ADA4430-1 Analog Devices, ADA4430-1 Datasheet - Page 11

• Manufacturer Part Number: ADA4430-1
• Description: Ultralow Power Video Filter
• Manufacturer: Analog Devices

THEORY OF OPERATION
OVERVIEW
The ADA4430-1 is designed for exceptional performance as
both a filter and a low power driver for portable video
applications. This performance is achieved by providing high
order filtering without trading off power consumption or device
size. While consuming only 1.85 mA quiescent supply current,
the ADA4430-1 provides video output on a single-supply as low
as 2.5 V. Such low power consumption and low supply operation
would normally indicate a single op amp with a 2- or 3-pole
roll-off; however, the ADA4430-1 achieves a sixth-order roll-off
in addition to a 10 MΩ input impedance for easy clamping and
lower DAC output power requirements. When not in use, the
ADA44330-1 can be shutdown to draw less than 1 μA of supply
current using the power-down pin, ( PD ). Additionally, the
ADA4430-1 is unique in that it is a high order filter that fits into
an SC70 package.
The ADA4430-1 provides a minimum 1 dB bandwidth of
5.5 MHz and a minimum stop-band rejection of 42 dB at
27 MHz. Phase response is not sacrificed in spite of the
exceptional filtering performance of the ADA4430-1, as
exhibited by its group delay, which varies by only 7 ns from
100 kHz to 5 MHz.
The ADA4430-1 is intended for use in applications that have
both ac- and dc-coupled inputs and outputs. The rail-to-rail
buffer on the ADA4430-1 output is able to drive 2 V p-p video
signals into two doubly-terminated video loads (150 Ω each) on
a single 2.5 V supply. The ADA4430-1 has a gain of 2 when the
SAG correction pin is tied directly to the output, which makes
up for the 6 dB termination loss. When the SAG feature is used
(see Figure 29), the ADA4430-1 has a low frequency gain of
2.5 (≈ 8 dB) and a high frequency gain of 2. Signal offsets and
supply levels must be considered when using the SAG correction
feature to ensure that there are no headroom issues.
The input range of the ADA4430-1 includes ground, while the
output range is limited by the saturation of the output devices.
Saturation occurs several tens of mV from the positive and
negative supply rails. For accurate reproduction of ground-
referenced input signals, an internal offset is used to shift the
output up by 95 mV.
The high input impedance and low input capacitance of the
ADA4430-1 offer advantages in a number of low power
applications. In reconstruction filter applications, the DAC can
be placed in its lowest power mode, allowing the use of a large-
valued load resistor. Using a large-valued load resistor does not
interfere with the frequency response of the ADA4430-1.
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The internal buffer at the ADA4430-1 input isolates the source
resistance feeding the ADA4430-1 from the internal filter networks.
High input impedance is also advantageous when using video
clamping circuits.
The output buffer feedback network used to create a gain of 2 is
connected internally to the GND pin and has a nominal impedance
of 5.2 kΩ. The current required to drive this feedback network
causes the overall supply current to vary based on the output
level. The feedback impedance was chosen specifically to
minimize excess current consumption while maintaining
optimal frequency behavior.
POWER SAVINGS USING THE ADA4430-1
Using a series source termination and a shunt load termination
on a low supply voltage with the ADA4430-1 realizes significant
power savings compared with driving a video cable directly from
a DAC output. Figure 27 shows a video DAC driving a cable
directly. Properly terminating the line results in the DAC driving
two 75 Ω loads and requires in excess of 30 mA to reach a full-
scale level of 1.3 V. Figure 28 shows the same video load being
driven using the ADA4430-1 and a series-shunt termination. This
requires two times the output voltage to drive the equivalent of
150 Ω but only requires a little more than 15 mA to reach a full-
scale output. When running on the same supply voltage as the
DAC, this results in nearly a factor of two reduction in power
compared with the circuit in Figure 27. The high level of
filtering provided by the ADA4430-1 lowers the requirements
on the DAC oversampling ratio, realizing further power savings.
On any given DAC, 8× and 16× oversampling ratios can require
twice the power consumption of a 4× oversampling ratio.
ENCODER
VIDEO
DAC/
Figure 28. DAC Driving Video Cable Using the ADA4430-1
ENCODER
VIDEO
DAC/
3V
Figure 27. DAC Driving Video Cable Directly
R
L
ADA4430-1
75Ω
FILTER
G = +2
3V
0.1µF
75Ω
ADA4430-1
75Ω
75Ω