Op Amp Behavoir

[morizabal]

The above circuit represents my current configuration. The circuit is used to measure a ± 10.0 VDC signal.

R2 prevents my circuit from floating and provides a good high impedance.

R3 and R4 is a voltage divider that reduces the ±10.0 Volt signal.

Someone recommended on this forum, that I put a resistor on a Unity gain buffer. (R1)

The issue is that I am not getting full swing on "NODE 1" when the voltage is negative.

I've changed op amps

OPA2277 had the biggest difference of about 0.030 V
OP296GS had a better difference of 0.007 V

Basically at ± 10 I am getting +10.000 and -9.993 (OP297)

Can someone shed some light on the characteristics of opamps that I am somehow missing.

Thank You,

morizabal

 

[sreid]

It's probably Common Mode Rejection Ratio (CMRR). The inputs to the buffer both swing +/-10V for a +/- 10V input swing. It the CMRR is speced at 60 db (1000:1), a 20V swing would result in a 20 mv error. The CMRR is often spected for much smaller comon mode swings (+/-1 V).

 

[morizabal]

What would be a good op-amp to use?

Would an inverting configuration take care of the problem?

 

[Warpspeed]

There are three possible causes of trouble. As sreid says you could be exceeding the common mode input range, but that is unlikely. You are not swinging the input within seven volts of either supply.

Another possibility is output voltage swing. That is unlikely too. The output does not have to go within seven volts of either supply rail either.

The only other possibility is that the output load current sourced or sunk by the particular op amp is being exceeded. With a 100K load that is not likely either.

First check everything is exactly as shown in the circuit, and the resistor values are correct. If I was a gambling man, I might bet R3 was 100 ohms not 100K.

The circuit as drawn should work perfectly o/k.

 

[cbarn24050]

Hi, op amps have an offset voltage your circuit doesn't have any balancing components. DC coupled amps are prone to drift, though it shouldn't be a problem on this circuit. You could be measuring the ground point at the wrong place, any power supply current in the 0v line will give rise to a volt drop.