Stripboard Oscilloscope MkII

In the previous project we built a super cheap oscilloscope on stripboard (aka Veroboard) using a Raspberry Pi Pico, an LM358 op-amp and some passive components. In this short article we will describe how to improve on the original design by a addition of a couple of ceramic capacitors.

While the performance of the Stripboard Oscilloscope is quite good when measuring sine waves (considering the ridiculously low price of this 'scope), those of you who built the oscilloscope may have noticed significant overshoot when viewing square waves - as shown below.

This overshoot is a sure sign of op-amp instability. We won't go into the theory of op-amp stability in this article but if you want the gory details see this excellent TI article:

Op amp stability and input capacitance

Fortunately for us this is easy to fix with the addition of a couple of capacitors across the R2 and R3 resistors in the feedback network of the op-amp.

To choose the values of the capacitors we use an empirical approach ie. try random values until we find the ones that seem to work :)

It turns out that 5pF across R2 (470k) and 15pf across R3 (100k) work nicely. Here's the result:

Done. In this article we've shown how to improve on the Stripboard Oscilloscope by simply adding two capacitors in the feedback path of the LM358 op-amp.