Designing a differential amplifier

[ans94]

I want to design a circuit using a differential OpAmp to de-amplify the voltage. The OpAmp input voltage is 250V which has to be reduced to 3.3V. Can I use Texas Instruments INA149 opamp in differential mode (by connecting external resistors) for the same? Will there be any stability issues with it since the gain will be less than 1.

 

[KlausST]

Hi,

Why not using two simple reistors as voltage divider?

It will be hard to find an OpAmp with 250V input voltage range.

Klaus

 

[Audioguru]

The datasheet for the INA149 shows that its inputs can survive 500V(!) for 10 seconds and operate normally with a common mode voltage as high as plus or minus 275V if its supply voltage is plus and minus 15V. It has a gain of 1 and it is used to show a small signal on high differential voltages.
It is stable. You cannot reduce its gain.

 

[ans94]

Thanks Audioguru. But i am still not understanding that why cannot i reduce the gain (make it lesser than 1)? Since the gain depends on external resistors, changing the value of them such that the gain is less than 1 should work...right?

 

[FvM]

The resistor circuit also allows a configuration with a gain of 0.05, by connecting RefB to Vout. It's not specified in the datasheet, so you don't know for sure if the amplifier is compensated for a feedback factor of about 1. The moderate bandwidth of INA149 suggests it is.



The advantage of using only internal resistors is in it's excellent matching.