Implementing PI controller with non-ideal op amps

[hassanzia]

Hi all.

I'm trying to implement a PI controller using op amps. I want to know what considerations I should keep in mind when designing the controller in hardware regarding the non-idealities of the 741 op amp.

 

[crutschow]

Frequency response and input DC offset are likely the most important characteristics to consider.

 

[LvW]

Hi all.

I'm trying to implement a PI controller using op amps. I want to know what considerations I should keep in mind when designing the controller in hardware regarding the non-idealities of the 741 op amp.

No other opamp choice possible?

 

[FvM]

I have seen a lot of PI temperature controllers built with 741. Without knowing specific design parameters, it's hard to guess if 741 limitations matter at all.

 

[schmitt trigger]

"Without knowing specific design parameters, it's hard to guess if 741 limitations matter at all."

Exactly what I was thinking....... it all depends on the requirements.
The 741 limitations are mostly voltage offset (which can be nulled), relatively low impedance and the relatively slow slew rate.

Those limitations can be tamed, but...........there are dozens and dozens of newer, low cost and widely available opamps, that offer a substantial improvement over a device which was introduced 40+ years ago.

 

[hassanzia]

Can you give an example of a new device for PI controller?

I might try to implement it after I have successfully done it with the 741

 

[schmitt trigger]

One key aspect in an integrator is low offset voltage, as this offset will be integrated and cause a substantial error.

The LM741 has a maximum uncorrected offset of 6 mV @ 25C, and 7.5 mV over the temp range.
The OP07C has a maximum uncorrected offset of 0.15 mV @ 25C, and 0.25 mV over the temp range.

And if you don't use the offset null pins, it is pin to pin compatible with the 741. Its supply voltage requirements are also similar. So you could replace it in your existing design.

This is one example, there several dozens similar to this.

 

[FvM]

OP07 is always a good standard that should be considered, although OP07(1975) isn't much newer than 741(1968).

If the PI controller is intended to implement large time constants, the low input currents of a FET OP might be more important than low offset. Ultimately, recent devices can provide both features simultaneously.

 

[schmitt trigger]

FVM, you are absolutely correct about the OP07C, there are many more modern amps with much improved performance.

However, I chose the OP07 as an example only;
and precisely because it is old, meaning that it is still sold in a DIP8 package, it is low cost, and should be available in most of the world.

And I will repeat to the original poster my admonition: this is one example, there are many others.
Meaning: start googling and reading datasheets. Datasheet reading and spec comparison are a key aspect of any design.