XRD9814B Datasheet, PDF (21/53 Pages) Exar Corporation – 3-Channel 14/16-Bit Linear CCD/CIS Sensor Signal Processors

XRD9814B/9816B
Minimum Capacitance (CDS Line Mode)
In general, the minimum value coupling capacitance is
limited by the amount of droop which can occur before
the input voltage range of the input amplifier is ex-
ceeded. The input capacitor droop is related to the
input bias current by:
Vdroop = ^{Ibias ⋅ n ⋅ T}
C
where I_bias = input bias current
n = number of pixels per line
T = pixel period
If the minimum input voltage is allowed to equal the 0V
input voltage of the XRD9814B/9816B, the maximum
allowable droop will be equal to the clamp level minus
the difference between the black and video levels. For
example, if Vc=4V, and the CCD video output is -2V
relative to the black level the maximum allowable
droop is equal to 2V.
Using the previous equation and assuming T=500ns,
n=3000
10nA ⋅ 3000 ⋅ 500 ns
C min =
= 7.5pF
2V
Note: These are the absolute minimum capacitor require-
ments. As stated for pixel-mode, a minimum value of 68pF
is recommended.
Minimum Capacitance (S/H Line Mode)
Unlike Line or Pixel CDS modes voltage droop across
a line appears as an absolute error and is the dominant
factor in determining the minimum coupling capacitor
size.
Ibias ⋅ n ⋅ T
C min =
Vε
where I_bias=input bias current
n=number of pixels per line
Assuming n=3000, T=500nS, I=10nA and Ve=12.5uV,
the minimum required capacitor is 1.2uF.
Maximum Capacitance (S/H Line Mode)
The maximum capacitance is determined by the
amount of time allowed to charge the coupling capaci-
tor. In order to minimize the charging time, the maxi-
mum capacitor can be set to the minimum value as
previously calculated. In this case the time required to
charge the capacitor is:
t
=
( Rs
+
Rc) ⋅
C
min ⋅
ln

Vr −
Vε
Vc

where t = clamp pulse - width ( SYNCH )
Rc = clamp resistance
Rs = signal source - resistance
Vr = input reference level
Vc = XRD9814B/9816B clamp voltage
Vε = error voltage
Cmin = coupling capacitor
Assuming that Vr=.5 Vc=0V, Vε=12.5uV, Rc=100Ω,
Rs=500Ω and C=1.2uF, the minimum clamp period is
equal to 1.9mS.
Rev. 1.00
21