xrd9816acv Exar Corporation, xrd9816acv Datasheet - Page 21

• Manufacturer Part Number: xrd9816acv
• Description: 3-channel 14/16-bit Linear Ccd/cis Sensor Signal Processors
• Manufacturer: Exar Corporation

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:
where I
If the minimum input voltage is allowed to equal the 0V
input voltage of the XRD9814/9816, 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
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.
where I
Assuming n=3000, T=500nS, I=10nA and Ve=12.5uV,
the minimum required capacitor is 1.2uF.
n=number of pixels per line
bias
bias
n = number of pixels per line
C
T = pixel period
Rev. 1.00
=input bias current
min =
= input bias current
C
Vdroop
min =
10
nA
=
Ibias n T
Ibias n T
3000 500
2
V
C
V
ns
= 7.5pF
21
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:
where t = clamp pulse - width ( SYNCH )
Assuming that Vr=.5 Vc=0V, V =12.5uV, Rc=100 ,
Rs=500 and C=1.2uF, the minimum clamp period is
equal to 1.9mS.
Cmin = coupling capacitor
Rc = clamp resistance
Rs = signal source - resistance
Vr = input reference level
Vc = XRD9814/9816 clamp voltage
V = error voltage
t
Rs
XRD9814/9816
Rc
C
min ln
Vr
V
Vc