This app note claims "direct digitization," but the circuits are based on voltage-to-frequency conversion rather than analog-to-digital conversion. Thus, I disagree with the title: V-to-F conversion isn't really a digitization technique; V-to-F conversion translates signals from one continuous domain (voltage) to another continuous domain (frequency). There's no quantization, so it's not really "digital". However, it is discretized in time (at the edges of the output waveform), so that's something.
Terminological quibbling aside, V-to-F conversion is an under-appreciated, but useful, trick in practical system design, particularly in instrumentation circuits, as demonstrated here. V-to-F isn't as sexy as A-to-D, and it doesn't get as much (if any) emphasis in college courses, but it's a life saver in some applications. Transmitting information via frequency (particularly over long cable runs) is often a good idea.
Figure 2 shows a V-to-F converter for a Type-K thermocouple. It's a good example of using a crude V-to-F converter in the forward path (the 74C04 inverter chain) and then linearizing it with a precision charge pump in the feedback path. Feedback is awesome. (Possible schematic error: is there a connection dot missing from the summing junction of A1? I think the 3300pF capacitor, the 33k resistor, and the 150k resistor should all be connected to the minus input of the op amp.)
The other applications are great, too. Figure 4 shows an acoustic thermometer (for those of you reading ahead, acoustic thermometry is also the topic of Jim's last app note twenty-five years later, App Note 131). Figure 6 shows a circuit for strain-gauge that produces a full-scale signal current of only 48 nA. Figure 8 shows a "Pease type charge pump" current-to-frequency converter with 100 dB of dynamic range (20 Hz to 2 MHz). Figure 11 shows a "humidity-to-frequency" converter (compare to Figure 8 in App Note 3). Figure 14 shows a level transducer with AC drive, and Figure 16 shows a circuit for a capacitive accelerometer. Again, the real story is the great assortment of sensors and transducers that Jim comes up with. I'm constantly jealous of his resourcefulness and his junk pile.
I think all these circuits are great, but the best circuit is the photodiode digitizer in Figure 8: Anything with a useful dynamic range of 100 dB deserves respect (and this result is just the tip of the V-to-F dynamic-range iceberg; stay tuned).
Best quote (page AN7-8): "At a minimum, careful layout and a clean PC board are required. The best practice is to use a Teflon stand-off for all summing point connections." Good advice. You know you're down in the picoamps when Telfon standoffs become necessary.