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Original question:

LPV821: Suitable op amp for voltage reference using REF5020

ADS1219: Suitable op amp for voltage reference using REF5020 to drive ADS1219

Vijay Kolaventy
Prodigy 50 points
Part Number: ADS1219
Other Parts Discussed in Thread: OPA320, REF5020, , OPA388, LMP91000

Hi

I am designing a gas sensor circuit. Please see circuit design below and advise.

The Amplifier support team has suggested change below and asked me to confirm with Data converter support. However, OPA320 is not available. Please suggest alternate parts that I can source easily.

Also please advise on the rest of the circuit. Mainly, I am not using any buffer for the ADC channel inputs. Is this OK? The circuit will have 4 gas sensors (Alphasense/Semeatech). Please links below for gas sensor specs.

Alphasense CO gas sensor

Semeatech CO gas sensor

Best regards

Vijay

  • 0
    TI__Guru** 109595 points

    HI Vijay,

    The REF5020 has plenty of drive capability.  Also the ADS1219 has an internally buffered reference input and is a Delta-Sigma ADC as opposed to a SAR.  So no external buffer is required for the reference.

    I'm not an expert on CO gas sensors, or even know how best to interpret the datasheet.  I will say that in the gas sensor circuit, the output of the op amp to the ADC input filter may cause some issues.  The time constant is very slow (100k and 10u = 1 second) and is very high impedance.  Any change in output from the sensor will not be recognized for a long period of time.  Also, as the impedance is high, the ADC will draw some bias current which would result in an error across the 100k resistor.  If you use these large values then I would suggest adding the buffer to the ADS1219 input as shown in the simplified schematic.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 50 points

    Thanks Bob. Are you suggesting that I should remove the 100K resistor also remove the op amp or should I reduce or increase the resistor value and change the op amp to OPA388 which has similar parameters to OPA320. I am simply trying to adapt the design below for Alphasense gas sensor with ADS1219.

  • 0 in reply to Vijay Kolaventy
    TI__Guru** 109595 points

    Hi Vijay,

    I'm not suggesting anything bur rather I'm simply stating what is already in the ADS1219 datasheet.

    The ADS1219 absolute input current can vary by 5nA as specified in the Electrical Characteristics Table of the datasheet.  If the maximum of 5nA is drawn, there can be an error of 500uV due to the high 100k resistance value.  This may or may not be an issue to you.  If it is an issue it may be possible to calibrate this out of the measurement without adding the additional buffer.

    Best regards,

    Bob B

  • 0
    Prodigy 50 points

    I am also not sure if the power supply design is ok. Can someone please into the AVDD and DVDD supply?

  • 0 in reply to Bob Benjamin
    Prodigy 50 points

    Sorry to bother. Like I said I am not an engineer. I am trying to build an opensource project that will provide air quality data by connecting these devices. I am more comfortable in software coding and design. I have been going back and forth with many design changes due to shortage of components. This is the main reason for trying to do this myself. Consultant engineers are not supporting design changes due to component shortages. Our original design was based on LMP91000 which is now not available for over a year. Any help in finalizing this circuit will be greatly appreciated.

  • 0 in reply to Vijay Kolaventy
    TI__Guru** 109595 points

    Hi Vijay,

    You are facing the same problem as every design and manufacturing company in the world, and that is supply issues.  What is available today may not be available tomorrow.  It will probably be another year or so before this settles out.  So I'm not sure what to tell you.  I too would have used the LMP91000.  For the ADS1219 you can use the same supply for both AVDD and DVDD.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 50 points

    I read that using a ferrite bead on AVDD supply is beneficial. I am not sure of the value of the ferrite bead that I should use. I was planning to use 100Ohm @ 100MHz 300mA ferrite bead. Is this fine?

  • +1 in reply to Vijay Kolaventy
    TI__Guru** 109595 points

    Hi Vijay,

    The ferrite is used to filter high-frequency noise.  We generally do not recommend using ferrites, especially on ADCs that are highly integrated due to the short duration/high current demands that can be choked by the ferrite inductance.  Instead of the ferrite we usually recommend using a small value of resistance 1 to 20 Ohms) instead which creates a low pass RC filter.  Most customer schematics I've seen use no ferrites or series resistance at all.

    In the case of the ADS1219 I doubt that the ferrite would have a negative impact.  However, if there is an issue you can replace with a resistor.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 50 points

    Thank you for the support Bob. I will rather remove the ferrite bead for now for the prototype. If there is an issue we any have to go for a final version. Just FYI, the amplifier forum suggested to use the REF5020 directly to drive the ADC, just as you suggested. Thanks again.