Computing stuff tied to the physical world

Op-amp limits

In Hardware on Dec 7, 2012 at 00:01

Let’s look at that AD8532 dual op-amp mentioned yesterday and start with its “specs”:

Screen Shot 2012 11 24 at 22 54 50

The slew rate is relatively low for this unit. Its output voltage can only rise 5V per µs. In a way, this explains the ≈ 0.1 µs phase shift in the image which I’ll repeat again here:


As you can see, the 500 KHz sine wave takes about 200 ns to rise 1 division, i.e. 0.5V, so it’s definitely nearing the limit of this op-amp. Let’s push it a bit with 1 and 2 MHz sine waves:



Whoa! As you can see, the output cannot quite reproduce a 1 MHz input signal faithfully (there’s an odd little ripple), let alone 2 MHz in the second screen, which starts to diverge badly in both shape and amplitude. The vertical scale is 0.5V per division.

Sine waves are “pure frequencies” – in a vague manner of speaking. It’s the natural way for things to oscillate (not just electrical signals, sine waves are everywhere!). The field of Fourier analysis is based on one of the great mathematical discoveries that all repetitive signals (or motions) can be re-interpreted as the sum of sines and cosines with different amplitudes and frequencies.

You don’t have to dive into the math to benefit from this. Most modern oscilloscopes support an FFT mode, an amazing computed transformation which decomposes a repetitive signal into those sine waves. One of the simplest uses of FFT is to get a feel for how “pure” signals are, i.e. how close to a pure sine wave.

Unfortunately, I have too many FFT scope shots for one post, so tomorrow I’ll post the rest and finish this little diversion into signal analysis. It’ll allow us to compare the above three signals in a more quantitative way.

  1. Hello, Very nice subject. I know I’m OT but what about Opentherm project?. Did you give up? I’m very involved in that problem. Could you give us some good news?


    Mirko Ugolini

    • I’m still collecting data, and I’m waiting for some parts to build an alternative setup.

      But at this point it looks like the OTG works fine with the OTG-tstat loop grounded separately. It seems to work better with a firm ground than through a 1.5 MΩ resistor – i.e. I’m seeing more packets sent out by the JeeNode.

      It’s also getting colder and currently snowing here – so that too might affect the amount of data.

Comments are closed.