there are others to consider, so you might want to do some looking.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 20 points

    Hello,

    Thank you for your quick reply. I wasn't aware that such LDO's existed. I will certainly look into that. Do you think an LDO would be enough or should more filters and such be added to provive a more stable supply ? I only really need 14 effective number of bits from the ADC. 

    Regards Sjordi

  • 0 in reply to Sjordi Van Damme
    TI__Guru* 94005 points

    Sjordi,

    I would imagine that the LDO would be enough if you are only trying to get 14 bits out of the ADC. I would pay special attention to 50/60Hz power line noise however. Often when connecting USB through a desktop computer or port, you'll get a lot of power line noise coming through.

    Joseph Wu