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INA129: Adding a CMOS (ADG1436 or ADG 1212) switch to adjust Instrument Instrment Opamp gain setting.

Byron Taradena
Prodigy 40 points
Part Number: INA129

I'm reviewing a change to a CCA were a changed was made to an INA129 Instrument Amplifier IA) Input Gain setting resistor circuit.

The change made is how the IA Gain Resistor is set by allowing the gain to be "adjustable" by using a serial of different resistors values being switch in or out using a CMOS Switches (e.g. CMOSADG1436 or ADG 1212). The IA Gain settable range is low (< 10 gain) measuring simple DC Voltages. The IA-Output is then feed to IC-Comparator as to trigger an event. This change seem very clever but I have some concerns this is a bad ideal.

Any thoughts are welcomed.

Byron

  • 0
    TI__Guru* 93105 points

    Hi Byron,

    Would it be possible for your to provide a screen-shot of a printed, or drawn, schematic for the INA129 circuit? Please include the resistor values, supply levels and input source details.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 40 points

    Thomas,

    The inputs signals inputs are of small DC Temperature Voltages not sure if they are RTD or TC type but either way they need the Gain to be usable at a IC-Comparators..

    Byron

  • 0 in reply to Byron Taradena
    TI__Guru* 93105 points

    Hello Byron,

    It is clear how the gain resistors are switched into the INA129 gain set circuit using the analog switches. This should work, but there are some things that need to be considered.

    The ON resistance of the analog switches will add in series with the 71.5 k resistor value. It appears that each switch can have an ON resistance of a couple hundred ohms, depending on supply and the voltage applied to them. If the 71.5 k ohms takes into account these resistances, then the error they introduce has been accounted for. The analog switch resistance isn't linear with applied voltage and that can add low-level distortion into the circuit.

    The switches do add a small amount of capacitance to the INA129 internal inverting op amp and that could affect the transient response of the circuit. However, the switch capacitances are very low so I expect the effect will be small.

    Regards, Thomas

    Precsion Analog Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 40 points
    Thanks for your help!