Datasheet AD8038, AD8039 (Analog Devices) - 14

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AD8038/AD8039 APPLICATIONS INFORMATION LOW POWER ADC DRIVER LOW POWER ACTIVE VIDEO FILTER 1kΩ 2.5V
Some composite video signals derived from a digital source
+5V 0.1µF 10µF
contain clock feedthrough that can limit picture quality. Active
3V
filters made from op amps can be used in this application, but
0.1µF 10µF
they consume 25 mW to 30 mW for each channel. In power-
1kΩ 8 3 REF
sensitive applications, this can be too much, requiring the use
1 50Ω V VINP IN 2
of passive filters that can create impedance matching problems
0V 1kΩ 1kΩ
when driving any significant load.
AD9203 AD8039
The AD8038 can be used to make an effective low-pass active
1kΩ 1kΩ 6
filter that consumes one-fifth of the power consumed by an
7 50Ω VINN
active filter made from an op amp. Figure 44 shows a circuit
5 4
that uses a AD8038 with ±2.5 V supplies to create a three-pole
1kΩ 0.1µF 10µF
Sallen-Key filter. This circuit uses a single RC pole in front of a 3 standard 2-pole active section. 04
–5V 1kΩ
1- 95 02
RF 680pF
Figure 43. Schematic to Drive AD9203 with the AD8039
1Ω +2.5V
The AD9203 is a low power (125 mW on a 5 V supply), 40 MSPS 10-bit converter. As such, the low power, high performance
0.1µF 10µF
AD8039 is an appropriate amplifier choice to drive it.
R1 R2 R3 V 200Ω 499Ω 49.9Ω AD8038 OUT R5
In low supply voltage applications, differential analog inputs
VIN 75Ω R4 C1 C3
are needed to increase the dynamic range of the ADC inputs.
49.9Ω 100pF 33pF 0.1µF 10µF –2.5V
044 Differential driving can also reduce second and other even-order 02951- distortion products. The AD8039 can be used to make a dc- Figure 44. Low-Pass Filter for Video coupled, single-ended-to-differential driver for driving these Figure 45 shows the frequency response of this filter. The ADCs. Figure 43 is a schematic of such a circuit for driving the response is down 3 dB at 6 MHz; therefore, it passes the video AD9203, 10-bit, 40 MSPS ADC. band with little attenuation. The rejection at 27 MHz is 45 dB, The AD9203 works best when the common-mode voltage at the which provides more than a factor of 100 in suppression of the input is at the midsupply or 2.5 V. The output stage design of clock components at this frequency. the AD8039 makes it ideal for driving these types of ADCs.
10
In this circuit, one of the op amps is configured in the inverting
0
mode, and the other is in the noninverting mode. However, to provide better bandwidth matching, each op amp is configured
–10
for a noise gain of +2. The inverting op amp is configured for a
) B –20
gain of −1, and the noninverting op amp is configured for a gain
d (
of +2. Each has a very similar ac response. The input signal to
IN GA –30
the noninverting op amp is divided by 2 to normalize its voltage level and make it equal to the inverting output.
–40
The outputs of the op amps are centered at 2.5 V, which is the
–50
midsupply level of the ADC. This is accomplished by first taking the 2.5 V reference output of the ADC and dividing it by 2 with
–600.1 1 10 100
5 a pair of 1 kΩ resistors. The resulting 1.25 V is applied to the 04 1-
FREQUENCY (MHz)
95 positive input of each op amp. This voltage is then multiplied by 02 Figure 45. Video Filter Response the gain of the op amps to provide a 2.5 V level at each output. Rev. G | Page 14 of 16 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS MAXIMUM POWER DISSIPATION OUTPUT SHORT CIRCUIT ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS LAYOUT, GROUNDING, AND BYPASSING CONSIDERATIONS DISABLE POWER SUPPLY BYPASSING GROUNDING INPUT CAPACITANCE OUTPUT CAPACITANCE INPUT-TO-OUTPUT COUPLING APPLICATIONS INFORMATION LOW POWER ADC DRIVER LOW POWER ACTIVE VIDEO FILTER OUTLINE DIMENSIONS ORDERING GUIDE