CS7620 datasheet(9/70 Pages) CIRRUS

9 / 70 page

CS7620

DS301PP2

3OPERATION

CDS/VGA (correlated double sampling/vari-

able gain amplification)

An idealized waveform of the CCD output is

shown in Figure 6.

The CCD output contains reset noise, thermal

noise, and 1/f noise generated in the CCD output

circuit. This degrades the S/N ratio and must be

cancelled. Since the noise during the active video

portion of the CCD signal is assumed to be corre-

lated with the noise during the feed through portion

of the signal, this noise can be cancelled by sub-

tracting the feed through level from the video level.

This operation is called correlated double sam-

pling. The active video signal is the difference be-

tween the feed through and video levels. The active

video signal varies according to light conditions. In

order to insure that the full dynamic range of the

ADC is utilized even under low light conditions,

the CCD output is amplified using a VGA. The

gain control is provided by a 2 bit control word

generated by an ADC after stage 1, which has a

gain of 1. Based on the input voltage, a gain of 1x,

2x, 4x, or 8x is subsequently applied to the signal.

The amount of gain is later adjusted in the digital

section. After the VGA, the signal gets digitized by

a 10 bit ADC. The 2 bit ADC output is used in

combination with the 10 bit ADC output to produce

a 13 bit output.

Adding more gain before the ADC does not offer

performance improvement because the noise of the

CCD (after gain is applied to it) begins to dominate

over the quantization noise. Any additional gain

should be done in digital since the performance is

the same as when the ADC output has the addition-

al gain applied.

In order to add more flexibility, the full scale input

range is programmable through register 07h. This

setting will determine what input level maps to the

highest ADC output code. Thus depending on the

saturation level of the particular CCD used in the

system, an appropriate full scale input level can be

chosen in the CS7620. The choices of full scale in-

put level are 1.6V, 1.07V, 0.53V with 1.07V the

default. In the remainder of this document, all the

figures and discussions assume a full scale level of

1.07 is used. If a different full scale level is used,

all the voltages scale up or down by x1.5 or x0.5 for

1.6V and 0.53V full scale levels respectively.

The transfer function of the VGA portion of the cir-

cuit is shown in Figure 7 with full scale level =

1.07. It is assumed that the CDS has already been

performed. If desired, the gain switching function-

ality can be disabled and forced to a fixed gain of

8x, 4x, 2x, or 1x. This way any dynamic range en-

hancement is lost and the digital output is only 10

bits. If a fixed gain of 1x is selected, DOUT[12:3]

is used as the output, a fixed gain of 2x will use

DOUT[11:2], etc. In order to use this mode, the

RESET LEVEL

FEED THROUGH

LEVEL

VIDEO LEVEL

PIXEL PERIOD

Figure 6. Idealized CCD output waveform

DARK

MAX. BRIGHTNESS

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