Download CIRCUIT FUNCTION AND BENEFITS

CIRCUIT FUNCTION AND BENEFITS
This circuit illustrates a digital-to-analog video converter paired
with a low cost, low power, fully integrated reconstruction video
filter. Although many video encoders (video DACs) such as the
ADV7393 can drive a video load directly, it is often very
beneficial to use a video driver at the output of a video encoder
for power savings, line driving, and additional circuit
protection. The video driver is typically configured as an active
filter, also known as a reconstruction or anti-imaging filter. The
purpose of the reconstruction filter is twofold: it blocks the
higher frequency components (above the Nyquist frequency)
that were introduced into the video signal as part of the
digitization process and provides gain to drive the external
75 Ω cable to the video display. For ac-coupled output
applications, the ADA4430-1 has an integrated SAG correction
network. Signal amplitude gain (SAG) correction is used to
provide low frequency compensation for the high-pass filter
formed by the 150 Ω video load of a back-terminated cable and
the output coupling capacitor. SAG correction reduces the size
of the traditionally large ac coupling capacitor (330 µF),
replacing it with much smaller 47 μF and 22 μF capacitors.
The ADA4430-1 and the ADV7393 are both recommended for
automotive application, which makes both products ideal for
infotainment and vision-based safety systems applications.
CIRCUIT DESCRIPTION
The ADV7393 is a low power, fully integrated digital video
encoder that converts digital 16-bit component video data, for
example from a CMOS rearview camera, into a standard analog
baseband video signal compatible with worldwide standards.
The three 2.6 V/3.46 V 10-bit video DACs provide support for
composite (CVBS), S-video (YC), or component (YPrPb/RGB)
analog outputs in either standard definition (SD) or high
definition (HD) video formats. The circuit in Figure 1 is
configured for low output drive operation, which reduces the
output current by 85% in all three DACs (RSET = 4.12 kΩ,
RL = 300 Ω). To further save power, the internal PLL is disabled,
and the encoder is operating in a 2× oversampling mode. The
16-bit video input port is configured to support SD video.
The ADA4430-1 is a single sixth-order, low-pass video filter
with a 9 MHz −3 dB cutoff and 50 dB of out-of-band rejection
at 27 MHz. The ADA4430-1 is a low power, low cost, rail-to-rail
output amplifier that has an operating supply range of +2.5 V to
+6 V and is ideal for SD video application. Combined with the
ADV7393 single chip video encoder, the ADA4430-1 enables
the most power efficient video output solution for automotive
applications. In Figure 1, the ADA4430-1 is configured as a
reconstruction video filter with ac-coupled output with SAG
correction. The compensation network includes C1, C2, and the
internal resistor network integrated into the ADA4430-1.
Traditional ac-coupling uses a large, expensive coupling
capacitor, making it costly and wastes valuable PCB space. SAG
correction allows two small, low cost capacitors to replace the
one large ac-coupling capacitor and still maintain the same field
tilt. Field tilt is a measure of the voltage droop (tilt) that occurs
on the ac-coupling capacitor when a constant luma signal is
applied. This droop is caused by the small discharge current
created by the 75 Ω load resistor. The SAG correction capacitor
values were chosen so as to achieve the equivalent field tilt of a
220 µF ac-coupling capacitor.
When using the SAG correction circuit, the gain from the input
to the immediate output of the ADA4430-1 is 2.5 (≈8 dB) at
extremely low frequencies, where the outer feedback loop
formed by the 22 μF capacitor effectively opens (see Figure 2)
and exhibits a second-order peak of approximately 11 dB at
approximately 5 Hz. This gain is approximately 7.5 dB at 30 Hz.
The extra gain must be accounted for when considering low
frequency input and output signal swings to keep them within
their specified limits. The gain from the ADA4430-1 input to
the load side of the 47 μF capacitor does not exhibit this
behavior; instead, it appears more like a single-pole high-pass
response.
