Sensing technique for self-contained charge-coupled split-electrode filters

A SELF-CONTAINED charge-coupled 55-tap split-electrode filter will be described in this paper. The major problem that had to be solved was that of transforming the signal from the sense electrodes into a usable output signal with a large dynamic range and a minimal harmonic distortion. The factors that must be considered in formulating a solution to this problem are: (1) eliminating the effects of depletion capacitance, (2) detecting a difference signal in the presence of a large common mode signal and (3) preventing reset noise in the detection circuitry. A practical method to avoid the effects of depletion capacitance under the split sense electrodes is to keep these electrodes at a fixed potential during the sensing process by using a feedback loop around an operational amplifier as indicated in Figure 1. This sensing circuitry is optimally used in conjunction with a voltage input to the charge transfer channel, where the charge packets are metered under an MOS electrode, (MW), the geometry of which is the same as that of the sense electrodes, and which is also kept at the same potential VSE; Figure 1. The overall transfer characteristic from the voltage applied to the input diode, (and thus the interface potential in the metering well MW) to the amount of image charge produced on the sense electrodes (and hence the output voltage V O ~ T ) can then be expected to be linear. Various possible ways to clamp the sense electrodes to a given potential and to extract the desired output signal have been discussed earlier’’2. A novel approach to extract this difference signal in the presence of the considerably larger common mode signal is shown in Figure 2. One amplifier (AD) performs the differencing operation, while the other amplifier ( Ac) is used to suppress the common mode signal on the two sense busses. AC operates by comparing the arithmetic mean of the sense electrode potentials to the sense voltage reference VSE and feeds back the same error signal to both sense busses through capacitors CCt and Cc. The feedback signal around AD through CDwll maintain the balance between the two sense busses, and the combined