Precision-matched capacitor pairs are commercially obtainable gadgets, however solely in a restricted vary of values, working voltage, and dielectrics.
Plus, typically an additional crucial software with additional tight tolerances (or an additional tight funds) can dictate a bit of (or so much) DIY. For instance, see “Inherently DC correct 16-bit PWM TBH DAC.”
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Determine 1’s matchmaker circuit might help make the in any other case odious chore of looking out by way of a batch of components for precisely matching pairs faster and a bit much less taxing. Better of all, it does precision matchmaking (doubtlessly to the ppm stage) without having for expensive precision reference parts.
Right here’s the way it works.
Determine 1 Flip-flop U2b generates complementary excitation of the A and B capacitors below check.
Complementary (equal however reverse) excitation of the A and B capacitors below check implies that if Ca = Cb, then the costs handed will precisely cancel out, yielding a null at OUTPUT. In the event that they differ, nevertheless, then an built-in output sign of fifty mV per % of mismatch will consequence if C3, C5, C4, and Cab (capacitors randomly chosen from the trial batch) are equal in worth, e.g., 0.68 µF. This sign is synchronously rectified by U1b, then built-in and buffered by the U1c and A1 suggestions loop.
If Ca > Cb, the “A > B” output polarity shall be optimistic relative to “B > A”. The reverse can also be true: If Cb > Ca, the “A > B” output polarity shall be adverse relative to “B > A”.
Decision of the match measurement will rely on the voltage decision of the digital voltmeter (DVM). If that’s 1 mV, then matching to ±1/50th of 1%, or ±0.02%, shall be attainable. If it’s 100 µV (typical of an ordinary 3¾ digit multimeter with a 300 mV scale), then matching to ±0.002%, i.e., ±20ppm, is doable. And so forth…
Measurement achieve is inversely proportional to C4. So, should you want extra decision, merely lower C4 to achieve achieve.
Determine 2 The U1aU2a multivibrator waveforms; the inexperienced waveform is the R3R4 junction, and the pink waveform is U2 pin 6. The frequency is 0.1mHz/C3.
Notice that the U1aU2a clock’s frequency precision and stability is considerably doubtful (even when we’re charitable). Fortunately, the accuracy of the final word Ca/Cb match doesn’t rely on a secure clock. Neither does match accuracy rely on the output impedances of D2b’s complementary outputs, not even on their symmetry!
Each insensitivities derive from the truth that it’s the transferred cost that varieties the premise of matchmaking precision, and subsequently neither present nor voltage issues very a lot.
Nevertheless, as a result of temperature sensitivity of some dielectrics, it’s most likely a good suggestion to deal with examined units with thermally insulating gloves. This can save time and frustration ready for them to equilibrate, to not point out attainable outright faulty outcomes. These are additionally recognized to trigger frustration!
My thanks go to frequent contributor Christopher Paul for suggesting the utility of capacitor precision matching, and as all the time, to DI editor Aalyia Shaukat for this marvelously productive DI EE ecosystem we inhabit.
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a good distance. Over 100 submissions have been accepted since his first contribution again in 1974.
Associated Content material
- Matchmaker
- Circuits assist get or confirm matched resistors
- Inherently DC correct 16-bit PWM TBH DAC
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