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


See my answer here that goes into more detail about TIA and noise gain. See also this more recent answer about photodiodes and TIAs.