INA125 design to use

Hi Lucian,

10V across a 400R load cell means a current of 25mA. That's more than pin 4 of INA125 can deliver (<5mA). As a remedy you could use the method shown in figure 4 of datasheet.

Kai

Hi,
There is a mistake. I mean is about mV not mA.

Hi Lucian,

Kai is not referring to your measured output, but to the supply of your load cell. The way you have it configured you are exciting your bridge with 10V. Since your bridge resistance is 400 Ohms, this is going to require 25mA from the INA regulator, which it is not capable of supplying. You can use the transistor trick as Kai has mentioned or add resistance at the top and bottom of your bridge to limit the current, though this will result in a lower voltage across the bridge resulting in less resolution.

With a 10 Ohm gain setting resistor you are operating in a gain of 6004. This is a very large gain and is going to amplify noise in your system as well as offsets. With a maximum offset voltage of +/-250uV, you could expect up to +/- 1.501V and this would still be considered within guaranteed operation. I suspect what you are actually seeing is the output saturated at the negative rail, as it can only swing within 150mV (typ) of the negative supply.

You are also using the device in single supply without a reference voltage, and this limits you to positive differential input signals.

The shield wiring should not be critical in this application since your bridge is very low impedance, but generally speaking it is best to drive the shield to the same potential as your common-mode voltage to minimize the effects of currents due to parasitic capacitance between the shield and the signal line. For this application you should be fine to just ground the shield.