# AD627 Common Mode Range calculator page

Hello,

I'm working on a weight measurement connected device.

I use an AD627 powered at 5v in single supply (final project will be powered with 3AAA).

The strain gauge is a full bridge (1k ohm for each resistor witch give a differential voltage for about 1mV/V at full scale) and I put the middle points of each half bridge in the AD627 inputs. The result is a 2.5v common mode voltage and a differential mode around mV.

My RG resistor is a 220 ohm. Important : I put 0v to VREF.

The receiving box on the strain gauge provide a differential input voltage of 0.9mV.

In summary, we have Vref = 0 ; Vcm = 2.5v ; Vdiff = 0.0009v ; Vcc = 5v ; Rg=0.2k

In this conditions, the calculator page (Instrumentation Amplifiers : AD627 Common-Mode Range / Gain Calculator | Design Center | Analog Devices) return an Out of range most of the time for my Vdiff comprise in [0.9mV ; 5.3mV]

However, my AD627 give me an output voltage ranging from close to 0V to a close to 5v (which is coherent for a rail to rail OpAmp). My weight system is very good in term of linearity and it cover the range I was looking for.

Then, is there somebody to give me an explanation about the 3.5V limitation for each OpAmp taken by this calculator page ?

Thanks for every coming help.

Eric

Parents
• Hi Scott,

thank you for your advices. I discovered and tested the Diamond Plot Tool yesterday and saw what you write but didn't get the time to post a comment. This is a very nice tool wich inspire me a new exercice for my student. I will illustrate it with your diamond tool and Analog instrumentation amplifier.

The second part a your message is important but not a problem in my design because the strain gauge is already charged with mechanicals stuffs that cause output exceding +0.2mV before the real charge take is place.

Best Regards,

Eric