Actions
Forum Thread Details
  • Replies 19 replies
  • Subscribers 299 subscribers
  • Views 3498 views
  • Users 0 members are here
Related
See a helpful answer?

Be sure to click 'more' and select 'suggest as answer'!

If you're the thread creator, be sure to click 'more' then 'Verify as Answer'!

amplify small dc signal sitting on large dc offset

davebullockmbe

Weekend brain teaser?

Hi experts,

I have a sensor that gives a varying dc output with sensed input.

My problem is that the sensor's output has a 'static' dc offset of around 1V d.c. but I need to use the sensor at the low end of it's range meaning that it's output will only swing a couple of millivolts d.c.

The couple of millivolts will then need to be amplified by something like x1000 to fully exercise an arduino ADC.

I am not sure how stable the 'static' dc offset is with time/temperature or how noisy the output is yet as I am just starting to tackle this problem.

Initially I am thinking differential amplifier and cancel the unwanted offset with a separate 1V input, but I imagine device dc drift will come into play?

Clearly it would be ideal if the dc offset could be tracked by the cancelling voltage but the 'wanted' dc signal will be cancelled too as they are both pretty static.

Before I spend time inventing the wheel or disappearing down rabbit holes.......

Has anyone solved any similar problems?

Discovered any suitable low drift devices etc.

Thanks in anticipation......

Dave

Parents

  • By far the best way to characterise the device is with an optical chopper as suggested by Shabaz.

    The data sheet is very light on useful detail (which usually means that the useful details of the performance are not controlled very well !) but it looks as if the response time is < 1ms. This is the response to an enable signal  but I would expect the optical response time to be at least as fast.

    An optical chopper can easily be made, small motor with a disc with holes in it. 600 rpm with 20 holes would give you 200Hz which is a nice frequency to work with.

    You can AC couple the signal and filter it with simple analogue band pass filters (followed up by any amount of fancy digital filtering in the processor.

    If you use a tiny stepper motor (might need to go for less than 600rpm)  and drive it with pulses from a micro-controller and use it's built in ADC then everything is nicely synchronised. The bits will cost < £20.

    If it's pure research and money is no object then you can just buy an optical chopper and a nice lock in amp from Stanford Research but it will cost a fortune.

    You may have a problem untangling drift and noise from the offset voltage, the chopper technique allows you to eliminate the drift of the offset and by making the bandwidth of the amplifier very small to get rid of sensor noise as well.

    Of course it's one thing using a mechanical chopper in the lab - quite different in the field or mass production.

    MK

  • in reply to anniel747

    Hi everyone,

    Thank you for the useful sharing of ideas .....true engineering spirit.
    Yes the same thought occurred to me about whether the output 'offset' would disappear when the enable pin is taken into 'Standby mode'.
    This is the first thing I will test now I'm back in my workshop.
    The mechanical chopper idea also looks possible and a variation on the earlier idea of employing a s/h to monitor the offset under momentary 'dark' conditions and keep this offset voltage tracked and subtracted, whilst reading the wanted dc in between.
    The 'Pin 10' idea could be the answer though if it really 'tracks' the unwanted offset voltage, as suggested a 'scaling' amplifier could be used to match this reference to the unwanted offset leaving the UV response unaffected.
    I will do some tests and report back.
    Thanks everyone :-)

    Dave

Reply
  • in reply to anniel747

    Hi everyone,

    Thank you for the useful sharing of ideas .....true engineering spirit.
    Yes the same thought occurred to me about whether the output 'offset' would disappear when the enable pin is taken into 'Standby mode'.
    This is the first thing I will test now I'm back in my workshop.
    The mechanical chopper idea also looks possible and a variation on the earlier idea of employing a s/h to monitor the offset under momentary 'dark' conditions and keep this offset voltage tracked and subtracted, whilst reading the wanted dc in between.
    The 'Pin 10' idea could be the answer though if it really 'tracks' the unwanted offset voltage, as suggested a 'scaling' amplifier could be used to match this reference to the unwanted offset leaving the UV response unaffected.
    I will do some tests and report back.
    Thanks everyone :-)

    Dave

Children
No Data