Charge amplifier

A charge amplifier is a common name in electrical measurement technology for a charge-voltage converter, which usually converts small charges into a voltage proportional to it. Since no gain factor can be defined due to the different units, strictly speaking it is not an 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, which are generated, for example, in electric field meters , piezoelectric sensors and photodiodes . The use of multi-channel CCD sensors always requires several charge amplifiers.

Advantages of the charge amplifier

The very low input impedance of this circuit short-circuits the capacities of these sensors, which is why longer shielded cables are possible between the sensor and the converter. Because the sensor is "virtually short-circuited", problems with high temperature and contamination are reduced. The reduction in the time constant compared to the simpler resistance load also allows high measuring frequencies.

In principle, sensors with a charge output can also be operated on voltage amplifiers with a very high input resistance. However, the decisive disadvantages are that the resulting RC low-pass filter attenuates high frequencies and that the output voltage depends on the capacitance lying parallel to the input, because the sensor voltage changes. Therefore, the sum of the internal capacitance of the sensor, the cable capacitance C _c and the input capacitance of the amplifier C _{inp must} be transferred. This dependency does not exist in the case of the charge amplifier, since these two capacitances are at the "virtual ground" of the operational amplifier and the sensor voltage always has the value zero.

circuit

Circuit of a charge amplifier

The charge amplifier is usually implemented using operational amplifiers , the special feature of which is the capacitive feedback. The non-inverting input (+) of the operational amplifier is at ground potential. The charge to be measured is introduced into the inverting input. The capacitances C _c and C _inp lying parallel to the input are discussed further above. The operational amplifier ensures that ground potential is also set at the inverting input (-) ( virtual zero point ). The voltage at the input of the charge amplifier ( u _inp ) becomes zero.

The charge q _in coming from the sensor and the capacitive charge q _f fed back from the output flow into the node at the inverting input . According to the knot rule , both charges compensate each other, that is

${\displaystyle q_{in}=-q_{f}}$.

The output voltage of the charge amplifier is calculated according to

${\displaystyle u_{out}={\frac {q_{f}}{C_{f}}}=-{\frac {q_{in}}{C_{f}}}}$

It is therefore proportional to the input charge q _in with an inverted sign .

The feedback capacitance C _f determines the gain.

This explains the basic function of the charge amplifier.

The resistor R _f across the feedback capacitor is used to produce a stable zero point voltage at the amplifier output. Without R _f , the DC voltage amplification of the circuit would be very high and bias currents of the operational amplifier inputs would appear highly amplified as DC voltage at the output, which could lead to overload . The resistance R _f determines the lower limit frequency of the charge amplifier:

${\displaystyle f_{u}={\frac {1}{2\pi R_{f}C_{f}}}}$

Due to the direct voltage influences described and the finite insulation resistances across the input and connections of C _f , a charge amplifier is not suitable for measuring static (standing) charges. However, since high-quality devices reach lower limit frequencies of less than 0.1 Hz, one speaks of quasi-static measurement. Charge amplifiers from some manufacturers have an operating mode in which R _{f is} replaced by a manually operated reset switch contact. This means that a defined DC voltage state can be generated at the output before the measurement.

Charge amplifiers implemented in practice often contain additional circuit stages, for example additional voltage amplifiers, high and low pass filters, integrators and circuits for level control.

particularities

The signals generated by sensors with a charge output may only be a few fC (FemtoCoulomb = 10 ⁻¹⁵ C). A disruptive effect of conventional sensor cables is the displacement of the smallest charges when the cable is deformed as a result of the triboelectric effect . Even slight movements of the cable can significantly falsify the measurement. For this reason, special cables with low interference voltage ("low noise") are used with a special conductive coating on the dielectric . These are significantly more expensive than conventional cables.

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

In order to bypass the problems mentioned with piezo sensors and to replace the often expensive charge amplifiers, sensors with charge output have been replaced for some time by IEPE- compatible sensors, which only require a voltage amplifier without special requirements for input impedance and a constant current supply. They consist of the sensor with an integrated impedance converter. Similar to an electret microphone , they are fed via the signal line.

There are also sensors with integrated charge preamplifiers (Remote Charge Converters) with IEPE output that require less voltage amplification.

Web links

• [1] Charge amplifier basics, Metra measurement and frequency technology
• Signal Conditioning Piezoelectric Sensors (PDF; 60 kB) application note, 2000, Texas Instruments