Charge Amplifiers and Charge Converters

Charge converters and charge amplifiers transform charge output signals with high-impedance to low-impedance voltage or current. Transducers connect piezoelectric, capacitive, or other charge-producing sensors to an amplifier that conditions the signal and outputs the voltage or current to a display such as a front panel, touch screen, or computer monitor. Charge converters and charge amplifiers are low-cost devices that are used in the measurement of variables such as acceleration, force, temperature, and dynamic pressure. Some charge converters and charge amplifiers include a voltage excitation supply or a current excitation supply that powers the sensor. Others include integral or pre-packaged sensors.

Charge converters and charge amplifiers vary in terms of maximum output voltage, accuracy, programmable gain, and signal isolation. They also provide different voltage, current, and relay outputs. Accuracy depends upon signal conditioning linearity, hysteresis, and temperature. Programmable gain measures amplification adjustments from a local interface such as a front panel, or from a computer interface. Signal isolation separates the signal’s direct electrical contact and can be achieved through optical isolation, capacitors, and magnetic induction. Charge converters and charge amplifiers can also include multiplexers that combine several inputs into one output for transmission. In terms of analog input channels, differential channels use the difference between two signals as an input.

There are several filter architectures for charge converters and charge amplifiers. Analog filters are designed with resistors and capacitors and are used in applications with low noise requirements. Digital filters are designed with solid-state components and include both finite impulse response (FIR) and infinite impulse response (IIR) filters. Filter functions are often programmable and may include anti-alias features that correct the signal misrepresentation that occurs during slow sampling. Low pass and high pass filters block or attenuate signals at frequencies above or below a specified cutoff frequency. Band pass filters block or attenuate signals at frequencies outside of the low pass and high pass cutoff frequencies. Band stop frequencies block only a portion of the frequency spectrum while all-pass filters allow all frequencies to pass.  

There are several form factors for charge converters and charge amplifiers. Some bolt into walls, cabinets, or other enclosures. Others mount in racks, slide into slots, or dock in modular bays. Stand-alone units are designed for benchtop use and often include a full case or cabinet and integral interface. Charge converters and charge amplifiers can also mount on standard DIN rails, plug into computer boards such as backplanes, or work with integrated circuits (ICs). 

Charge converters and charge amplifiers are used in a variety of aerospace, medical, and industrial applications. For example, these devices are used with pressure transducers and high intensity microphones in vehicle dynamics studies. They are also used in pacemakers that include an accelerometer to detect motion and translate this motion into an appropriate pacing rate.

• Paralleling the TC1121 to Reduce Output Resistance for Driving Higher Load Currents (TC1121)
  Microchip Technology's TC1121 is a charge pump voltage converter specified to have a 100mA output current capability. The TC1121 has the benefits of selecting different internal charge pump switching...
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