REV. 0

–8–

AD7482

220

BIAS

VOLTAGE

1

2

3

4

5

6

7

8

AD8021

50

AC

SIGNAL

220

10pF

–

+

10pF

Figure 2. Analog Input Circuit Used for 1 MHz Input Tone

For higher input bandwidth applications, Analog Devices’

AD8021 op amp (also available as a dual AD8022) is the

recommended choice to drive the AD7482. Figure 2 shows

the analog input circuit used to obtain the data for the FFT

plot shown in TPC 2. A bipolar analog signal is applied to

the terminal shown and biased up with a stable, low noise dc

voltage connected as shown. A 10 pF compensation capacitor

is connected between Pin 5 of the AD8021 and the negative

supply. As with the previous circuit, the AD8021 is supplied

with +12 V and –12 V supplies. The supply pins are decoupled

as close to the device as possible, with both a 0.1

µF and 10 µF

capacitor connected to each pin. In each case, the 0.1

µF capaci-

tor should be the closer of the two caps to the device. The

AD8021 logic reference pin is tied to analog ground and the

DISABLE Pin is tied to the positive supply as shown. Detailed

information on the AD8021 is available on the Analog

Devices website.

FREQUENCY – kHz

–20

10

100

–50

–70

–80

–40

1000

–30

–60

–10

0

100mV p-p SINE WAVE ON SUPPLY PINS

TPC 7. PSRR without Decoupling

1

2

3

4

5

6

7

8

AD829

1k

1k

100

AC

SIGNAL

BIAS

VOLTAGE

150

220pF

–

+

Figure 1. Analog Input Circuit Used for 10 kHz Input Tone

Figure 1 shows the analog input circuit used to obtain the

data for the FFT plot shown in TPC 1. The circuit uses an

Analog Devices AD829 op amp as the input buffer. A bipolar

analog signal is applied as shown and biased up with a stable,

low noise dc voltage connected to the labeled terminal

shown. A 220 pF compensation capacitor is connected be-

tween Pin 5 and the AD829 and the analog ground plane.

The AD829 is supplied with +12 V and –12 V supplies. The

supply pins are decoupled as close to the device as possible

with both a 0.1

F and 10

F capacitor connected to each

pin. In each case, 0.1

F capacitor should be the closer of

the two caps to the device. More information on the AD829

is available on the Analog Devices website.

TEMPERATURE – C

0.0004

–55

–25

5

35

95

125

–0.0004

–0.0008

–0.0012

–0.0016

–0.0020

0

65

TPC 8. Reference Out Error