Ultra low power data collection - reading strain guage

I have a customer that needs to digitize the output of a strain gauge.  Their power source can vary from 1.6 to 3.6V and is capable of supplying no more than 200 uA.  Ideally, they want to use a differential input A/D and want a SPI output.  Sample rate is super low.  Maybe one sample per minute.  8 bit accuracy is acceptable.  The design is high volume 100-200K units and very cost sensitive  The Maxim MAX1391 is being considered.  As an alternative, I offered the AD7468 + AD8237.  I have a few questions:

  • Do we have a differential input, low cost A/D that can operate at 1.6V?
  • The 7468 datasheet recommend using the ADR318 as a reference/power supply for the ADR318 as a 1.8V reference.  This part is EOL.  I don't see another 1.8V reference to choose from.  Please offer suggestions of 1.6 or 1.8V references.  The customer does not want to use a higher voltage reference.
  • I assume that if the reference can source enough current, there would be no problem powering the AD8237 with it either.  Do you agree?
  • Can you propose any lower power or cheaper away to solve this problem?

Attached is my initial idea

A couple new thoughts.  Perhaps the AD8236 is a better fit for the Instrumentation amp.  It is much lower power.  Regarding the reference, I looked for something cheaper than the Maxim part and considered the following:

Both are very low power

Scan_Doc0008.pdf
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  • 0
    •  Analog Employees 
    on Feb 6, 2014 7:26 PM

    Hi Lionell,

    No worries on keeping commenting. It always help because, it can be give a better understanding of the problem.

    I found an ADC that is differential that is supplied from 2.5V to 5.5V. The AD7687 one of the candidates I found. The reference pin is external and can be connected with the power supply. The reference input voltage range from 0.5V to VDD+0.3. There is a circuit note for the AD7687 that you might want to check CN0225, http://www.analog.com/en/circuits-from-the-lab/CN0225/vc.html, Analog Front signal conditioner for industrial process control.

    From what I understand, the stimulus or analog voltage measured on  strain gauge are not dynamic. Usually for application that are not dynamic input voltage,sigma-delta ADCs are recommended.

    I would recommend, you may check Sigma Delta ADC I found which is a single chip solution for strain gauge application. To mention are the AD7793 ( has on chip reference), AD7798/99 and AD7190.

    Regards,

    Jonathan

Reply
  • 0
    •  Analog Employees 
    on Feb 6, 2014 7:26 PM

    Hi Lionell,

    No worries on keeping commenting. It always help because, it can be give a better understanding of the problem.

    I found an ADC that is differential that is supplied from 2.5V to 5.5V. The AD7687 one of the candidates I found. The reference pin is external and can be connected with the power supply. The reference input voltage range from 0.5V to VDD+0.3. There is a circuit note for the AD7687 that you might want to check CN0225, http://www.analog.com/en/circuits-from-the-lab/CN0225/vc.html, Analog Front signal conditioner for industrial process control.

    From what I understand, the stimulus or analog voltage measured on  strain gauge are not dynamic. Usually for application that are not dynamic input voltage,sigma-delta ADCs are recommended.

    I would recommend, you may check Sigma Delta ADC I found which is a single chip solution for strain gauge application. To mention are the AD7793 ( has on chip reference), AD7798/99 and AD7190.

    Regards,

    Jonathan

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