App Note 22

"A monolithic IC for 100MHz RMS-DC conversion." 16 pages.

App Note 22 is a first chapter in the sad story of an underappreciated design. This app note discusses the LT1088, an IC building block for RMS-to-DC conversion. Jim advocated, designed, and was the primary evangelist of this product. In addition to this app note, Jim also co-authored the 1986 ISSCC paper that described its function and fabrication. For someone with a instrumentation background, an RMS-to-DC converter is an important functional block. If you look in future app notes, you see that he often used this part in his circuits, and he referenced this app note (and the ISSCC paper) quite a few times. It should have been a popular part (and I think that Jim was proud of it), but unfortunately the LT1088 was a poor seller and has been discontinued.

The first part of the app note discusses challenges for RMS-to-DC conversion, and explains how the feedback approach (Figures 2 and 3) is the preferred solution. The text on page AN22-4 (and Figures 5 and 6) discusses the problems and proprietary solution to the thermal conductivity of the die attach (the complexity of which may be a contributing factor to the part's discontinuance). (The box section on page AN22-15 details a method for measuring the thermal resistance of the die attachment.) Figure 7 is a die photo, showing the extreme symmetry necessary for good performance. (Also, there's an inside joke here about "Counts' Theorem" (that 9 does not equal 10) that I don't fully appreciate. Someone is going to have to explain it to me. Anyone?)

Figure 8 shows a complete application circuit for the part, and its performance is shown in Figures 11 through 16. Figure 16 shows the stereotypical response for a thermal-computation system such as this one, where the positive-going and negative-going transient responses are different (because of the differences in heating and cooling time constants). Figures 20 and 21 show protection circuitry to prevent damage to the LT1088.

As Jim says, "some applications may require buffering the LT1088's relatively low input impedance." Several buffers are shown in Figures 23, 24, and 25. Figures 24 and 25 are from App Note 21's collection of composite amplifiers.

I think the best (most interesting) circuit is Figure 27, the RF leveling loop, using a AD539 wideband multiplier, a wideband discrete amplifier (from App Note 21 Figure 6), and a LT1088 RMS-to-DC converter.

Best quote (from page AN22-1): "Applications such as wideband RMS voltmeters, RF leveling loops, wideband AGC, high crest factor measurements, SCR power monitoring, and high frequency noise measurements require the advantages of thermally based conversion." There are a large number of applications; it's too bad it wasn't a more popular part.