21 September 2011

App Note 43 part 1

"Bridge circuits: Marrying gain and balance." 48 pages.

At 48 pages, this is the longest app note so far (however, the epic App Note 47 is just around the corner). This app note has three major sections: instrumentation bridge circuits, Wien-bridge oscillators, and the (five) appendices. This app note is important for two reasons: one, it is a succinct and useful collection of signal conditioning circuits for bridge circuits, and two, it includes an in-depth discussion of low-distortion sine-wave oscillators, inspired by his obsession with the HP200A oscillator.

I'll cover each section in turn.

The first section (just about half of the app note) discusses instrumentation bridges, including pressure transducers, strain gauges, and temperature sensors (mostly using RTDs). The table in Figure 4 is a good summary of various interface circuits for bridge circuits and sensors. The schematics in the following figures (Figures 5, 6, 7, 8, and on), show specific implementations of the circuits in Figure 4. The various common-mode suppression tricks are very useful. The single-supply circuits in Figures 9 and 10 use the LTC1044 and LT1054 voltage inverters, which have been previously discussed (and Figure 9 is copied from App Note 11 Figure 7).

Figure 11 shows a high-precision scale that is sensitive enough to measure weight fluctuations due to a person's heart beat. See the "ballistocardiograph" in Figure 12. I've found this example makes a great undergraduate laboratory project, because it's really exciting to see it working, and students love playing with it.

Several of the following circuits have been discussed in previous app notes. Figure 13 shows an instrumentation amplifier with a 200V common-mode range using HP HSSR-8200 opto-switches (App Note 6 Figure 2 has a 300V common-mode range, but perhaps those opto-switches are harder to get).

Linearized platinum RTD bridge circuits are shown starting with Figure 14. The switched-capacitor signal conditioner in Figure 15 is copied from App Note 3 Figure 6. Figure 16 uses digital linearization, which is the reason for the three-and-a-half pages of 68HC05 code (from page AN43-14 to AN43-17).

Power reduction is explored starting with Figure 19, which uses low-voltage drive. Figure 20 uses strobed bridge power (and is copied from App Note 23 Figure 5). Figure 21 uses a sampled output, and Figure 23 adds a sample-and-hold circuit for continuous output voltage (Figures 23 and 24 are very similar to App Note 23, Figures 3 and 4.) Figure 25 claims higher resolution with 50-volt drive.

The lock-in amplifier in Figures 27 and 28 is copied from App Note 3 Figures 4 and 5, and the level transducer circuits in Figures 29A, 29B, and 30 are copied from App Note 7, Figures 13, 14, and 15.

The next section of the app note starts discussing oscillators, which I'll cover on Friday.

Best quote (from the footnote on page AN43-1): "Wheatstone had a better public relations agency, namely himself."


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