# Why is a shielded cable not effective with high impedance sensor?

Could some one simplify what this means? Why is the shielded cable not effective with a high impedance source/sensor?

• They are saying that the shield capacitance and leakage limit bandwidth and sensitivity respectively. Apr 29 '16 at 13:52
• I would also like to know, Why does that happen with the high impedance source? :) Apr 29 '16 at 13:54
• High impedance means the source has high resistance and low capacitance. The cable is in parallel so it affects the source. Apr 29 '16 at 13:57
• A common circuit for this application, mentioned in a question earlier today, uses a remote preamp to avoid the issue of excess capacitance in the cable. Apr 29 '16 at 17:50

Shields are not effective with high impedance?

The title of this question is about a TIA and photodiode. The photodiode is, at low frequencies, modeled as a current source but, as frequency increases its self-capacitance plays a killer role in determining the likely bandwidth of the TIA circuit. And of course, a shield adds to the capacitance between the two wires and makes a bit of a mess of things. You can't distinguish between diode self capacitance and that extra produced by the shield.

This is why capacitor C$_{COMP}$ is needed to straighten things out and of course that knocks the bandwidth of the TIA down significantly. So to understand the assertion you need to recognize the following: -

• The circuit is a photodiode amplifier and the photodiode has high impedance at low frequencies.
• It's a TIA and any capacitance to ground at the inverting input creates what is known as noise-gain - effectively this means that the op-amp's internal noise gets amplified by many, many dB due to the ratio of R$_{FB}$ to X$_{diode\space capacitance}$. We can be talking 20 dB of noise gain for common or garden TIA circuits.
• Capacitance is added to by the shield - this can make the noise gain totally intolerable so C$_{COMP}$ is added to reduce noise gain
• Adding C$_{COMP}$ can ruin the bandwidth of a TIA and you end up with no choice but to put the TIA right up with the photodiode or use fibre optics and have the photodiode with the TIA.

A high impedance sensor produces only a small amount of current since $$I=\frac{V}{R}$$ Thus any noise or interference which is magnetically coupled into the line can result in a signal that is a much larger fraction of your signal of interest than in the case with a lower impedance, higher signal current sensor.