Charge amplifier

A charge amplifier in the electrical measurement, a common name for a charge-voltage converter which converts charges into a generally low voltage proportional thereto. Since no gain is defined because of the different units, it is strictly speaking no amplifier. The circuit is similar to an integrator with no input resistance.

Applications

This converter is used when it is necessary to measure extremely small amounts of charge that are generated, for example, in electric field meters, piezoelectric sensors and photodiodes. The use of multi-channel CCD sensors always requires more charge amplifier.

Advantages of the charge amplifier

The very low input impedance of this circuit shorts the capacities of such sensors, which is why long shielded cable between sensor and converter are possible. Since the sensor is " short-circuited virtual" problems at high temperature and pollution are reduced. The reduction in the time constant compared to the simpler resistive load also allows high measurement frequencies.

Sensors with charge output can be operated in principle also to voltage amplifiers with very high input resistance. Important disadvantages of this are, however, that the resulting RC low- pass filter dampens high frequencies and that the output voltage of the input plane parallel capacity is dependent because changes the sensor voltage. Therefore, the sum of sensor self-capacitance, the cable capacitance Cc and the input capacitance of the amplifier CINP must be reloaded. When charge amplifier, this dependency does not exist, since those two capacities at the " virtual ground " of operational amplifier are and the sensor voltage always has the value zero.

Circuit

The charge amplifier is typically implemented using operational amplifiers, with its special feature is the capacitive feedback. The non-inverting input ( ) of the operational amplifier is connected to ground potential. To the inverting input of the charge to be measured is introduced. In the plane parallel to the input capacitances Cc and CINP will be discussed further above. The op-amp ensures that the inverting input (- ) is also ground potential sets ( virtual zero ). The voltage at the input of the charge amplifier ( uinp ) is thus zero.

The node at the inverting input, the flow coming from the sensor and the charge Qin capacitive feedback from the output of charge QF. According to the node rule, both charges compensate, ie

The output voltage of the charge amplifier is calculated according to

It is thus proportional to the input charge qin with inverted sign.

The feedback capacitance Cf determines the gain.

Thus, the basic function of the charge amplifier is explained.

The resistance Rf to the feedback capacitor is used for producing a stable zero -point voltage at the amplifier output. Without Rf the DC voltage gain of the circuit would be very high and bias currents of the operational amplifier inputs would be highly amplified appear as a DC voltage at the output, which could lead to clipping. The resistor Rf determines the lower cutoff frequency of the charge amplifier:

Due to the DC voltage influences described and finite isolation resistors on the input and the connections of Cf, a charge amplifier is not the measurement of static ( stationary ) is adapted to charge. However, since achieve high-quality devices lower cutoff frequencies less than 0.1 Hz, it is called quasi-static measurement. Charge amplifier some manufacturers have a mode of operation is where Rf is replaced by a manually operable reset switch contact. So that a defined DC voltage condition can be generated at the output before the measurement.

Practically realized charge amplifier often contain additional circuit stages, such as additional power amplifier, high pass and low pass filters, integrators and circuits for level control.

Special

The signals generated by sensors with charge output may be only a few fC ( femtocoulombs = 10 -15C ). A disturbing effect customary sensor cable is the smallest shift cargoes during deformation of the cable as a result of the triboelectric effect. Even slight movements of the cable can significantly distort the measurement. Therefore used störspannungsarme ("low noise" ) special cable with a special conductive coating on the dielectric. These are considerably more expensive than conventional cable.

Due to the low impedance of the transmission is sensitive to magnetic fields. The cable length between sensor and amplifier for sensitive measurements should therefore not exceed a few meters.

To get around in piezo sensors, the mentioned problems and to replace the often costly charge amplifiers, sensors with charge output will be for some time supplanted by IEPE -compatible sensors that only need to run a voltage amplifier with no special requirements on the input impedance and a constant current supply.

A special form is the charge preamplifier (Remote Batch Converter ) with IEPE output which allows the connection of sensors with charge output to a simple voltage amplifier.