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ADS1258: Amplification of Input Signal / AINCOM Reference

Sandro Ragusa
Prodigy 20 points

Part Number: ADS1258
Other Parts Discussed in Thread: ADS1261EVM

Hello

I am likely to use your multi-channel ADC "ADS1258" in my next design.
Now I just want to make sure that I correctly understood the functionality of this ADC.

The ADC would have a 5V analog supply and a 3.3V digital supply voltage.
Let's assume that I will have to measure 10 sensors, each with an output signal between 1.45...1.55V.
The sensors have a supply voltage of 3.0V.
I would use the single-ended Auto-Scan Mode and provide 1.50V to AINCOM (Accurate 1/2 voltage divider from sensor supply)

This way the differential voltage at MUXOUT will be -0.05...0.05V right?
With an external amplification circuit (like on page 45 of the datasheet but with Gain=20) the differential ADC input voltage would than be -1.00...1.00V.

So I would scan the 10 sensors and measure voltages with the ADC between -1.00...1.00V on each channel. Is this correct?
With the reference set to 3.0V -> 1LSB = 381.5nV this corresponds to -2’621’440...2’621’440LSB reading (two's complement already considered)

An additional question:
Do you see problems with USB operation? So I would supply the ADC with 5V bus voltage (maybe filtered with a LC-filter) and generate proper and quiet 3V with an LDO and some extra filter effort for the Sensor supply and ADC reference.

Please let me know if I misunderstood something or if you have anything to mention.
Thank you very much!

Regards,
Sandro

  • 0
    TI__Guru 56611 points

    Hi Sandro,

    Your assumptions at the top of your post are correct. I didn't check the math on the LSB size, but keep in mind that the ADC's input range is +/-1.06*VREF. This is a little different compared to most ADCs (where it is usually +/-VREF or +/-2*VREF), so make sure you account for that extra scaling factor. In general a +/-1 V signal using a +/-3V VREF results in ~1/3 of the available code range (or ~5 million codes for a 24-bit ADC, which is approximately what you have shown).

    The ADC needs a stable 5V supply to operate, so I guess it is up to you to determine if the USB voltage is stable enough. An LDO will not be possible, unless you use a boost converter first. You can take a look at some of our newer EVMs that are USB powered to see an example e.g. ADS1261EVM.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 20 points

    Hi Bryan

    Thank you very much for your prompt reply!
    This suits perfectly for my application and I will definitely look into your newer EVMs for application hints and circuit examples.

    Have a nice day
    Best Regards,
    Sandro