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5 Resistor Difference Amplifier

Peter Baxter
Intellectual 420 points

Other Parts Discussed in Thread: INA149, INA117

The INA117 (INA149 and a number of others) is a Difference Amplifier with 5 resistors- rather than the usual 4.

The 5th resistor is the one that goes from the Op-amp inverting input - to a package pin - which is often show going to that same point as the REFERENCE pin.

I'm not succeeding in finding a good description of why that 5th resistor is there.

Might someone be able to explain what that 5th resistor is about?

  • 0
    Guru 21975 points

    Peter,

    The INA117 and INA149 have very wide common-mode range. To do this they must divide the large input voltage down with resistive dividers. A conventional 4-resistor difference amplifier with this wide common-mode voltage range would attenuate the difference voltage as well as the common-mode voltage. The 5th resistor creates voltage gain of the difference voltage to restore it to a unity-gain transfer function.

    Regards, Bruce.

  • 0
    TI__Guru* 93105 points

    Hi Peter,

    Tagging on to what Bruce explained about the 5-resistor difference amplifier I did a little head scratching and came up with the ideal gain equation. Using the resistor identifications and pin voltages shown in INA117, Fig. 1:

    Vo = V2 (-R2 / R1) + V3 [ 1+ R2 / ((R1xR5) / (R1+ R5)) ] [ R4 / (R4 + R5) ]

    Vo = -V2 (380 / 380) + V3 [ 1 + 380 / ((380 x 21.1) / (380 + 21.1)) ] [ 20 / (20 + 380) ]

    Vo = -V2 (1) + V3 [ (1 + 19) (1/20) ]

    Vo = -V2 (1) + V3 (1)

    Vo = V3 - V2

    Looking into R5 and R1 inputs you find that they terminate at the summing node. Their parallel resistance is 20 k-Ohms, the same value as R4.

    Regards, Thomas

    PA - Linear Applications Engineering

     

     

  • 0
    TI__Mastermind 25865 points

    Hello Peter,

    You may be interested in the following app note:  http://www.ti.com/lit/an/sboa001/sboa001.pdf