INA114 voltage output range

Hey Dillon,

I see a couple of design choices that may cause some problems with your circuit: driving the low impedance node of Vref with high impedance and varying the voltage seen at Vref to create a common mode voltage violation.

First, I have posted the internal simplified schematic of the INA114 given in the datasheet.

As you can see, the Ref pin (5) leads directly to two laser trimmed 25kΩ resistors that need to match each other as closely as possible in order to deliver the gain provided in the datasheet. Adding a voltage divider and other resistors directly to this pin will change the impedance at that node and give you unexpected gain.

You can fix this by adding a buffer to the reference pin as shown in Figure 2 of the datasheet:

Second, while using the potentiometer to adjust your reference voltage is a great application when correcting for offset error that can occur from your buffer, you must consider that turning the dial one way or another can result in ±15V going into the Vref Pin if you are using the voltage regulator correctly.

Assuming your common mode voltage (or the bridge excitation voltage) on the inputs is 0V, your common mode voltage range may be severely limited if the voltage on the ref pin goes towards either supply of ±15V. This will cause an unexpected value at your output. In fact, you encountered this exact problem before you when you had 5V and 0V on your supply voltages.

One last thing to note is that you mention getting an output of 5V or higher when the bridge is “weighted.” By “weighted” I assume you mean having one of the resistors in your bridge change which creates a differential voltage.

I bring this up because the Vout of an instrumentation amplifier is given by the equation:

So be sure to measure the differential voltage when the bridge is weighted and use an appropriate gain and Vref that will cause you to get the 5V or more that you specified in your post.

If you have any questions about the factors that can limit your common mode voltage in instrumentation amplifiers, you can learn more about them from these series of articles: https://e2e.ti.com/support/amplifiers/precision_amplifiers/w/design_notes/3373.understanding-instrumentation-amplifier-vcm-vs-vout-plots

If you would like a tool that can help calculate your common mode voltage range—which is covered in more detail by the linked series of articles—you can find it here: http://www.ti.com/tool/ina-cmv-calc

Also, if you wish to learn more about the basic concepts behind these problems or get a refresher, you should check out the series of lectures provided by TI Precision labs which can be found here: http://www.ti.com/lsds/ti/amplifiers-linear/precision-amplifier-precision-labs.page

Best,

-Cole