Bilateral comparison of a piezoelectric charge amplifier transfer function inthe range 1 to 10000 Hz

Project Description

The objective of this project is to verify the Laboratories capabilities in a charge amplifier calibrating:

• Piezoelectric transducers used as reference standards are generally of the kind which requires the connection to an high quality charge-amplifier to obtain, from this so called measuring chain, the most stable output signal to be sent to the measuring instrumentation.
• the absolute calibration of the piezoelectric transducer alone (by itself) requires obviuosly calibrating also the charge-amplifier in conditions simulating the connection to the transducer.
• indirect data on charge amplifier calibration as extracted from previous piezoelectric transducer comparison campaigns often shows rather unjustified amplification scattering which could be due either to poor calibration or to instability of the circulating instrument.
• the proposed comparison will allow to check directly calibration and uncertainties agreement between the two laboratories and to confirm stability of the specific instrument employed.
• the comparison could be extended to other laboratories, using a very stable amplifier, to ascertain their calibration capabilitites and avoiding circulating such an instrumznt in future piezoelectric transducer calibration.
• the calibration technique employs a few standard capacitors (from 1 pF to 1000 pF), connected to the charge-amplifier input, by measuring its output/input voltage ratio and phases when supllying the capacitor with a suitable sinusoidal voltage.

Final Report 200-04-04

The objective of this project was to verify the Laboratories capabilities in a charge amplifier calibrating.

The results of the comparison between IMGC and PTB, identifying the behaviour of the chargeamplifier, lead to the following conclusions:

• The comparison measurements were carried out at IMGC and PTB during a period of about 6 weeks in which the charge amplifier showed a very good long-term stability
• The intercomparison results show differences, which, with a few exceptions, are within the resolution of the instrumentation employed. The excellent agreement of better than 0,1% in sensitivity measurement and 0,1° in phase shift measurement over the whole range of frequencies (1 Hz to 10000 Hz) demonstrates the high level of two well-equipped Laboratories
• The results of the investigation of the charge amplifier, carried out at the IMGC before and after the comparison period (see 1) have demonstrated that the effects from long-term instability, power supply (battery) and tracking (i.e. repeated settings of switches) are potential sources of significant errors and uncertainties, whereas the effect from humidity turned out to be negligible
• Looking at the instability (see 3) of the charge amplifier, it is recommended that in intercomparisons for the quantity of acceleration only a very stable accelerometer circulates, without any charge amplifier.
• To calibrate piezo-electric accelerometers, the laboratories must be equipped with up-to-date electrical means (e.g. standard capacitors) for the calibration of their own charge amplifier to be used.
• To achieve high accuracy, e.g. expanded uncertainty of 0,2% (e.g. in international comparisons in accelerometry), the electrical calibration of the charge amplifier should be carried out shortly before the accelerometer calibration, using appropriate instrumentation and suitable procedure.
• As only one charge amplifier was used for the comparison measurements and investigations, the results may not be representative for other charge amplifiers.