Take-Back-Half precision diode charge pump

Nearly four decades ago (in his Designs for High Performance Voltage-to-Frequency Converters), famed designer Jim Williams cataloged five fundamental techniques for voltage to frequency conversion. One of those five is reproduced in Figure 1.

Figure 1 Precision charge pump closes feedback loop to make “crude V→F” accurate from “Designs for High Performance Voltage-to-Frequency Converters”.

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Williams concisely summarizes how this famous topology works: “The DC amplifier controls a relatively crude V→F. This V→F is designed for high speed and wide dynamic range at the expense of linearity and thermal stability. The circuit’s output switches a charge pump whose output, integrated to DC, is compared to the input voltage The DC amplifier forces V→F operating frequency to be a direct function of input voltage.”

Earlier in “Designs for…” William had presented several terrific VFC designs embodying Figure 1’s concept, that utilized a variety of different charge pumps. Two of these were diode types. More examples of Williams VFC designs incorporating diode pumps are detailed in his fascinating (and entertaining!) narrative of his creative design process: “The Zoo Circuit” (Chapter 18).

The success of these and other diode-pump equipped designs proves the utility of diodes in precision applications. However, an inherent challenge in working diode pumps into VFCs is accommodation of the inconvenient fact that no (real) diode is ideal. Diodes incur non-linear and temperature-dependent voltage drop, shunt capacitance, reverse recovery charge, and other “charming” idiosyncrasies. Inspection of any good VFC with a diode pump (including Williams’s excellent designs) will reveal a significant fraction of circuitry and part count dedicated to mitigating these quirks. Figure 2 sketches where some of these errors arise and their effects on pump accuracy.

Figure 2 The realities of a diode pump where errors can arise such as non-linear and temperature-dependent voltage drop, shunt capacitance, reverse recovery charge, and more.

 If the diodes in Figure 2’s pump were perfect, then each Vpp cycle of the input frequency would output a dollop of charge Q = -VC, and we’d therefore have Vout = FVCR. But since they’re not, forward voltages (Vd), shunt capacitances (Cs), etc. subtract from the net charge pumped leaving Q = – (VC – 2Vd(C + Cs)) making Vout = F(VC – 2Vd(C + Cs))R.

 Traditional circuit tricks for (at least partially) canceling these errors and nulling out (most) of the tempco they introduce (e.g., 2mV/^oC for each Vd) include adding strings of diodes in series with VFC voltage references and calibration trims in input networks. Although they can be made to work, fine tuning these remedies in a given design can be complex and none of it is particularly elegant or easy.

Figure 3 shows an approach that’s entirely different from reference tweaking: “Take-Back-Half”, or TBH!

Figure 3 TBH adds a half-amplitude reverse-polarity pump that subtracts error terms.

 TBH adds a new opposite-polarity pump in parallel with the usual diode pair, driven by a 1:2 ratio capacitive voltage divider with the same total capacitance. The result is to generate opposing charge packets that have half the nominal signal amplitude but equal error signal amplitude. Consequently, when the charges are summed, half the desired signal is “taken back” from the net pump output, but all the error goes away.

This leaves only the original, ideal-diode-case output:  Q = -VC and Vout = FVCR.

This verbiage might sound garbled and confusing (I know) but the analog algebra is simple and (I hope) clear. Please see Figure 3.

Stephen Woodward’s relationship with EDN’s DI column goes back quite a long way. Over 100 submissions have been accepted since his first contribution back in 1974.

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