lesson
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If you connect a high-impedance pH probe or piezoelectric sensor to a standard three-meter coaxial cable, your measurement bandwidth and noise immunity can suddenly collapse.
Even though the outer braided shield blocks external electrostatic noise, the cable itself adds roughly 100ย pF of parasitic capacitance per meter directly between your signal conductor and ground.
๐Interactive schematic showing a high-impedance source (Vs with Rs = 10 Megohms) connected through a coaxial cable to an op-amp. The cable is shown with core wire and grounded outer braid. Distributed parasitic capacitors (Ccable = 300pF total) bridge the core to the grounded shield. Current arrows animate leaking AC signal current from core to ground. A frequency response graph below shows a severe low-pass roll-off with a cutoff frequency fc = 1 / (2 * pi * Rs * C) at just 53 Hz.
Can we keep the outer shield for interference protection without letting its capacitance steal our signal?
The Driven Guard Principle
A driven guard neutralizes cable capacitance by actively forcing the shield to the exact same electrical potential as the inner signal wire.
Electric charge only moves across a capacitor when a potential difference exists: q=Cโ
ฮV and current i=Cdtd(ฮV)โ.
If a unity-gain buffer (Avโ=1) drives the shield with the input voltage Vinโ, the voltage across the parasitic capacitance becomes ฮV=VinโโVguardโ=0ย V.
๐Comparative visual diagram showing two cable setups side-by-side. Left card: 'Grounded Shield' showing Vin = 5V peak, Shield = 0V, Delta V = 5V, heavy displacement current i(t) leaking into ground, effective capacitance Ceff = 100%. Right card: 'Driven Guard' showing Vin = 5V peak, Shield driven by buffer amplifier to 5V peak, Delta V = 0V, displacement current i = 0A, effective capacitance Ceff ~ 0%. Clear equations and green highlights showing zero charge transfer.
Why does this make the cable behave as if it has no capacitance at all?
Deriving Effective Capacitance
Consider a buffer amplifier with a practical voltage gain Avโโ0.999. When the core wire carries signal v(t), the shield is driven to Avโโ
v(t).
The signal source only needs to supply current across the physical cable capacitance Ccableโ for the remaining voltage drop: