MAX1101 Datasheet, PDF (10/12 Pages) Maxim Integrated Products – Single-Chip, 8-Bit CCD Digitizer with Clamp and 6-Bit PGA

Single-Chip, 8-Bit CCD Digitizer
with Clamp and 6-Bit PGA
pixel during the CCD output waveform’s reset phase. To
compensate for the CCD black level, CLAMP is activated
during the black-pixel portion of the linear array, as
shown in Figure 10. Each of these modes requires a dif-
ferent value of C_EXT, as described in the following sec-
tion.
Choosing C_EXT for CDS
In CDS applications, C_EXT = 4nF. This value is the best
compromise to minimize errors due to the CLAMP switch
resistance/C_EXT time constant and switch charge injec-
tion. The following equation represents the error due to
incomplete charging of C_EXT during integration time:
ε = ∆V_RESET x e^-t/RC
where ∆V_RESET = the maximum change in reset level
from one pixel to the next, t = CLAMP pulse width, and
R = CLAMP switch resistance (150Ωmax). At a sample
rate of 670kHz, with t = 750nsec, a 4nF capacitor
removes at least 70% of the change in reset voltage
level. Typically, R = 60Ω, which corresponds to a 96%
cancellation of the change in reset level.
The offset due to switch charge injection is represented
by 13pC / 4nF = 3mV. Note that this error will behave
like any DC offset; that is, it will be constant from pixel
to pixel.
Choosing C_EXT in Black-Level Compensation
In activating CLAMP once per line to compensate for the
CCD black level, the recommended value of C_EXT is
governed by the following equations:
C_EXT ≥ 12nF
and
C_EXT ≤ N x t x 760pF/µsec
where N is the number of light-shielded cells, and t is
the width of the CLAMP pulse in µsec.
The second equation ensures that the time constant
formed by R x C_EXT is small enough that the black level
is captured to within 0.5mV during the dark pixel
phase. For example, in an array with 27 dark pixels at a
670kHz sample rate, with t = 750nsec, the second
equation becomes C_EXT ≤ 15nF. Capacitors smaller
than 12nF can be used; however, offset increases due
to switch charge injection, as explained in the section
Choosing C_EXT for CDS.
DIGITAL
OUTPUT
11. . .111
11. . .110
11. . .101
100. . .000
00. . .111
00. . .110
00. . .101
00. . .100
00. . .011
00. . .010
00. . .001
00. . .000
-3
256
V_FS
-1
256
V_FS
1
256
V_FS
3
256
V_FS
-4
256
V_FS
-2
256
V_FS
2
256
V_FS
V_VIDEO
124
256
V_FS
249
256
V_FS
251
256
V_FS
250
256
V_FS
V_FS
=
V_REF+ - V_REF-
G_PGA
V_VIDEO = VOLTAGE DIFFERENCE BETWEEN THE
VIDEO LEVEL AND THE PRE-CHARGE (RESET) LEVEL.
Figure 11. Transfer Function for CCDIN
DIGITAL
OUTPUT
111. . . . 111
111. . . . 110
111. . . . 101
100. . . 000
000. . . 011
000. . . 010
000. . . 001
000. . . 000
1
256
V_FS
3
256
V_FS
V_REF-
2
256
V_FS
4
256
V_FS
V_FS = V_REF+ - V_REF-
Figure 12. Transfer Function for AIN
V_{AIN_}
253
256
V_FS
255
256
V_FS
254
256
V_FS
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