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ADS1248: I would like to ask you to check the circuit of my customer.

Part Number: ADS1248
Other Parts Discussed in Thread: INA129

Hi,

I have got the circuits (INA129+ADS1248) of my customer and I would like to ask you to check it.

Could you tell me your e-mail to ask?

Thank you for your support.

Best Regards,

  • Takumi-san,


    You can send the schematic to me to my email if you have that. If not then send me a message through the e2e messaging icon (at the top left corner of the e2e page).

    I would note that generally you don't want to put an instrumentation amplifier in front of the ADS1248. This may give you larger gain, but you will get better noise performance from the PGA in the ADS1248.


    Joseph Wu
  • Takumi-san,


    Just so that it's clear on the forum, I sent you a direct message back to you about the circuit.

    Of the comments, I'd say that the important things that I covered are:

    1. The INA probably just adds more error to the measurement (gain error, offset error and noise). It also disrupts the ratiometric measurement.

    2. The photoMOS devices that are used for switches all have some amount of leakage. This also can cause a gain error (that might be significant over temperature). This should be considered as an error by the customer.

    3. With the ±10V input measurement a simple voltage divider still has the ADC AINN input set to ground. This is outside the PGA input range of operation for the ADS1248. Negative inputs would also be outside the range of operation also. Using the INA here to shift the input to mid supply would be good here.

    Other basic comments include: input filtering may be useful, series input impedance can cause errors if the impedance is high (>10k), watch for the unintended resistance in parallel with the reference resistor.


    Joseph Wu
  • Hi Joseph-san,
    Thank you so much for your kind support.
    Let me consider your idea.
    Best Regards,
  • Takumi-san,


    I thought I'd post a quick reply to your last message. There weren't replies to all of my points, but I thought I'd respond to the specific comments you got back from your customer.

    1. In general I would recommend the ADS124S0x instead of the ADS1248. The ADS124S0x is a newer device with better specifications.

    However, I generally wouldn't recommend using an INA in front of the ADC's PGA. There is a significant contribution of offset and gain error by using the INA which I think would be very difficult to eliminate. This would be for any INA, not just the INA129. I would also point out that I understand that the noise performance is important and that they want to get to 16 NFB. However, I don't think that the customer is considering the additional noise contribution of the INA129 in their measurement. The INA129 will add noise and degrade the NFB performance. Has the customer calculated the noise of the INA129 and what it will add to the measurement?

    3. The leakage of the photoMOS relays might impact the measurement with leakage current. This current may not be a problem for all measurements, but it might be significant if the circuit is operating at high temperatures,

    4. Any time there is large series impedance, the measurement might have some non-linearity because of the input sampling. I think they can make the measurement, however near an input of 0V there may be some error. I think this is something they should test before committing to a design.

    6. I don't know what kind of environment the device will be used in, but simple RC filtering is commonly used to help limit the noise and reduce the effect of EMI/RFI. RTDs often have differential and common-mode filtering as described in the link below.

    www.ti.com/.../sbaa201.pdf


    Joseph Wu
  • Hi Joseph-san,

    Thank you so much for your support!

    Best Regards,