[sdiy] Calculating cable capacitance effect

Bernard Arthur Hutchins, Jr bah13 at cornell.edu
Sat Apr 17 19:31:41 CEST 2021


What about patch cord capacitance and those 1k series resistors often used on synthesizer module outputs:  PATCH CORD CAPACITANCE may well matter.

The 1k series output resistor was not really for protecting the outputting op-amp from damage (very unlikely), nor was the automatic “mixing” (often touted) of much importance. [It did assure that something well-defined happened contrary to an ill-designed chance occurrence of two op-amp outputs being (inadvertently) directly connected! Also, a somewhat similar moderate output impedance (600 ohms) was common in audio signal work. And the 1k’s were obviously never suggested for the lines carrying a main-control-voltage (volts /oct.) .]

Two useful things: First, the SDIY experimenter frequently (typically) has his/her ”synthesizer” (finished modules) driving an external breadboarded module under test (MUT). For example, a finished VCO might be modulating a new VCO MUT. Things are not working – no surprise. Then to your horror you see that your good VCO has also now stopped - what have you done!

Well, if you had the 1k series resistor your finished VCO would at least be running happily regardless of what is going on in the connected VCO MUT. (Op-amps generally drive anything if isolated by 1k). Without the 1k, a wiring error (perhaps a breadboard short to ground) may well get back into your finished VCO, causing confusion as well as anxiety.

The second useful function of the 1k series resistors is that they “decouple” capacitive (typically shielded cables at perhaps C=100 pfd/meter) loads from op-amp outputs thus preventing spurious high-frequency (MHz) oscillations WITH ASSOCIATED DC SHIFTS. Connecting a cable (even just to a scope) directly to an op-amp output forms an RC low-pass (R being the inherent INTERNAL op-amp output resistance of perhaps 100 ohms). Such an oscillation is HF but low level (slew limited) and non-symmetric (non-symmetric slew limiting). The result is a fuzzy looking scope trace (looks out of focus) and is only there when a cable IS attached, and as a VCO control can cause a small but noticeable pitch shift.

The oscillation occurs because the R is internal to the op-amp and the RC is INSIDE any feedback loop and contributes excessive phase shift. With the 1k series the RC (R now 1k) is OUTSIDE the op-amp’s feedback loop.  Fuzzy trace and pitch shift gone.

(previously posted – in large part here, or on  MW?)

- Bernie


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