13 July 2011

App Note 3

First, we've got an oscilloscope photo with eight traces and two different time scales in Figure 16. It must be a dual-beam Tektronix 556 with two 1A4 plugins. Excellent.

This app note discusses the LTC1043, a board-level switched-capacitor building block, with applications involving platinum RTD temperature sensors, capacitive humidity sensors, and LVDT position sensors. You can see Jim's history in physics and biomedical instrumentation rear its head when he starts discussing instrumentation amps (Figures 2 and 3), and the lock-in amplifier (Figure 4). Unless you have background in experimental science, you probably don't even know what a lock-in amplifier is! (Hint: it's a powerful technique for extracting useful signals from noisy environments. It is the hidden ace in many experimental laboratories. See page 10 of the Analog Devices AD630 datasheet, or read the book by Meade (Lock-In Amplifiers: Principles and Applications, 1983).)

Another early appearance of a classic Williams theme: the schematic in Figure 10 (the LVDT signal conditioner) includes a Wien bridge oscillator (misspelled here as "Wein"), with a FET resistance for amplitude control. (We'll see this circuit again and again, I assure you.) More classic hardware: the tuning procedure for Figure 7 suggests a General Radio 1432-K precision resistor decade box (page AN3-7). Good luck finding one (Hello, flea market).

Frequency-to-voltage and voltage-to-frequency converters are shown in Figure 12. More foreshadowing of future topics, here. Another clever trick in these circuits is the use of a resistor with a temperature coefficient that is opposite of the tempco of the polystyrene capacitor. A single-slope analog-to-digital converter (first of many) is shown in Figure 15.

I think the best circuit is either the lock-in amplifier in Figure 4 (previously discussed) or the the temperature-compensated crystal oscillator with varactor in Figure 21, but there's not much discussion of the latter. Too bad.

Best quote (page AN3-4): "In this [lock-in amplifier] application, the signal source is a thermistor bridge which detects extremely small temperature shifts in a biochemical microcalorimetry reaction chamber." Wait, what?


Joe Sousa said...

About a month before Jim passed away, I asked him why he usually specified film capacitors (polystyrene, polypropylene, Teflon) when dielectric absorption or other fine performance linear qualities were desired, instead of the ceramic COG/NP0 types which usually have very low (<10ppm) Dielectric Absorption, along with low temperature coefficient, and very low (<10ppm) voltage coefficient. Jim's answer for his film cap preference is that the COG/NP0 types are too easily confused with high K ceramic types, such as X5R, Y5V, X7R, which have terrible voltage coefficients (80% capacitance loss at rated voltage for Y5V), along with very high Dielectric Absorption (several percent) and and much higher temperature coefficients.

Doctor Analog said...

I certainly agree with that reasoning: as an applications engineer, he had to minimize confusion! However, some films are better than C0G/NP0 ceramics. I think that Teflon has the best behavior. Polypropylene is usually better than any ceramic, but it varies by manufacturer. There’s probably more variation between good polypropylene and bad polypropylene than there is between good polypropylene and good C0G/NP0.

There’s an EDN article by Bob Pease that basically says (paraphrased), "Do you care about capacitor performance? Then measure it yourself!" Bottom line is C0G/NP0 might be good, polypropylene might be good, Teflon is probably good, but you should always check for yourself.

And yes, I always avoid Y5V. I don’t even use it for bypassing!