Voltage Divider vs. Op-Amp

[luissizzle]

Hey all,

I needed to read a 5V signal with a 3.3V MCU. I used an inverting op-amp with a VCC bias to map 0 to 5V to 3.3V to 0, respectively. The circuit works great (picture attached). Someone asked me why not just use a voltage divider, and I didn't have a good answer to that. A voltage divider with a zener voltage clamp would protect the input and accomplish the same thing. In this application, are there advantages to using the op-amp circuit that i'm not seeing?

[Benta]

Well, you're inverting the input signal, so there must be a reason for that.

[luissizzle]

Well, you're inverting the input signal, so there must be a reason for that.

that's a consequence of the op-amp circuit. The inversion is handled with the MCU

[GerryR]

A simple divider would have worked, and you wouldn't have had to handle the inversion in the processor, but your solution is fine, just more work.

[Kim Christensen]

In this application, are there advantages to using the op-amp circuit that i'm not seeing?

None that I can see. A voltage divider would do just as well.
The only advantage I can see is future flexibility. ie: In the future, if you wanted to change a 0 - 1.5V signal to 3.3V - 0V all you'd need do is swap out some resistors on your PCB and thus avoid a PCB redo/mod. (Or some other ratio where you wanted to increase the voltage swing)

[rstofer]

When you use an op amp, you are likely using high value divider resistors,  Perhaps on the order of 100k or so.  But the output impedance of the op amp will be quite low and this is sometimes required to drive the ADC.

The alternatives:  Low value resistors consuming excessive power sized to meet the ADC's input requirements or modest value resistors with a capacitor creating a low pass filter on the ADC input that limits high frequency response but provides a relatively low impedance to drive the ADC.

It's a tradeoff.  I would probably use the op amp if I had room and power was a consideration.

[ledtester]

...
A voltage divider with a zener voltage clamp would protect the input and accomplish the same thing.
...

You haven't said what other rails are available or what guarantees there are about the input signal, but it seems that if you need to use a zener voltage clamp with the voltage divider you would also need some sort of input protection with the op-amp.