Cable length in charge systems
To provide accessibility to charge converters we are running soft cable lengths between 85' to 130' between the accelerometers and the charge amplifiers. We will be replacing Endevco #3090C-600 soft cable with Endevco #3097M1. I know you can't run these forever in distance but I have not seen it documented anywhere in your literature on what the recommended cable lengths are for best performance (naturally the shorter the better) and the maximum lengths before we see partial and complete loss of signal due to weakness. Are there standard numbers for these or is each cable unique? Do you have this data for both cables and can you send it to me for comparison in our design change?
First, you are correct that the "shorter, the better" holds true. A long length of cable acts like an antenna and picks-up undesirable noise. In the case of a charge system, this is a concern.
To best determine the maximum cable length, three factors must be taken into consideration:
- Source capacitance of the charge amplifier (Cs)
- Capacitance of the accelerometer (Ca)
- Capacitance of the cable (Cc)
The sum of the accelerometer's capacitance and the capacitance of the cable can not exceed the source capacitance of the charge amplifier.
In the case of a 2771B charge converter the source capacitance is 30,000pF. This value is on the charge converter specification sheet as "Input Source Capacitance."
The 3097M1 has a capacitance of 39pF/ foot. Thus if the cable length is 100 feet, the cable capacitance will total 3,900pF. You will need to add in the capacitance of the accelerometer to the 3,900pF. If this total is less than 30,000pF you are good to go.
The cable you are replacing has a capacitance of 40pF/ft, so there is little difference in the cable capacitance. In general, the accelerometer capacitance is the major factor. The actual capacitance of the accelerometer is measured, at the factory, and included in the calibration certificate. The nominal capacitance is available on the product data sheet.
You will have little signal loss from the longer cable, but there can be an increase in noise. The advantage of charge amplifiers over voltage amplifiers is the fact that signal level is no influenced by cable length. Once the cable and accelerometer capacitance exceed the amplifier's source capacitance, attenuation of the signal will occur.
I hope this information will prove helpful. I don't know what accelerometer model you are using thus I am unable to provide real numbers. Please let me know if we can be of further assistance and thanks for your question.
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