VSP2272

SLES020 – DECEMBER 2001

9

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PRINCIPLES OF OPERATION

introduction

The VSP2272 device is a complete mixed-signal IC that contains all the key features associated with the

processing of CCD imager output signals in a video camera, a digital still camera, security camera, or similar

applications. A simplified block diagram is shown on page 3 of this data sheet. The VSP2272 device includes

correlated double sampler (CDS), programmable gain amplifier (PGA), analog-to-digital converter (ADC), input

clamp, optical black (OB) level clamp loop, serial interface, timing control, and reference voltage generator.

An off-chip emitter follower buffer is recommended between the CCD output and the VSP2272 CCDIN input.

The PGA gain control, the clock polarity setting, and the operation mode can be selected through the serial

interface. All parameters are reset to their default values when pin 45 (RESET) goes low asynchronously from

the clocks.

correlated double sampler (CDS)

The output signal of a CCD imager is sampled twice during one pixel period; once at the reference interval and

again at the data interval. Subtracting these two samples extracts the video information of the pixel and removes

any noise that is common—or correlated—to both intervals. Thus, the CDS reduces the reset noise and the low

frequency noises that are present on the CCD output signal. Figure 1 shows the simplified block diagram of the

CDS and input clamp.

–

+

CLPDM

VSP2272

OPA

C1 = 10 pF

C2 = 10 pF

SHP

SHD

CCDIN

SHP

CM (1.5 V)

CIN

CCD Output

Figure 1. Simplified Block Diagram of CDS and Input Clamp

the dc level of the CCD output signal is usually too high (several volts) for the CDS to work properly. A 0.1-

µF

Also, an off-chip emitter follower buffer is recommended to drive more than 10 pF, because the 10-pF sampling

capacitor and a few pF of stray capacitance can be seen at the input pin. The analog input signal range at pin 30

(CCDIN) is 1 Vp-p, and the appropriate common mode voltage for the CDS is around 0.5 to 1.5 V.