ADA4430-1

Rev. C | Page 12 of 20

THEORY OF OPERATION

OVERVIEW

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 ADA4430-1 is designed for exceptional performance as

both a filter and a low power driver for portable video applica-

tions. 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

normally indicates a single op amp with a two- or three-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 shut down to draw less than 0.1 μA of supply

current using the disable pin (DIS). Additionally, the ADA4430-

1 is unique in that it is a high order filter that fits into an SC70

package.

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.

The ADA4430-1 provides a minimum 1 dB bandwidth of

5.5 MHz and a typical stop-band rejection of 50 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.

3V

VIDEO

DAC/

ENCODER

75Ω

75Ω

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.

Figure 27. DAC Driving Video Cable Directly

0.1µF

3V

RL

VIDEO

DAC/

ENCODER

ADA4430-1

FILTER

G = +2

75Ω

75Ω

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.

Figure 28. DAC Driving Video Cable Using the ADA4430-1