[SOLVED] Strain Gauge amplifier doesn't work

[daltonvono]

Hello Everybody!

I'm tring, without success, amplify the signal of a 20KN strain gauge, 1mV/V, part number C2C from HBM Test and Measurement (datasheet attached), using the MAX4194 from Maxim. I'm no having any output variation. The output is ever in 17mV even applying a force of 2 tons using a hydraulic press.
The circuit is very simple.

Following the datasheet, the RG = 249Ohms to get a voltage gain = 500, and the Reference voltage is in GND. Vcc = 3.3V and Vbridge = 10V.

thanks in advance for any help!

DVono

 

[dick_freebird]

So what do you know about what's coming directly off the bridge?

What do you know about how the bridge voltage common mode
relates to the amplifier input common mode (and per-pin single
ended) range?

As amplifier VEE is evidently ground, maybe something just gets
bound up, and VCC is anybody's guess.

Measuring and sanity checking is a good way to fill the empty
hours before any better guesses come.

 

[FvM]

Can't work. Bridge output is around 5V, MAX4194 common mode range ends at Vcc-1.1V.

 

[KlausST]

I agree with FvM.
For a first test try to supply the bridge with 3.3V.

Still some issues:
* the bridge may not be offset corrected, thus you may expect a relatively high amplifier output offset, maybe even at the supply rails. In this case apply external offset correction
* if the AMP´s Ref is a t GND as you say, then you can´t measure safely down to zero (here an offset shift may also help) and you can measure in one direction only. You need to ensure enough headroom at AMP output to both supply rails for the full (sensor) input range.

Don´t forget about ESD protection and signal low pass filtering.

Klaus

 

[dvono]

Thank you guys! You are right. Changed the cell supply to 3.3V, now it´s working!