AD817

REV. B

–11–

A HIGH PERFORMANCE ADC INPUT BUFFER

High performance analog to digital converters (ADCs) require

input buffers with correspondingly high bandwidths and very

low levels of distortion. Typical requirements include distortion

levels of –60 dB to –70 dB for a 1 volt p-p signal and band-

widths of 10 MHz or more. In addition, an ADC buffer may

need to drive very large capacitive loads.

The circuit of Figure 36 is useful for driving high speed convert-

ers such as the differential input of the AD733, 10-bit ADC.

This circuit may be used with other converters with only minor

modifications. Using the AD817 provides the user with the op-

tion of either operating the buffer in differential mode or from a

single +5 volt supply. Operating from a +5 volt power supply

helps to avoid overdriving the ADC—a common problem with

buffers operating at higher supply voltages.

SINGLE SUPPLY OPERATION

Another exciting feature of the AD817 is its ability to perform

well in a single supply configuration. The AD817 is ideally

suited for applications that require low power dissipation and

high output current and those which need to drive large capaci-

tive loads, such as high speed buffering and instrumentation.

Referring to Figure 37, careful consideration should be given to

the proper selection of component values. The choices for this

particular circuit are: R1+ R3//R2 combine with C1 to form a

low frequency corner of approximately 300 Hz.

R2

10k

Ω

C3

0.1

µF

R1

9k

Ω

R3

1k

Ω

C2

0.1

µF

C1

0.1

µF

C

L

200pF

R

L

150

Ω

SELECT C1, R1, R2 & R3

FOR DESIRED LOW

FREQUENCY CORNER.

(R2 = R1 + R3)

C

OUT

+V

S

V

OUT

V

IN

AD817

7

6

3

2

4

0.01

µF

3.3

µF

Figure 37. Single Supply Amplifier Configuration

Combining R3 with C2 forms a low-pass filter with a corner

frequency of 1.5 kHz. This is needed to maintain amplifier

the AD817’s exceptional output drive capability. In this con-

figuration, the output is centered around 2.5 V. In order to

eliminate the static dc current associated with this level, C3 was

52.5

Ω

0.1

µF

1k

Ω

0.1

µF

–V

S

1k

Ω

1k

Ω

1k

Ω

AD773

10-BIT

18MHz

ADC

V

INA

V

INB

ADREF43

VOLTAGE

REFERENCE

+2.5V

50

Ω

COAX

CABLE

V

IN

500mVp-p

MAX

+V

S

AD817

7

6

3

2

4

26

27

0.1

µF

0.1

µF

–V

S

+V

S

AD817

7

6

3

2

4

+5V

COMMON

–5V

100

µF

25V

100

µF

25V

+V

S

–V

S

Figure 36. A Differential Input Buffer for High Bandwidth ADCs