 Pin Compatibility
with the Existing CXA2096N |
|
The CXA3796N is a bipolar IC that was
developed as a CCD camera head amplifier
appropriate for low-illumination imaging with
security cameras and similar products. After
applying noise exclusion by correlated double
sampling (CDS) to the pixel output signals
from the CCD, the signals are amplified with
an automatic gain control (AGC) amplifier
and output to the A/D converter in the DSP in
the later stages of the system.
The block diagram of the CXA3796N includes
the CDS circuit, the AGC amplifier for the
CCD output signal, a camera sample-and-hold
(S/H) circuit for the external A/D converter,
A/D converter reference voltage output circuits,
and a CCDLEVEL output circuit for
monitoring. (See figure 1.) The CXA3796N
is pin compatible with the existing CXA2096N, and can directly replace that device with no
changes. (The MAXGAIN and GAINSHIFT
pins correspond to NC and PS (Power Save)
pins in the CXA2096N, and operate in
CXA2096N mode when the CXA3796N
replaces the CXA2096N with no changes to
the application circuit.) |
| Nine Gain Curves |
|
Any of nine gain curves can be selected by the
voltages applied to the MAXGAIN and
GAINSHIFT pins. (See table 1 and figure 2.)
Since the CXA3796N is designed in particular
to also handle negative gains, it can provide a
long-desired and effective solution to the color
rolling phenomenon. |
Noise Reduced by 4 to 5 dB
|
|
The AGC amplifier circuit structure was
revised in the CXA3796N to achieve noise
reduction. The inclusion of a blanking function
that disables the CCDLEVEL output during
blanking also contributes to reduced noise.
Figure 3 shows the gain and output noise
characteristics. When compared at the same 26
dB gain, the CXA3796N provides a noise
reduction effect of 4 to 5 dB. (Note: This value
differs depending on the camera system.) |
| Black Level Set by CDS Clamping |
|
While the CCD pixel outputs have large
differences in the DC levels due to
temperature, sample-to-sample variations, and other factors, the CXA3796N can continuously
match this DC level to a reference voltage
black level using the externally provided
CLPDM signal. This makes it possible to
always maintain a fixed black level even under
different usage conditions.
Since an A/D converter is connected to the
CXA3796N in the following stages, the
CXA3796N output includes a sample-and-hold
function that is applied to each pixel.
This reduces problems such as signal
degradation due to inadequate settling. |
| Low-Illumination Imaging |
|
Photograph 1 shows an example of imaging
using this device. Excellent images can be
obtained even under low illumination due to
the reduced noise. |
CCD Camera Head Amplifier
with High-Gain/Low-Noise Amplifier
Vol.52