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Impedance Matching

Ali Movahedi
Intellectual 840 points
Other Parts Discussed in Thread: MSP430F5438A

Dear all,

I am using an oxygen sensor that has 10K impedance.  I need to interface to an op amp and ADC of MSP430F5438A.  When I interface to an op amp to amplify the output of the O2 Sensor do I need to add 10K to the input terminal of the op amp to match the O2 sensor impedance please?

Thanks,

mmsya.

  • 0
    TI__Genius 9380 points

    mmsya,

      Traditionally, no you do not need to match impedances between the sensor and the input of an amplifier. When the manufacture tells you the output impedance of the sensor what they are telling you is that if you load the output of the sensor you can see a drop in the reported value. This can cause an error in measured concentration of oxygen.

    The opamp will have very high input impedance and should not effect the output of the sensor. The idea is that the opamp has a low impedance in its output and a high impedance in the input. The opamp will act like a buffer protecting the sensor from loading as well as add gain or filtering based on your design. 

  • 0 in reply to Gordon Varney
    Intellectual 840 points

    Dear Gordon,

    Thank you for your reply.  Sorry for not being very knowledgeable on this matter please.  So, in what case do we need to do an impedance matching please.

    Thanks, mmsya.

  • 0 in reply to Ali Movahedi
    TI__Genius 9380 points

    mmsya,

      Impedance matching is usually used in transmission lines. When a signal, typically RF, is sent over long distances, it is important to match the output impedance with the transmission line impedance and the load impedance. If they all match then you will not get a signal reflection. Impedance matching can play a part in logic signals that exceed the wire length and coupled capacitance, that will also reflect.

    Remember we are not talking about resistance. Resistance is an Ohmic value measured at DC. Impedance is an Ohmic value at a frequency. You can't measure an impedance with an Ohm meter.  

  • +1 in reply to Gordon Varney
    Intellectual 840 points

    Dear Gordon, thank you for your prompt respond and it does make sense.

  • 0 in reply to Ali Movahedi
    Intellectual 840 points

    Dear Gordon,

    One further question please.  If the O2 sensor has 10K impedance (DC Resistance) and I like to interface it to an op amp with gain of 20 to 21, would it matter what input and feedback resistor I use.  For example, can I use 10k and 200K, or preferred to use 1K and 20K.  Is there an advantage of using one over the other please?

    Thanks,

    mmsya.

  • 0 in reply to Ali Movahedi
    TI__Genius 9380 points

    mmsya,

    There is always a reason to select one value over the other. In this case you really would not see much difference. 

    The lower the resistor values the higher the bias current in the amplifier. High bias current even if only a few mA's can cause linear error. To high of resistor values and you will become sensitive to noise influences.

    The rule of thump is to try to balance the bias input currents on the positive and negative inputs unless you have a reason to do otherwise.

    The image below is the true ratio you need to get 20x. Do not use 123 Ohm resistor, the values are for setting the ratio so get as close as you feel the need.. Since you have a probe with a 10k impedance use a 12.3k resistor and a 234k resistor. In this application you are not matching impedances you are balancing the bias currents.