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INA128 output problem

Heidi Wilkinson
Prodigy 30 points
Other Parts Discussed in Thread: INA128, INA129

Hi,

I want to use an INA128 to amplify signals from a pressure sensor in the signal range +/-50mV to an output of +-10V. I have built a circuit according to the design in the data sheet using +/-15V supply and a potentiometre to fine tune the gain to 200. The problem is this, the gain varies a lot depending on what signal i put to the amplifier, approximately between 178 and 205 or so. I have tried to use different supply sourses ans different singal sourses. What can be causing this?

Hope you can help!

  • 0
    TI__Genius 12325 points

    Heidi,

    Potentiometers aren't a great way to set the gain of an instrumentation amplifier because their resistance can change quite a bit depending on if any pressure is applied to the leads or the wiper knob. I'm unable to see the picture you've uploaded, and it would help a lot of its your schematic. Could you try uploading it again?

    If you want the gain to be exactly 200, one option is to switch to the INA129 (exactly the same but with a different gain calculation). The required gain resistance is now 247 ohms which can be achieved by placing  237 and 10 ohm resistors in series. Using resistors with tight tolerances will give a much more precise solution than a potentiometer.

    For the INA128, the gain resistor required is 251.256, and 249 and 2.26 ohm resistors in series will give almost 200, but not as close as the other solution I mentioned.  

  • 0 in reply to John Caldwell
    Prodigy 30 points

    Hi, I have tried using different combinations of fixed resistors, the problem is still there. With an input of 10-50mV the gain varies with about 10 %, (G_highest-G_lowest=20). I have built the circuit after figure 1 in the data sheet:

    http://www.ti.com/lit/ds/symlink/ina128.pdf

    What can this mean?

  • 0 in reply to Heidi Wilkinson
    TI__Genius 12325 points

    Is your input signal AC or DC? The gain should not change at all with the input signal. If this is a DC measurement have you taken into account the offset of the circuit itself? With the inputs both connected to ground, what is the output voltage of the INA128?

  • 0 in reply to John Caldwell
    Prodigy 30 points

    Hi John,

    I have now tried to connect both inputs to ground, this gives an output of -13 mV. My input signal is DC. Any good ideas?

  • 0 in reply to Heidi Wilkinson
    TI__Genius 12325 points

    Heidi,

    Are you accounting for this offset when you calculate gain? For example, with a 10mV input the output would be 10mV * 200 - 13mV = 1.987V, without accounting for the offset this would appear to be a gain of 198.7.

    More importantly, this is an unusually high offset for the INA128. Are you using dual supplies in this application (+V and -V) or a single power supply? Also is this circuit built on a PCB or a breadboard? Verify all the connections of the circuit, and if it is built on a PCB make sure the board has been properly cleaned to remove any residue from assembly. Finally, have you replaced the INA128 with another one to check if the device is damaged?