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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

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  • The datasheet suggests that the offset is 1V to within +-50mV, so it might just be simplest to remove a fixed 1V offset and then only have a gain of 20, i.e. 2 mV +- 50 mV would fit in a 2V range for a high-res ADC. which could be (say) 24-bit since you mention the wanted signal is very static too. This might all be simpler than trying to sample-and-hold, although if there are periods where it's known that there is no input, (could even be a shutter, perhaps motor spun) then the sampling could be done frequently.

    If up to 16-bit is sufficient then ADS1115 or MCP3426 etc is very low-cost and has a built-in amplifier.

    Otherwise, I don't know how slowly-changing the signal is; is a filter ruled out?

    One method is a bit like a data slicer, i.e. using just an op-amp with a RC filter on one input, and the op-amp set to a gain. 

  • in reply to shabaz

    Yes well I am just gathering ideas how to tackle the problem and the only tests I have carried out is to determine that very low levels of UVA can be detected/measured with the ML8511. 
    The main issue is that even attempting to test the linearity at low UV levels was hampered by the 1V offset overloading the measuring equipment.
    The choice of A/D is already decided as the code is written and operational using an arduino microprocessor and so I need to try to interface the ML8511 to this platform.
    It may be that just using a low noise differential amplifier will be OK but I just thought I would 'throw it open' to the community in case someone had encountered a similar problem and come up with a novel solution :-)

    Dave

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  • in reply to shabaz

    Yes well I am just gathering ideas how to tackle the problem and the only tests I have carried out is to determine that very low levels of UVA can be detected/measured with the ML8511. 
    The main issue is that even attempting to test the linearity at low UV levels was hampered by the 1V offset overloading the measuring equipment.
    The choice of A/D is already decided as the code is written and operational using an arduino microprocessor and so I need to try to interface the ML8511 to this platform.
    It may be that just using a low noise differential amplifier will be OK but I just thought I would 'throw it open' to the community in case someone had encountered a similar problem and come up with a novel solution :-)

    Dave

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