Almost 4 many years in the past (in his Designs for Excessive Efficiency Voltage-to-Frequency Converters), famed designer Jim Williams cataloged 5 elementary strategies for voltage to frequency conversion. A kind of 5 is reproduced in Determine 1.
Determine 1 Precision cost pump closes suggestions loop to make “crude V→F” correct from “Designs for Excessive Efficiency Voltage-to-Frequency Converters”.
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Williams concisely summarizes how this well-known topology works: “The DC amplifier controls a comparatively crude V→F. This V→F is designed for prime pace and huge dynamic vary on the expense of linearity and thermal stability. The circuit’s output switches a cost pump whose output, built-in to DC, is in comparison with the enter voltage The DC amplifier forces V→F working frequency to be a direct operate of enter voltage.”
Earlier in “Designs for…” William had introduced a number of terrific VFC designs embodying Determine 1’s idea, that utilized a wide range of completely different cost pumps. Two of those have been diode sorts. Extra examples of Williams VFC designs incorporating diode pumps are detailed in his fascinating (and entertaining!) narrative of his inventive design course of: “The Zoo Circuit” (Chapter 18).
The success of those and different diode-pump outfitted designs proves the utility of diodes in precision purposes. Nonetheless, an inherent problem in working diode pumps into VFCs is lodging of the inconvenient indisputable fact that no (actual) diode is good. Diodes incur non-linear and temperature-dependent voltage drop, shunt capacitance, reverse restoration cost, and different “charming” idiosyncrasies. Inspection of any good VFC with a diode pump (together with Williams’s wonderful designs) will reveal a big fraction of circuitry and half depend devoted to mitigating these quirks. Determine 2 sketches the place a few of these errors come up and their results on pump accuracy.
Determine 2 The realities of a diode pump the place errors can come up reminiscent of non-linear and temperature-dependent voltage drop, shunt capacitance, reverse restoration cost, and extra.
If the diodes in Determine 2’s pump have been good, then every Vpp cycle of the enter frequency would output a dollop of cost Q = -VC, and we’d subsequently have Vout = FVCR. However since they’re not, ahead voltages (Vd), shunt capacitances (Cs), and many others. subtract from the web cost pumped leaving Q = – (VC – 2Vd(C + Cs)) making Vout = F(VC – 2Vd(C + Cs))R.
Conventional circuit methods for (a minimum of partially) canceling these errors and nulling out (most) of the tempco they introduce (e.g., 2mV/oC for every Vd) embody including strings of diodes in sequence with VFC voltage references and calibration trims in enter networks. Though they are often made to work, superb tuning these cures in a given design will be advanced and considered one of it’s notably elegant or simple.
Determine 3 exhibits an method that’s completely completely different from reference tweaking: “Take-Again-Half”, or TBH!
Determine 3 TBH provides a half-amplitude reverse-polarity pump that subtracts error phrases.
TBH provides a brand new opposite-polarity pump in parallel with the standard diode pair, pushed by a 1:2 ratio capacitive voltage divider with the identical complete capacitance. The result’s to generate opposing cost packets which have half the nominal sign amplitude however equal error sign amplitude. Consequently, when the fees are summed, half the specified sign is “taken again” from the web pump output, however all the error goes away.
This leaves solely the unique, ideal-diode-case output: Q = -VC and Vout = FVCR.
This verbiage would possibly sound garbled and complicated (I do know) however the analog algebra is easy and (I hope) clear. Please see Determine 3.
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Over 100 submissions have been accepted since his first contribution again in 1974.
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