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ADS8688A: [ Part Suggestion ] ADS8688A, but differential

Jefferson Lee
Prodigy 40 points
Part Number: ADS8688A
Other Parts Discussed in Thread: ADS8568, ADS114S08B, ADS8686S, ADS114S08

I'm currently working on a data acquisition system and the ADS8688A looks like an incredible device. 

I'm currently trying to sample:

  1. Load Cell (10V excitation)
  2. Pressure Transducers (0-5V)
  3. Thermocouples (+- 6 mV)

The ADS8688A's multiple range selection is incredible, and the analog front end's low pass filter helps with skimping out on some RC filters, but the problem is that the GND inputs all must be within 0.1V of each other. This makes it difficult to use the load cell, where the output is differential between its leads, but the leads individually to ground may be up to the excitation voltage of 10V.

Is there a chip that that has all the multiple ranges and analog front end of ADS8688A but is differential (so that the negative ends don't all need to be equal-ish to GND and each other)? If not I think this chip will be really useful for DAQ systems where a whole lot of voltage ranges are needed, but the number of each is not known at design time.

  • 0
    TI__Mastermind 35820 points

    Hello Jefferson,

    Thanks for choosing TI ADS8688A. I totally understood your challenge on designing DAQ system for differential input signal. Unfortunately, right now we do not have an ADC to support true differential input and other features similar as ADS8688A, this demand has caught our attention. However, there is one solution based on existing ADCs which can solve your challenge by using an instrumentation amplifier or precision amplifier on the load cell channel before the ADS8688A. Please see app notes:

    Circuit for driving a high voltage SAR with an instrumentation amplifier

    Circuit for driving high-voltage SAR ADCs for high voltage, true differential signal acquisition

    These app notes were designed for ADS8568 HV ADCs, and they can be used for ADS8688A ADC family as well.

    Best regards,

    Dale

  • 0 in reply to Dale Li
    Prodigy 40 points

    Hi Dale,

    Here is a more elaborate list of what I need to meet:

    • Load cells @ excitaton voltage to 10V
    • Thermocouples in the millivolts
    • Pressure transducers 0-5V
    • Pressure transducers 4-20mA (I'm planning on just gluing on a shunt resistor to make them 0-5Vish)

    I'll start working on a multi-chip solution, but my dreamboat IC is one that has:

    • 8+ differential channels (16 pins) connected to an instrumental amp so both input legs can be 10ishV relative to AGND
    • Each instrumental amp is connected to an Analog front end with low pass filter and PGA for 1x->100x gain
    • Each Analog front end multiplexes to a 14+bit ADC that can sample @ 250ksps, with auto cycling capability
    • Integrated temperature sensor for cold junction for thermocouples

    Hopefully this turns out to be a common use case (and I'm not the only EE out there working with finicky aerospace requirements). Wouldn't mind getting a free tape down the line if you need application testers :D

    Thanks,

    -Jefferson Lee

  • 0 in reply to Jefferson Lee
    TI__Mastermind 35820 points

    Hello Jefferson,

    Thank you for your feedback. I forgot another solution based on an existing ADC: using two channels to sample one differential signal, thus you can get the benefits:

    1. Be able to capture the differential signal.
    2. Achieve 2~3dB higher SNR since the dynamic range is increased.

    ADS8686S has total 16 channels, similar structure as ADS8688A including bipolar direct input with different ranges on each input channel, integrated analog-front-end (AFE) including PGA and ADC driver, single power supply, higher sampling rate 1Msps with 8-to-1 internal multiplexer. Also, this ADS8686S has two ADC cores so that you can use one channel of core 1 and one channel of core 2 to sample your differential analog signal synchronously. 

    Please let me know if you have any questions about this solution, thanks.

    Best regards,

    Dale

  • 0 in reply to Dale Li
    Prodigy 40 points

    Hi Dale,

    I believe that with this approach we won't be able to use a PGA on the differential voltage. At max pressure and 10V excitation we are expecting a 30mV difference, so amplification to ADC's full scale range would be useful.

    My current plan is to just add 2 unitary gain instrumentation amplifiers and feed their outputs into a ADS114S08B. The ADS114S08B will then take care of amplification with its PGA and also has an internal temperature sensor for cold junction compensation.

    Does this plan seem OK? (I'm currently an undergrad and PCB design is a very new thing for me)

    Thank you,

    -Jefferson

  • 0 in reply to Jefferson Lee
    TI__Mastermind 35820 points

    Hi Jefferson,

    You are right, you have to amplify your small small signal externally to meet the input range of ADS8686S.

    The ADS114S08 is a good solution. Also, high resolution and simultaneous sampling Delta-Sigma ADCs which support differential input directly can be considered if needed please see ADS131M08 and ADS131E08.

    Best regards,

    Dale