04 January 2012

App Note 74 part 1

"Component and measurement advances ensure 16-Bit DAC settling time: The art of timely accuracy." 48 pages.

This app note discusses the settling time of digital-to-analog converters. The measurement of settling time is a topic that Jim has discussed before (see Appendix B in App Note 47), but this app note is the most exhaustive treatment so far. This app note is also very, very dense.

There is a plethora of good advice here, included here for several reasons. First of all, the problem is hard. As Jim explains on the first page
In particular, the settling time of the DAC and its output amplifier is extraordinarily difficult to determine to 16-bit resolution... Measuring anything at any speed to 16 bits (0.0015%) is hard.
Secondly, and most importantly, he solves the problem FOUR times. Not content to simply update the settling-time test circuit from App Note 47 (compare App Note 47 Figure B2 with App Note 74 Figure 6), but he also verifies the measurement three more times. One, a bootstrapped clamp in shown in Figure 19. Two, a direct interface to a classical sampling oscilloscope is shown in Figure 26. And three, a unique differential amplifier is employed in Figure 28. These four methods are summarized on page 15.

These results are the fruits of a monumental effort. These four similar scope traces probably represent months of work, and it is an amazing testament to his laboratory skill that they all match so well.

Out of these four measurements, I think the two most interesting ones are the settling-time test circuit and the direct interface to a sampling oscilloscope. The settling-time test circuit in Figure 6 is an update to the circuit discussed in App Note 47. He has made a few improvements to the circuits and replaced a few of the amplifiers with better components. The big improvement is in the choice of the diode bridge; instead of four Schottky diodes, he uses a monolithic array of vanilla diodes with a temperature-control loop. (The temperature control is similar in concept to his temperature-stabilized transistor array in National App Note 299.)

The other interesting measurement uses his Tektronix 661 sampling scope in Figure 26. As he says on page 3, "The only oscilloscope technology that offers inherent overdrive immunity is the classical sampling 'scope." In the footnote, he comments about Appendix B and some of the references and, in particular, "Reference 15 is noteworthy; it is the most clearly written, concise explanation of classical sampling instruments the author is aware of. A 12 page jewel." This reference can still be found on the Tektronix website.

Two more great quotes are worthy of mention. On page 7, he discusses clamp diodes that protect the diode array from damage. In the footnote, he confesses,
This can and did happen. The author was unfit for human companionship upon discovering this mishap. Replacing the sampling bridge was a lengthy and highly emotionally charged task.
On page 17, he discusses the General Radio model 1422-CL precision variable air capacitor. Again, the great quote is the description in the footnote,
A thing of transcendent beauty. It is worth owning this instrument just to look at it. It is difficult to believe humanity could fashion anything so perfectly gorgeous.
There is a similar model capacitor currently listed on eBay for $3,000!

I'll cover the appendices next time.


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