OP AMP Behavoir

[morizabal]

What would cause two seperate op-amps to behave different?
Problem: I have an instrumentation amplifier configured in the following manner. The first stage amplifies a differential input by G=25. eg, 0.020V * 25 = 0.5V
The next stage takes the difference of the two signals and amplifies by G = 10 eg, 0.5 * 10 = 5.0. A voltage divider is then used on this output to trim the output. The output is then amplified by G=2 to get a 10V final output. Here's the problem. When I use op-amp LT1013CN8 the circuit works fine, when I replace with OPA2277P the circuit doesn't operate as designed. The voltage is pushed close to the negative rail. The OPA2277 are new, so I think they are in working order. What is it that I am ignoring.

http://www-s.ti.com/sc/ds/opa2277.pdf

(opa2277)

http://www.linear-tech.com/pdf/10134fb.pdf

(lt1013)

Any help would be appreciated, thanks

 

[jbartos]

Suggestion. To have two stages of amplifiers with the feedback stable you probably need an additional inverter to make the feedback stable as well as the amplifier stages' output versus input stable. Normally, an odd number of amplifiers with the feedback are stable.

 

[morizabal]

How would the number of op-amps cause instability. I am using 4 opamps (3 for initial, 1 for final output) in total configured as standard 3 opamp instrumentation amplifier. The first two affect the initial differential output of my sensor, the third subtracts these outputs and multiplies by a gain. A voltage divider using digital pots trims that initial output, and a final stage multiplies by 2.

Thank You

 

[KD7CDG]

I glanced at the data sheets, and noticed that the common mode range is quite
different between the two parts.
The OPA2277 specifies (V+)-2V to (V-)+2V
The LT1013 specifies (V+)-1.5V to (V-)
If something in your circuit puts a voltage outside the range on the inputs,
no telling what exactly will happen.
If you run split supplies and the input is close to ground, either one should work
fine.
If you run single supply, and the input is close to ground, then the 1013 will
work where the 2277 will not.