參數(shù)資料
型號(hào)： LM9812
廠商： National Semiconductor Corporation
英文描述： LM9812 30-Bit Color Linear CCD Sensor Processor
中文描述： LM9812 30位彩色線陣CCD傳感器處理器
文件頁(yè)數(shù)： 32/37頁(yè)
文件大?。?/td> 479K
代理商： LM9812
32
http://www.national.com
3.0 ANALOG INTERFACE
3.1 Voltage Reference
The V
REF IN
pin should be connected to a 2V±5% reference volt-
age. The LM4041-ADJ adjustable bandgap reference is recom-
mended for this application, as shown in Figure 6. The
inexpensive “D” grade meets all the requirements of the applica-
tion and is available in a TO-92 (LM4041DIZ-ADJ) package as
well as a SOT-23 package (LM4041DIM3-ADJ) to minimize board
space. To reduce noise, the V
REF IN
pin should be bypassed to
AGND with a 0.1μF monolithic capacitor.
The LM9812 generates three internal reference voltages for its
analog front end: V
REF MID
, V
REF HI
, and V
REF LO
. V
REF MID
is
equal to 0.49*V
A
, or 2.45V with a nominal 5V supply. V
REF IN
drives a transconductance amplifier with two output currents, -
V
REF IN
/2 and +V
/2. These currents go into the V
and
V
reference voltage buffers, which generate a nominal
3.45V (V
= V
+ V
/2), and 1.45V (V
=
V
- V
/2). To minimize noise, the reference amplifiers
sense (IN) inputs are brought out to the chip for bypassing. Each
buffer’s output and input should be tied together and decoupled to
AGND through a 0.1μF monolithic capacitor.
3.2 Clamp Capacitor Selection
This section is very long because it is relatively complicated to
explain, but the answer is short and simple: A clamp capacitor
value of 0.01μF should work in almost all applications. The rest of
this section describes exactly how this value is selected.
The output signal of many sensors rides on a DC offset (greater
than 5V for many CCDs) which is incompatible with the LM9812’s
5V operation. To eliminate this offset without resorting to addi-
tional higher voltage components, the output of the sensor is AC
coupled to the LM9812 through a DC blocking capacitor, C
CLAMP
(the CCD’s DOS output, if available, is not used). The value of
this capacitor is determined by the leakage current of the
LM9812’s OS input and the output impedance of the sensor. The
leakage through the OS input determines how quickly the capaci-
tor value will drift from the clamp value of V
REF MID
, which then
determines how many pixels can be processed before the droop
causes errors in the conversion (±0.1V is the recommended
limit). The output impedance of the sensor determines how
quickly the capacitor can be charged to the clamp value during
the black reference period at the beginning of every line.
The minimum clamp capacitor value is determined by the maxi-
mum droop the LM9812 can tolerate while converting one sensor
line. The following equation takes the maximum leakage current
into the OS input, the maximum allowable droop (100mV), the
number of pixels on the sensor, and the pixel conversion rate
(f
MCLK
/12 for triple output sensors or f
MCLK
/8 for dual output sen-
sors) and provides the minimum clamp capacitor value:
For example, if the OS input leakage current is 20nA worst-case,
the sensor has 2700 active pixels, the conversion rate is 2MHz
(f
MCLK
= 24MHz, triple output sensor), and the max droop desired
is 0.1V, the minimum clamp capacitor value is:
RS
OS
CDS
(Internal)
Diagram 5: x4 Preview Mode Timing
Hold Ref
Hold Signal
1
(Even/Odd Mode)
2
(Even/Odd Mode)
1
(Standard Mode)
2
(Standard Mode)
1
1.5 1.5
7.5
1
1
1.5
1.5
6
0.5
6
7.5
All lengths given in units of MCLK periods
p
n
p
n+1
p
n+2
p
n+3
I=V
REF IN
/2R
Reference
Amplifiers
LM4041-ADJ
0.1μF
0.1μF
Figure 6: Voltage Reference Generation
LM9812
V
REF IN
V
REF LO OUT
V
REF LO IN
V
REF MID OUT
V
REF MID IN
0.1μF
V
REF HI OUT
V
REF HI IN
0.1μF
10.1k
10.5k
V
A
R
R
3k
4.7k
V
A
3k
-I
+I
DOS
OS
OS
SENSOR
NC
C
CLAMP
Figure 7: OS Clamp Capacitor and Internal Clamp
2.450V
LM9812
CCLAMP MIN
dV
Equation 6: C
CLAMP MIN
Calculation
--- i
=
lemax droop(V)
conversion rate (Hz)
=
CCLAMP MIN
20nA
270pF
0.1V
2MHz
=
=
Equation 7: C
CLAMP MIN
Example
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