AD8038/AD8039

Rev. G | Page 14 of 16

APPLICATIONS INFORMATION

LOW POWER ADC DRIVER

8

1

0.1µF

10µF

0.1µF

10µF

+5V

7

0.1µF

10µF

–5V

3

2

6

5

4

AD8039

1kΩ

1kΩ

1kΩ

1kΩ

VINP

VINN

REF

50Ω

50Ω

AD9203

1kΩ

1kΩ

1kΩ

1kΩ

VIN

0V

3V

2.5V

Figure 43. Schematic to Drive AD9203 with the AD8039

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

AD8039 is an appropriate amplifier choice to drive it.

In low supply voltage applications, differential analog inputs

are needed to increase the dynamic range of the ADC inputs.

Differential driving can also reduce second and other even-order

distortion products. The AD8039 can be used to make a dc-

coupled, single-ended-to-differential driver for driving these

ADCs. Figure 43 is a schematic of such a circuit for driving the

AD9203, 10-bit, 40 MSPS ADC.

The AD9203 works best when the common-mode voltage at the

input is at the midsupply or 2.5 V. The output stage design of

the AD8039 makes it ideal for driving these types of ADCs.

In this circuit, one of the op amps is configured in the inverting

mode, and the other is in the noninverting mode. However, to

provide better bandwidth matching, each op amp is configured

for a noise gain of +2. The inverting op amp is configured for a

gain of −1, and the noninverting op amp is configured for a gain

of +2. Each has a very similar ac response. The input signal to

the noninverting op amp is divided by 2 to normalize its voltage

level and make it equal to the inverting output.

The outputs of the op amps are centered at 2.5 V, which is the

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

a pair of 1 kΩ resistors. The resulting 1.25 V is applied to the

positive input of each op amp. This voltage is then multiplied by

the gain of the op amps to provide a 2.5 V level at each output.

LOW POWER ACTIVE VIDEO FILTER

Some composite video signals derived from a digital source

contain clock feedthrough that can limit picture quality. Active

filters made from op amps can be used in this application, but

they consume 25 mW to 30 mW for each channel. In power-

sensitive applications, this can be too much, requiring the use

of passive filters that can create impedance matching problems

when driving any significant load.

The AD8038 can be used to make an effective low-pass active

filter that consumes one-fifth of the power consumed by an

active filter made from an op amp. Figure 44 shows a circuit

that uses a AD8038 with ±2.5 V supplies to create a three-pole

Sallen-Key filter. This circuit uses a single RC pole in front of a

standard 2-pole active section.

0.1µF

+2.5V

10µF

–2.5V

0.1µF

10µF

C3

33pF

R3

49.9Ω

RF

1Ω

680pF

R5

75Ω

R2

499Ω

C1

100pF

R1

200Ω

R4

49.9Ω

AD8038

VIN

VOUT

Figure 44. Low-Pass Filter for Video

Figure 45 shows the frequency response of this filter. The

response is down 3 dB at 6 MHz; therefore, it passes the video

band with little attenuation. The rejection at 27 MHz is 45 dB,

which provides more than a factor of 100 in suppression of the

clock components at this frequency.

FREQUENCY (MHz)

0.1

110

–10

10

100

0

–20

–30

–40

–50

–60

Figure 45. Video Filter Response