Underwater pressure measurements during explosive performance testing relyon piezoelectric (turmalin) free-field sensors, where integrated electronics piezo-electric sensors with built-in amplifiers have to a large extent replaced traditionaltourmaline crystals connected to external charge amplifiers. Field measurementswith the new sensor type have shown unexplained deviations in the slow decayphase of the pressure waveform, as well as oscillations near peak pressure, withoutit being possible to determine whether these deviations originate from the amplifi-er electronics or other parts of the measurement chain. The objective of this thesiswas to characterise the built-in amplifier of the sensor so that its contribution tomeasurement deviations could be separated from other error sources. A chargeinjection rig was constructed to electrically excite the amplifier under controlledconditions, replacing the tourmaline crystal with a known injection capacitor. Theamplifier was characterised in the time and frequency domains with respect to di-scharge time constant, high-frequency transfer function, output impedance, andbehaviour under capacitive load. The results showed a discharge time constant ofat least 1.628 seconds at unloaded input, a second-order high-frequency responsewith a resonance frequency of 4.98 megahertz and an upper cut-off frequency of10.8 megahertz, an output impedance of 20.0 ± 2.3 ohms, and an effective peakcurrent of approximately 24 milliamperes. The high repeatability, with a relativestandard deviation below 1 percent, indicated that the amplifier introduced a sys-tematic but not stochastic error. This suggested that the observed field variationsoriginated from either the test object, the test environment, or the tourmalinecrystal, rather than from the amplifier electronics.