25 July 2011

App Note 11

This app note contains some clever tricks for circuits powered from a single 5V rail, most of them using LT1013/14 low-voltage op amps and LT1017/18 low-voltage comparators. The box section on page AN11-15 tells a little bit of the history of linear supply voltages, starting with the 300-volt supplies used in vacuum-tube analog computers, and going down from there. (How low can you go? Stay tuned: single-cell circuits are coming up in App Note 15.)

Again, the circuits here have interesting sensors. The platinum RTD in Figure 1 and the thermocouple in Figure 5 are old news, but Figure 2 shows a unique methane detector with a voltage-to-frequency output. A little translinear circuit is used to compensate for the sensor's square-root behavior. Figure 7 shows a strain-gauge circuit that uses a voltage inverter (LTC1044) and an op amp to servo the left side of the bridge to zero volts. (There are many more bridge interfaces coming up in App Note 28.)

The schematics in Figure 6 show instrumentation amplifiers for 5V operation, one using the standard-topology three-op-amp instrumentation amp with a LT1014 quad op amp, and the other circuit using the LTC1043 switched-capacitor block and LTC1052 chopper amp from App Note 9 Figure 11.

Figure 8 shows a "tachless" motor speed controller that senses the motor's back EMF to determine speed. This approach makes me nervous. Sensing the voltage this way is really noisy! The design of windings (and commutator brushes) is very different for a motor than for a tachometer. I think the key phrase here is "Q3 turns off and the motor's back EMF appears after the inductive flyback ceases." You can see all of the signal junk in Trace B of Figure 9, and the residual junk after filtering in Trace D. I'm always impressed when an engineer can get this noise-plagued approach to work even this well.

Circuits for systems requiring galvanic isolation are shown in Figures 11, 12, and 14. The best circuit is Figure 14, a fully isolated single-slope analog-to-digital converter. As in Figure 12, the transformer is used both to power the isolated circuits and to transmit data (in this case, the pulses from the A/D conversion) across the isolation barrier.

Best quote (from page AN11-1): "The difficulties encountered in maintaining the lowest possible levels of noise and drift in an analog system are challenging enough without contending with a digitally corrupted power supply."

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