CS7622 datasheet(11/36 Pages) CIRRUS

11 / 36 page

CS7622

DS322PP1

During fixed gain mode the time constant is a little

different.

In order to achieve no ringing in the settling use,

for offset range = 1, and

for offset range

= 0.

The 9 MSBs of the black level accumulator can be

read or written through a register. If written, the

LSBs are set to zero. The black level is set to “8” in

a 10-bit digital output representation. In a 13-bit

representation, it is set to “64.” The power-up de-

fault value in the accumulator is at mid level.

Also note that the black level adjust loop can be

disabled. In addition, the black level can be pro-

grammed through the serial port.

3.3 Gain Adjust Block

In order to increase the dynamic range of the ADC,

a variable gain, whose value is determined by the

signal level, is applied to each pixel. This allows

for 13 bits of dynamic range and 10 bits of resolu-

tion after accounting for the significance of the

ADC output bits. The gain applied in the analog is

illustrated in the transfer curve in Figure 7. Once

the signal is digitized, the gain adjust block uses the

gain information for a given pixel word and shifts

its bits accordingly. For example, using a full scale

level of 1.0 V, if Vin = 0.3 V, the VGA would

choose a gain of 2X so the ADC input is 0.6 V. The

10-bit output of the ADC (with no black level) is

(0.6/1.0) × 1024 = 614, or “1001100110.” in bina-

ry. The gain adjust block will take this value plus

the bits representing the 2x gain and divide the out-

put by two (shift right by 1). The output of the gain

adjust block is then “0100110011.000.” Note that

the decimal point is virtual, having no existence in

silicon. It is representing the fact that we keep 3 ex-

tra bits of lower significance in the output. In the

same manner, if Vin = 0.75 V, a gain of 1X would

be chosen and the output of the gain adjust block

For a fixed gain of 1:

For a fixed gain of 2:

For a fixed gain of 4:

For a fixed gain of 8:

ADC

Z-1

DAC

BINARY

THERM

Z-1

FU = UPDATE FREQUENCY

FP = PIXEL FREQUENCY

CLIP

CDS/VGA

VIN

FROM SERIAL INTERFACE

BLK LVL LOOP

‘64’

Figure 10. Black level adjustment loop

GAIN REG

-------

–





----------------------------

–







-----





-------

–





----------------------------

–







-----





-------

–





----------------------------

–







-----





1nK

–

()

---------------------------

–



 1

-----





----

≤

-------

≤

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