LM9801 datasheet(28/34 Pages) NSC

28 / 34 page

Applications Information (Continued)

TLH12814 – 40

FIGURE 13 Parallel Output CCD Timing

To maximize the integration time for the Red Green and

Blue photodiodes the transfer (TR) pulses should be stag-

gered as shown in

Figure 13 This is done by a demultiplex-

er (Mux 2) between the TR output of the LM9801 and the

transfer gate inputs of the CCD If the CCD’s transfer gate

input capacitance is relatively low (see the CCD datasheet

for this specification and the requirements for TR pulse rise

and fall time) then the other half of the 74HC4052 may be

used to switch the TR pulses as shown If the TR gate input

capacitance is so large that the minimum TR rise and fall

times can not be met because of the 200X max on resist-

ance of the 74HC4052’s switches then the 74HC4052 can

not be used to multiplex the TR output and should be re-

placed with an active device such as the 74HC155 dual 2-

to-4 demultiplexer

Two signals (A and B) must be generated to choose which

color is going to be digitized and receives the TR pulse

These signals can be as simple as the output of a two bit

counter that counts from 0 to 2 (0 1 2 0 1 2 etc) This

counter should be incremented after the end of the previous

line and before the first transfer pulse of the next line Also

since each color will need a different VGA gain the appro-

priate VGA gain value for each color should be sent to the

LM9801 during this time

72 Parallel Output CCD Three LM9801s

Figure 14 uses three LM9801s to achieve a 75 Mpixelsec

(25M RGB pixelssec) pixel rate The three LM9801s are

synchronized by applying the same MCLK and SYNC sig-

nals to all three devices One LM9801 provides the clock

signals required for the CCD Since the coefficient data for

all three LM9801s will be latched simultaneously on the ris-

ing edge of CCLK the correction coefficient bus must either

be at least 24 bits wide (8 correction coefficient bits by 3

LM9801s) or run at a 75 MHz rate and be latched into a

buffer between the correction coefficient databus and each

LM9801 Similarly the output data for all three LM9801s will

be available simultaneously at the 3 output databusses

Since each LM9801 is dedicated to one color the VGA gain

does not change during line scan

TLH12814 – 41

FIGURE 14 Parallel Output CCD Three LM9801

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