I am using a very sensitive transimpedance amplifier on a 3m long cable to a photodiode. Signal levels are about 70 nA - 700 nA. Bumping into the cable causes a large voltage spike on the output of the amplifier. The insulating material is FEP.
Could this be the piezoelectric effect in the cable insulaton?
Edit: Here's a typical voltage spike. This happens when I bend, shake or tap the cable.
Triboelectric effect probably.
You can find cable made for musical instruments that is screened with a low resistance carbon impregnated layer (They call it semi conducting!) between the screen and the insulation of the centre conductor, this is helpful when dealing with high Z sources as it minimises the noise due to movement.
Adding to Jack's answer:
Triboelectric effects are just like every day static electricity: rubbing dielectrics together rips some electrons out and creates charge.
This creates a current (i=dq/dt)/
How this will affect your measurement is proportional to the impedance of the circuit driving the cable. The error voltage created is Z*i according to ohm's law. If the cable is driven from the low impedance output of an amplifier, then the effect will usually not matter at all.
But you are measuring tiny photodiode currents, so here it matters.
There is another effect: your coax is a capacitor. If it is charged to a DC voltage (not 0V) then bending it will modify its capacitance. Since the charge inside the cable's capacitance is constant, changing the capacitance changes the voltage. Again, this only matters in a high-impedance setting... like yours.
Solutions:
...and the obvious one, although not always possible:
If you want non-triboelectric coax, the keywords you have to google are "low noise coaxial cable". This is a type of coax with a graphite/carbon based resistive layer between the insulation layers, which dissipates any charge created by rubbing. However, they are hard to find and expensive. They are used in electrocardiograms, or for vibration measurement using piezo sensors, for example.
I also second the use of stage microphone cable: musicians like cables which don't produce a THUMP in the speakers when someone steps on the cable. If you walk in any music/guitar shop and ask for such cable, the sales person will know what you're talking about. They may not know about the physics involved, but they will know about clients returning cables because thy're microphonic...
These cables usually have a cotton filler around the wires, which does not generate charge when bent.
I use balanced stage microphone cable for audio measurements: they're cheap, easily available, and work very well. No need for fancy audiophile stuff.
If you want coax, try guitar cable. They're designed for the same setting: tiny signals, lots of spastic movement, high-Z setting... These won't be specified for HF impedance though.
This sounds like microphonic effects in the cable. It arises from several sources, some of which have been mentioned in other answers, including
It could also be capacitive or inductive pick-up. Though this is less likely if you already have good quality shielded cable with a grounded shield.
Common solutions include:
Cables designed for on-stage audio use might be good here, as they are often non-microphonic. There is a lot of overpriced garbage in the audiophile market, but there are also some nice products which - though vastly overengineered for audio use - are great for precision lab equipment.
Copper doesn't exhibit any appreciable piezoelectric effect.
What is probably happening is that small charges are getting moved around on the outside of the insulation of the cable. The insulation is basically the dielectric of a capacitor, with the signal wire one side of the capacitor. As the cable gets moved around and touches different things that may have various static charges on it, small currents are caused thru the capacitor.
Normally these currents are too small to matter, but in your case you are specifically amplifying them.
The solution is to use a shielded cable. The insulation capacitor is then between the shield and whatever is outside. The center conductor sees a relatively constant capacitance to the shield, which is held at a constant voltage by your circuit.
If the cable has a high DC voltage, then any capacitance change in the cable will result in a charge flowing and, therefore, a signal. To test this, disconnect your transducer and see if the effect is still present. If so, perhaps your photodiode is ALWAYS disconnected, so check if it is sensitive to an optical signal. To see this kind of signal due to capacitance change in cable requires an abnormally high impedance, so perhaps something is not connected.
If everything is connected and the photodiode responds to optical signals, you can try
What you are seeing, is not piezoelectric effect. It is a capacitance effect due to the "distortion" of the dielectric material. To fix the problem, one could try to reduce the charge generated by the bending (expensive), or one can prevent the "distortion" by "encasing" the twisted pairs inside metal conduit (like EMT), which gives them rigidity, protection from bends and bumps, and additional shielding. The conduit would, of course, need to be anchored so that it does not move.
