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
  • 0
    •  Analog Employees 
    on Feb 10, 2014 10:28 PM

    Hi Lionel,

    Do you know if the bridge will generate both positive and negative voltages? Or are they interested in the positive swing only? In any case, it may be necessary to shift the output to account for bridge offsets and/or negative voltages. I noticed this is missing from your sketch.

    Also, how much gain do they need and how large is the bridge offset? The following article may shed some light into why I'm asking

    http://www.analog.com/library/analogdialogue/archives/48-01/bridge_sensors.html

    Regards,

    Gustavo

  • Hello

    I'm pretty new in the forum I don't have much experience in fact this is my first cuestion in the forum, I was wondering if you could help me to find a sollution a similar problem that Lionelwallace has exposed time ago.

    I need to digitize the output of a strain gauge.  Their power source normally will be close to 3.6 but it should be possible use a step up for use with 5 v  the consumption is critically I mean must be very, very low consumption this circuit need the positive swing only, and the resollution must be  at least 10 bits  Ideally 12,  SPI output.  The maximum sample rate is 15ms.


    Could you please help me to find a sollution


    Thanks in advance.