17 October 2011

App Note 47 part 4

The final two application sections (pages AN47-51 to AN47-66) cover data-conversion circuits and some miscellaneous things. As footnote 15 on page AN47-51 says, "Seasoned readers of LTC literature, a hardened corps, may recognize this and other circuits in this publication as updated versions of previous LTC applications. The partial repetition is justified based on improved specifications and/or simplification of the original circuit." Jim undersells the application circuits here; some of the improvements (utilizing these high-speed op amps) are a big deal.

Figure 118 shows a sine-wave VCO, again using a AD639 as a triangle-wave-to-sine-wave converter (see Figure 21 in App Note 13). As I've said before, the AD639 was a Barrie Gilbert's brilliant Universal Trigonometric Function Converter, and using it as a triangle-to-sinusoid converter is like using a Lamborghini to go get your groceries.

Figure 121 is a 1Hz-to-10MHz V-to-F converter. Ten megahertz is pretty fast, but remember that App Note 14 includes the 100-MHz "King Kong" V-to-F converter. Of course, the advantage here is that with the high-speed op amps, there's no more need for the scope-trigger circuits and exotic 10H ECL parts. I especially like the caption of Figure 123 "(Whoosh!)".

Figure 124 is a high-speed (100 ns!) sample and hold, using a four-diode gate with transformer drive. This topology is an elegant solution for a high-speed S&H. Compare to the discrete 200-ns sample-and-hold circuit in Figure 23 of App Note 13.

Figure 131 is a trigger circuit with adaptive threshold, basically the adaptive subcircuit from Figure 97 (I wonder why he didn't present these two circuits in the opposite order?).

Figure 132 is a simple pulse-width-to-voltage converter, using a current source charging up a capacitor, like the single-slope converter in Figure 33 of App Note 13. Despite (or, perhaps, due to) being simple, the performance is very fast: able to resolve 1% accuracy on 250-ns pulses. The drive circuitry for Q3, including the Baker clamp and the speed-up capacitor, is especially instructive.

Figure 137 shows another application circuit for his LT1088 RMS-to-DC converter. This circuit is the same as Figure 8 in App Note 22, except the LT1223 is used instead of the discrete buffer suggested in App Note 22. Figure 139A shows a RF-leveling loop (from App Note 22 Figure 27), using the RMS-to-DC converter from Figure 137. (Figure 139B show a much simpler RF-leveling loop.)

Figure 140 shows a voltage-controlled current source (basically, it's Figure 8 from App Note 45 with much faster op amps). Figure 142 shows a higher-power version of the current source, using a discrete output stage.

Figure 144 shows a high-speed (18 ns!) circuit breaker (compare to the 12-ns version in Figure 40 of App Note 13, which required a floating load).

The best circuit in these sections is a toss up between the elegant sample-and-hold circuit in Figure 124 or the high-speed pulse measurement in Figure 132. (I still think the best circuit in the whole app note is the AM radio station in Figure 116.)

Best quote (page AN47-58): "Digital methods of achieving similar results dictate clock speeds of 1GHz, which is cumbersome." Understatement?


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