An integrated current-controlled current source with programmable gain for charge amplifier applications

We present an integrated current-controlled current source (CCCS) for an n-well 1.2 μm-CMOS-process with 5 V supply voltage allowing positive and negative current values. The gain is digitally programmable within a range of 1/16 to 1 by switching four binary weighted input branches on or off, which facilitates automatic gain control. Its output resistance is higher than 10 GΩ due to active-feedback cascode techniques. Linearity is better than 0.25%, the bandwidth varies from 1 MHz for unity gain to 10 MHz for a gain of 1/16 without any gain overshoot, as simulations with Cadence/SpectreS show. With an on-chip capacitor connected to the output, a precise current integrator ASIC can be created. We discuss its application as a charge amplifier for capacitive sensors. Sensitivity can be adjusted to different values of the sensor capacitance by simply changing the CCCS-gain. The charge amplifier shows a bandwidth ranging from 3.5 Hz to 350 kHz and an output voltage settling time of 2 μs for a 1 V_pp output voltage