Hi,

How can you use and InAmp for high or low side sensing? Here I assume one of the inputs is essentially connected to one of the rails.

Looking at the common mode vs output voltage of the INA333 but also many other INAxxx by Texas Instruments, when the differential input is small and output is near the Ref voltage you can add a little bit of bias to the input(s) as suggested here.

But suppose I want to sense 0-10A on the high side for a 0-5V output. At 10A the common mode is (Vcc + Vcc - 0.1)/2 and the output at 2.5V of 5V depending on Vref. How do you avoid violating the common mode range?

 

Something like an INA180 or INA181 has a common mode range that extends to +26V even off a 3.3V supply.
(But it doesn’t do the same on the negative side)

 

Something like an INA180 or INA181 has a common mode range that extends to +26V even off a 3.3V supply.
(But it doesn’t do the same on the negative side)

Indeed I've seen some that have CM well beyond the rails.
Does this mean to do high or low side sensing one must use an INA181 etc and not an INA333, INA350 etc? Or that an INA333 for high/low sensing must be used in conjunction with a transformer of some type?

Edit: I'm looking up the INA180, it's even called a high side / low side current sens amplifier.

 

Texas Instruments uses the INA prefix for all its instrumentation amplifiers. Not all of them can be used as current sense amplifiers!

 

Does this mean to do high or low side sensing one must use an INA181 etc and not an INA333, INA350 etc? Or that an INA333 for high/low sensing must be used in conjunction with a transformer of some type?

If the amp's common-mode range doesn't go completely to the plus supply rail then it's not suitable for direct, high-side current sensing.

A transformer does not work for DC.

 

Even instrument amplifiers are limited to provide correct operation within their published specification limits. Thus an amplifier operated outside of the published limits may not deliver the desired results. This is why reading and understanding the data sheets is vital.