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FERRITE BEAD
VDD_IO
33µF
0.1µF
10µF
GND_IO
GND_IO
GND_IO
FERRITE BEAD
VDD_IO POWER
SUPPLY
DECOUPLING
0.01µF
GND_IO
PVDD
33µF
0.1µF
10µF
PGND
PGND
PGND
FERRITE BEAD
PVDD POWER
SUPPLY
DECOUPLING
0.01µF
PGND
VAA
33µF
0.1µF
10µF
AGND
AGND
AGND
FERRITE BEAD
0.01µF
33µF
DGND
0.1µF
10µF
DGND
DGND
VAA POWER
SUPPLY
AGND DECOUPLING
1µF
AGND
VDD
0.01µF
DGND
VDD_IO = 3.3V (1.71V TO 3.63V)
PVDD = 1.8V (1.71V TO 1.89V)
VAA = 3.3V (2.6V TO 3.465V)
VDD = 1.8V (1.71V TO 1.89V)
VDD POWER SUPPLY
DECOUPLING FOR
EACH POWER PIN
VAA
VDD_IO
P0
P1
P2
P3
P4
P5
P6
P7
PVDD
VDD
VDD
VAA
2.2nF
COMP
RSET
ADV739x
4.12kΩ
AGND
VAA
PIXEL PORT INPUTS
P8
P9
P10
P11
P12
P13
P14
P15
POWER-DOWN CONTROL
ADV7392/
ADV7393
ONLY
1
6
1x
×1
4
GND
2
HSYNC
VSYNC
3
VOUT 47µF 75Ω
DAC 1
2.6kΩ
AGND
SAG
CLOCK INPUT
5
VS+
VIN
DAC 1
300Ω
CONTROL
INPUTS/OUTPUTS
0.1µF
PD
ADA4430-1
2.6kΩ
1.3kΩ
2.6kΩ
CLKIN
22µF
MPU PORT
INPUTS/OUTPUTS
SDA/SCLK
SCL/MCSI
SFL/MISO
ALSB/SPI_SS
ADA4430-1
75Ω
DAC 2
DAC 2
LPF
RESET
300Ω
EXTERNAL LOOP FILTER
AGND
ADA4430-1
12nF
EXT_LF
150nF
170Ω
75Ω
DAC 3
DAC 3
LPF
LOOP FILTER COMPONENTS
SHOULD BE LOCATED
AGND PGND DGND DGND GND_IO
CLOSE TO THE EXT_LF
PIN AND ON THE
SAME SIDE OF THE PCB
AS THE ADV739x.
300Ω
AGND
AGND PGND DGND DGND GND_IO
Figure 1. Low Cost, Fully Integrated Reconstruction Filter using the ADA4430-1(Simplified Schematic)
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08373-001
PVDD
Figure 2 illustrates the SAG frequency response immediately at
the ADA4430-1 output and at the load side of the 47 μF
capacitor.
10
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
8
Data Sheets
6
ADA4430-1 Data Sheet.
4
GAIN (dB)
Kester, Walt. 2005. The Data Conversion Handbook. Analog
Devices. Chapters 8 and 9.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of AGND and DGND. Analog Devices.
12
2
ADV7393 Data Sheet.
0
–2
REVISION HISTORY
–4
7/09—Revision 0: Initial Version
AT ADA4430-1 OUTPUT
AT LOAD SIDE OF 47µF CAPACITOR
–8
1
10
100
1k
10k
08373-002
–6
–10
LEARN MORE
100k
FREQUENCY (Hz)
Figure 2. SAG Correction Frequency Response at ADA4430-1 Output and at
the Load Side of the 47 µF Capacitor
Buffering is a function that is often overlooked, but is very
important. For example, many automotive customers will use
low cost amplifiers to protect more expensive and complex
devices, such as video decoders and encoders. Amplifiers with
ac-coupled outputs as shown in Figure 1 help protect such
devices from overvoltage and ESD damage.
COMMON VARIATIONS
There are a few options for ac-coupled output configuration.
SAG correction, as shown in Figure 1, uses 22 μF and 47 μF
capacitors. The traditional ac-coupled output uses a single
220 μF capacitor; in this configuration, the SAG pin is
connected directly to the output pin before the output
capacitor. For dc-coupled configurations, the SAG pin is also
connected directly to the output pin. In both dc-coupled and
traditional ac-coupled configurations, connecting the SAG pin
directly to the output pin results in a gain of +2 buffer at all
video frequencies.
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CN08373-0-7/09(0)
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