Maximum cable length for charge-mode piezoelectric accelerometers
Users of piezoelectric accelerometers have a choice of both voltage mode (Isotron®) and charge mode piezoelectric accelerometers. Voltage mode devices have a built-in electronic impedance converter providing the maximum convenience and eliminate the need for special cables.
Charge mode piezoelectric devices offer advantages such as high temperature operation, the flexibility of the remote charge amplifier and extremely high MTBF. When using charge-mode devices, it is necessary to use low-noise cable and the cable length is somewhat limited. The very high impedance charge-mode circuit is very susceptible to noise including the nose generated by cable motion (called triboelectric noise) therefore, cable with a special low-noise treatment must be employed.
This article provides practical background information about how to determine the maximum cable length between the accelerometer and charge amplifier (or in-line charge converter) and maintain optimum low-noise performance.
In the early days of shock and vibration measurements, very high impedance electrometer voltage amplifiers were used to interface accelerometers with readout electronics. The amplitude response of the measurement was so sensitive to the cable length, continuous calibration was necessary and measurement results were often in doubt. Anytime the accelerometer cable was replaced, or the length changed, the entire system had to be recalibrated.
Voltage amplifiers have since been replaced with charge amplifiers and Endevco was a pioneer in this technology with the introduction of the first solid state charge amplifier. A charge amplifier is basically a very high impedance amplifier where the gain is controlled by a feedback capacitor within the amplifier circuit. The design is such that the capacitance of the accelerometer and cable capacitance has no noticeable effect on the gain of the amplifier. As can be seen, changing cable length and accelerometers requires no re-calibration of the accelerometer system, but there are some limitations.
The actual limit is specified on the charge converter, or amplifier's "Source Capacitance" specification. For example, the Endevco 2771C Charge Converter and the model 133 Charge/Voltage amplifiers have a source capacitance of 33,000 pF. When selecting an accelerometer and cable, the total combined capacitance of the cable and accelerometer can not exceed the amplifier's source capacitance. The amplifier datasheets will specify the source capacitance. Typical coaxial cable will have a capacitance of 30pF/ foot. The capacitance of the accelerometer may vary from approximately 170pF to 4,000 pF, depending on the design and type of crystals used in the construction.
To determine the capacitance of the accelerometer, refer to the calibration certificate provided with the accelerometer. If the calibration certificate is unavailable, the product datasheet provides a nominal value, which can be used for this calculation.
The next step is to determine the total capacitance contributed by the cable. Cable capacitance is available from the manufacturers data sheet and is specified in pF/foot (and pF/meter). Simply multiply the capacitance per foot times the total length of cable required.
To determine the total source capacitance, simply add the accelerometer capacitance to the total cable capacitance. If the total capacitance is lower than the amplifiers source capacitance, you should be good to go, but consider the below precautions.
- Since there are manufacturing tolerances involved with the cable, the total combined cable and accelerometer capacitance should be about 20% lower than the maximum charge amplifier source capacitance. As an example, the Endevco model 133 has a source capacitance of 33,000 pF thus a maximum input of 26,000 would be the recommended maximum input capacitance.
- "Shorter is better". Keep the high impedance cables as short as possible! Since we are dealing with a very high impedance system, noise pick-up can be a problem. One way to keep the high impedance cable short is to use an Endevco charge converter as close to the accelerometer as possible, then continue the cable run using the low impedance output from the charge converter. As a "rule-of thumb", a cable run of 50 feet or less, between the charge amplifier (or RCC) and the accelerometer will generally cause little increase in the noise level. When cables exceed 100', there can be a considerable noise increase.
The Endevco 2771C Converts a high-impedance Piezoelectric charge into a low impedance Isotron® output. No additional charge amplifiers are required.
[ Return to front page ]
- ALWAYS use low noise cable between charge mode accelerometers and the charge amplifier / converter input. Cables must be secure since movement can produce signal distortion.