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4.2.3 Correlated-Clamp-Sample-and-Hold (CCSH) Amplifier with Charge
Detection Nodes:
Charge is converted into a video signal by transferring the charge onto a floating diffusion
structure in detection node 1 that is connected to the gate of a MOS transistor Q1. The
proportional charge-induced signal is then processed by the circuit shown above.
This circuit consists of a low pass filter formed by Q1 and C2, coupling capacitor C1, dummy
detection node 2, which restores the DC bias on the gate of Q3, sampling transistor Q5, holding
capacitor C3, and output buffer Q6. Transistors Q2, Q4 and Q7 are current sources for each
corresponding stage of the amplifier. The parameters of this high-performance signal processing
amplifier have been optimized to minimize noise and maximize the video signal.
The signal processing begins with a reset of detection node 1 and a restoration of the DC bias on
the gate of transistor Q3 through the clamping function of dummy detection node 2. After the
clamping is completed, the new charge packet is transferred onto detection node 1. The resulting
signal is sampled by the sampling transistor Q5 and is stored on the holding capacitor C3. This
process is repeated periodically and is correlated to the charge transfer in the registers. The
correlation is achieved automatically since the same clock lines used in shift registers S2 and S3
for charge transport serve for reset and sample. The multiple use of the clock lines significantly
reduces the number of signals required to operate the sensor. The amplifier also contains an
internal reference voltage generator that provides the reference bias for the reset and clamp
transistors. The detection nodes and the corresponding amplifiers are located some distance
away from the edge of the storage area. Therefore 11 dummy elements are incorporated at the
end of each serial register to span the distance. The location of the dummy elements, which are
considered to be a part of the amplifier is shown in the block diagram.
The above sequence of events helps to remove the noise (called kTc noise) that would otherwise
have been introduced due to the "floating" node. By clamping it to a "known" reference between
pixel values, and then charging it with the "delta" or the difference of potential due to the new
pixel value, a significant source of noise is removed.
Figure 4-4: TC-245 CCSH Amplifier Circuit (

Texas Instruments, 1994)