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# INA114: Two simultaneous signals on the oscilloscope

Part Number: INA114

I've been having an issue where the circuit outputs two simultaneous signals on the oscilloscope, one that is the correct reading, and one that stays at 0 volts.

Here is a basic circuit schematic. The 100 ohm resister should mean the gain on the instrumentation amplifier is 501.

Here is an example oscilloscope output. The top bar, despite some noise, gives the correct output. The bottom bar stays at 0 volts.

The load cell is a strain-based wheatstone bridge, Omega Engineering LC302-25.

Why is this happening, and is it something I can fix?

• Hi Colin,

Why is this happening, and is it something I can fix?

In addition to the noisy signal at Vout, I have a feeling that you did not configure the INA114 properly.

With single supply rail, Vm=2.5Vdc, Gain=501 and Vref=GND=0V, the linear INA114's output region is from 1.3V to 1.994V according to the engineer calculator, see the triangle boundary. You may download the analog engineer calculator as shown below.

https://www.ti.com/tool/ANALOG-ENGINEER-CALC

If you have Vref configured at 2.5V, the image below is the Vout linear response region.

Per your strain gauge sensor's datasheet, the strain gauge will generate approx. 5mV differential signals from the Wheatstone bridge. With Vref = 2.5V, enclosed is a Tina simulation. The Vout's transfer function is: Vout = Gain*(Vin+ - Vin-) + Vref. As in the example below, Vout = 167.67*(-3.52613mV) + 2.5V = 1.90877V

INA114 E2E 10222021.TSC

You may still use your current setup with Vref=0. Since your Vout range is limited, you need to lower your gain from 501 V/V to approx. 350V/V range.

Regarding to the output noise, please use low ESR decoupling capacitor next to 5V supply rail. The excitation reference voltage needs to be low noise and stable reference voltage. Assumed the application's 5Vdc is generated from LDO, otherwise, you will implement LPF to get rid of regulator's noise.

Best,

Raymond

• Hi Colin,

We have not heard from you for a few days. We hope that your issues have been resolved.

I am going to close this inquiry. If you have additional questions, you may still re-open this thread or create a new one via E2E forum.

Best,

Raymond