Scope Sunday 39

Currently I am teaching an industry seminar on analog-to-digital converter technology. I was doing a little "pre-assignment" reading (reading text before assigning it to my students) of Chapter 5 in the Analog Devices Data Conversion Handbook. Normally, I wouldn't draw attention to an error in another author's work (I have plenty of my own errors to worry about!), but this one was particularly egregious. On page 5.15 I came across the following discussion of measuring settling time and oscilloscope overdrive:

Modern digital storage scopes (DSOs) and digital phosphor scopes (DPOs) are popular and offer an excellent solution for performing settling time measurements as well as many other waveform analysis functions... These scopes offer real-time sampling rates of several GHz and are much less sensitive to overdrive than older analog scopes or traditional sampling scopes. [pg. 5.15]

Unfortunately, this statement is backwards. Traditional sampling scopes are virtually immune to overdrive (see part three of my series "Vintage scopes are better"), but modern DSOs and DPOs can be just as sensitive to overdrive as older analog scopes.

The text continues:

From a historical perspective, older analog oscilloscopes were sensitive to overdrive and could not be used to make accurate step function settling time without adding additional circuitry. Quite a bit of work was done during the 1980s on circuits to cancel out portions of the step function using Schottky diodes, current sources, etc. [pp. 5.15-16]

This statement seems to be an indictment of Jim's careful settling-time-measurement work in application notes such as App Note 74, but the objection is erroneous. The authors qualify their statement in the first sentence of the following paragraph:

Even with modern DSOs and DPOs, overdrive should still be checked by changing the scope sensitivity by a known factor and making sure that all portions of the waveform change proportionally. Measuring the mid-scale settling time can also subject the scope to considerable overdrive if there is a large glitch. The sensitivity of the scope should be sufficient to measure the desired error band. A sensitivity of 1-mV/division allows the measurement of a 0.25-mV error band if care is taken (one major vertical division is usually divided into five smaller ones, corresponding to 0.2 mV/small division). [pg. 5.16]

The first sentence is good advice (ALWAYS verify your measurement chain!), but the last sentence is terrible advice. You cannot measure the fine settling time of a DAC by simply cranking up the vertical sensitivity. It doesn't matter if your oscilloscope is vintage analog or a modern DSO, if ANY part of the waveform is off the screen, you can't trust the results.

Here's a quick example, using the same TDS3012B (and the same four-volt square-wave oscillator) from my post on aliasing last month. The falling edge of the square wave exhibits a little undershoot. At 500 mV/div, the undershoot appears to be about 800 mV (at the bottom of the screen). Note that some of the four-volt waveform is off the top of the screen.

However, if we move the trace up to the top of the screen, now the undershoot appears to be about 1400 mV (note that the vertical scale is still 500 mV/div).

The only change between these two screenshots is a small rotation of the vertical position knob. Clearly, oscilloscope overdrive is a concern, even in a modern DSO.