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參數(shù)資料
| 型號： | LM9801CCV |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 模擬信號調(diào)理 |
| 英文描述： | Greyscale/24-Bit Color Linear CCD Sensor Processor |
| 中文描述： | SPECIALTY ANALOG CIRCUIT, PQCC52 |
| 封裝： | PLASTIC, LCC-52 |
| 文件頁數(shù)： | 23/34頁 |
| 文件大?。?/td> | 520K |
| 代理商： | LM9801CCV |
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Applications Information
(Continued)
4.4 Correlated Double Sampler (CDS)
Figure 4 shows the output stage of a typical CCD and the
resulting output waveform:
TL/H/12814–31
FIGURE 4. CDS
Capacitor C1 converts the electrons coming from the CCD’s
shift register to an analog voltage. The source follower out-
put stage (Q2) buffers this voltage before it leaves the CCD.
Q1 resets the voltage across capacitor C1 in between every
pixel at intervals 2 and 5. When Q1 is on, the output signal
(OS) is at its maximum. After Q1 turns off (period 3), the OS
level
represents
the
residual
(V
RESIDUAL
). V
RESIDUAL
includes charge injection from Q1,
thermal noise from the ON resistance of Q1, and other
sources of error. When the shift register clock (
w
1) makes a
low to high transition (period 4), the electrons from the next
pixel flow into C1. The charge across C1 now contains the
voltage proportional to the number of electrons plus
V
RESIDUAL
, an error term. If OS is sampled at the end of
period 3 and that voltage is subtracted from the OS at the
end of period 4, the V
RESIDUAL
term is canceled and the
noise on the signal is reduced. (
[
V
SIGNAL
a
V
RESIDUAL
]
b
V
RESIDUAL
e
V
SIGNAL
). This is the principal of Correlated
Double Sampling.
voltage
across
C1
The LM9801 implements CDS with two switched-capacitor
S/H amplifiers. The S/Hs acquire a signal within a 50 ns
window which can be placed anywhere in the pixel period
with 25 ns precision. See Diagrams 7 and 8 for more de-
tailed timing information.
4.5 CIS Mode
The LM9801 provides some support for CIS (Contact Image
Sensor) devices by offering a sampling mode for capturing
positive going signals, as opposed to the CCD’s negative
going signal.
TL/H/12814–32
FIGURE 5. CIS vs CCD Output Signals
While CIS devices do not usually have a reference level with
which to perform correlated double sampling, many have a
very repeatable reset level which can be used as a black
reference allowing the LM9801 to perform pseudo CDS on
the signal. When the Signal Polarity bit is set to a zero, the
LM9801 expects a positive going signal, typically from a CIS
device. When the Signal Polarity bit is set to a one, the
LM9801 expects a negative going signal, typically from a
CCD sensor.
4.6 Offset DAC
The 4 bit plus sign Offset DAC is used to compensate for
DC offsets due to the correlated double sampling stage.
The offset can be corrected in 31 steps of 0.42 ADC LSB
size between
b
6.3 and
a
6.3 LSBs. Note that the DAC
comes betore the PGA, so any offset errors at this stage are
multiplied by the gain of the PGA. The calibration procedure
described in Section 5.0 demonstrates how to use the DAC
to eliminate offset errors before scanning begins.
Note that this DAC is programmed during LM9801 calibra-
tion/configuration and is not meant to compensate for pixel-
to-pixel CCD offset errors. CDS cancels the pixel-rate offset
errors.
4.7 Programmable Gain Amplifier (PGA)
Gain
The PGA provides 8 bits of pixel-to-pixel gain correction
over a 0 dB to 9 dB (x1 to x3) range. After the input signal is
sampled and held by the CDS stage, it is amplified by the
gain indicated by the data (‘‘PGA Code’’) on the CD0–CD7
databus using the formula:
V
V
e
1
a
PGA code
256
1.95
4.8 Offset Add Bit
In addition to the Offset DAC, there is a bit in the configura-
tion register which, when set, adds a positive 2 LSB offset at
the output of the PGA. This offset ensures that any offset
between the output of the PGA and the ADC is positive, so
that no dark level information is lost due to negative offsets.
The calibration procedure described in Section 5.0 demon-
strates how to set this bit.
http://www.national.com
23
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相關代理商/技術參數(shù) |
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|---|---|
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| LM9810 WAF | 制造商:Texas Instruments 功能描述: |
| LM9810CCWM | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:LM9810/20 10/12-Bit Image Sensor Processor Analog Front End |
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