When used with normal, single ended transducers, the microplug adaptor JJ 0207, supplied with Type 2634, allows the use of normal, low-noise coaxial cables; this adaptor automatically grounds one of the input poles.
The input amplifier is a differential charge amplifier consisting of a dual, low-noise FET and two, highgain IC operational amplifiers. The lower limiting frequency of 1 Hz is determined by a filter network around the input amplifier which provides a 40 dB/decade fall-off in response over the decade 1 Hz to 0.1 Hz. This eliminates the influence of low frequency noise on the measured signal, for example due to the pyroelectric effects of some transducers in fluctuating temperature conditions.
The output amplifier has a low output impedance which is suitable for driving long cables. The influence of load capacitance on the high frequency response of the amplifier is shown in Fig. 2. By means of an internal 10 turn potentiometer, the gain of the amplifier can be adjusted between 0 and 20 dB.
The Power Input and Signal Output of the 2634 is supplied with a 3.5 m long cable AO 0198 fitted with matching connector. Wherever possible power should be provided by a dual polarity supply with voltage between ± 6 and ± 24 V. This ensures that the output signal is centred at ground potential with negligible DC offset and that power supply noise and common mode signals ar more effectively suppressed.
When the 2634 is used with normal, single ended transducers, the microplug adaptor JJ 0207 supplied allows the use With single polarity power supplies, the output signal is DC offset to approximately + 6 V. Voltages of + 12 V to + 28 V may be used. When the Charge Amplifier is used in conjunction with one of the Brüel & Kjær measuring amplifiers or frequency analyzers, it is convenient to use the + 12 V power supply provided at the 7 pin preamplifier input socket on these instruments. This may be applied to the 2634 using the power and signal cable WL 0309 which is available on special order.
Power Supply and output signal can be obtained from the 5 pin Lemo socket on the Amplifier 2634. Both differential (balanced) type and normal, single ended type piezoelectric accelerometers may be used with the 2634. The differential types will be used especially in conditions of severe electromagnetic interference. With differential transducers, where both poles of the piezoelectric element are isolated from the case, and therefore from the machine frame, ground loop interference problems are largely eliminated.