®

VSP2000

9

BLACK LEVEL AUTO-ZERO LOOP

The black level auto-zero loop amplifies the difference

between the output of the output amplifier and a reference

signal during the dummy pixel interval. This difference

signal is amplified and fed back into the output amplifier to

correct the offset. In doing so, the output level of the entire

CCD channel can be controlled to be approximately –FS +

32LSBs under zero signal conditions. The black level auto-

zero loop is activated by the OB timing signal.

Figure 5 shows a block diagram of the black level auto-zero

loop. The loop time constant is given by:

A/D CONVERTER

The A/D converter utilizes a pipline architecture. The fully

differential topology and digital error correction guarantee

10-bit resolution. The A/D converter circuitry includes a

reference circuit that provides bias voltages for the entire

system.

DECOUPLING AND GROUNDING

CONSIDERATIONS

The VSP2000 has several supply pins, one of which is

dedicated to supply only the digital output driver (pin 17,

case, divided into analog and digital supply pins since they

are internally connected on the chip. For this reason, it is

recommended that the VSP2000 be treated as an analog

component and to power if from the analog supply only.

Digital supply lines often carry high levels of wide band

noise which can couple back into the VSP2000 and limit

performance.

Figure 6 shows the recommended decoupling scheme for the

VSP2000. In most cases, 0.1

µF ceramic chip capacitors are

adequate to keep the impedance low over a wide frequency

range. Their effectiveness largely depends on the proximity

to the individual pin. Therefore, they should be located as

close as possible to the pins. In addition, one larger capacitor

(1

µF to 22µF) should be placed on the PC board in proxim-

ity of the VSP2000.

DEMONSTRATION BOARD

A demonstration board, DEM-VSP2000, is available to

assist in the inital evaluation of the circuit performance

using the VSP2000. The schematic and board layout of the

DEM-VSP2000 are shown in Figure 6 and Figures 7a

through 7d, respectively.

T =

C

where C is the external filter capacitance applied to pin 24

(C), G

black level auto-zero loop is in operation. The duty cycle (D)

must be considered as the loop operates in a sampled mode.

Operation of the black level auto-zero loop is activated by

the OB signal that happens once during each horizontal line

interval.

OB

To ADS

32LSB

Error

Amplifier

From

VCA

C

C

FIGURE 5. Black Level Auto-Zero Loop.