Read-out circuitry for resistive biosensor

Discussion in 'The Projects Forum' started by asibio, Jan 20, 2013.

  1. asibio

    Thread Starter New Member

    Jan 20, 2013
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    Hello,

    I have a biosensor. It's resistivity changes while sensing the analyte I want.
    Resistivity of biosensor is around 10 Ohms.

    I normally use Keithley 2400 sourcemeter for experiments. I apply a constant voltage across the biosensor, then measure the resulting current using same probes.

    I need to make this amperometric measurements using microcontroller. I'm applying same potential using pwm after filtering and buffering.

    My intention is to apply a constant voltage to biosensor, have a small shut resistors connected to biosensor in series, and measure the voltage over the shunt resistor using uC's ADC.

    But since both biosensor and shunt resistor have small values, it drains a lot of current which microcontroller is not able to handle.

    The voltage that I'm applying goes directly to zero.

    What should I do to be able to measure the current in this system? I have been thinking to use instrumentations amplifiers, but I still have the same main problem, sinking huge current to ground which makes the applied potential zero.

    I would appreciate any kind of help. Please ask for any clarifications.

    Thank you,
    -asibio
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    Your problem and question are not clear. Can you post a diagram?
     
  3. asibio

    Thread Starter New Member

    Jan 20, 2013
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    I have attached the diagram.
    Thanks,
    -asibio
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    You need a power amplifier that can drive a 10Ω load.
     
  5. asibio

    Thread Starter New Member

    Jan 20, 2013
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    Can you explain more? That would be really helpful since I don't have much experience with this kind of design. Thanks.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    Let's suppose your supply voltage is 15V. If you want to feed 15V into 10Ω you would need to supply at least 1.5A. Can your buffer amplifier supply 1.5A?
     
  7. wayneh

    Expert

    Sep 9, 2010
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    The diagram suggests the "normal" voltage is 0.5V across, at a minimum, 10Ω. So 50mA max current, right?

    What sort of biosensor is this?
     
  8. DickCappels

    Moderator

    Aug 21, 2008
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    What part are your using for the buffer?

    Also, have you looked at the PWM signal to see if it is present? If you sent PWM to 50% you should be able to read 50% of Vcc with a DVM (assumptions: the PWM switches between Vcc and Vdd and that PWM frequency is greater than several tens of Hz.)

    Is the voltage going into the buffer the same as the voltage out of the PWM output (while the PWM is more than 0%)?
     
  9. asibio

    Thread Starter New Member

    Jan 20, 2013
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    0
    Biosensor is carbon nanotube (CNT) based. When I add the analyte to the solution, it binds to the CNTs and decreases its conductivity.

    I believe my approach was not correct. Instead of trying to have constant voltage and measuring current change, I just realized that would be better if I have a constant current source and measure the voltage change on the sensor.

    Since chemistry of the sensor doesn't need any voltage requirements, this would be better approach.

    Do you have any circuit suggestions for this purpose?
     
  10. wayneh

    Expert

    Sep 9, 2010
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    You could build an op-amp based, constant current supply. Just Google those last 3 words and look for simple (small, low current) examples.

    [update] Here's an example I have already drawn up. It would need adaptation for your application, but maybe it will give you an idea. The principles - use an op-amp to compare voltage across shunt to voltage reference, adjust transistor - are widely applicable.

    [​IMG]
     
    Last edited: Jan 22, 2013
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