AD8051/AD8052/AD8054

–14–

REV. B

AD8051

+5V

0.1 F

10 F

10 F

0.1 F

AVDD

SELECT

INA-I

10pF

CLK

SLEEP

D9

D1

D2

D3

D4

D5

D6

D7

D0

DVDD

AVSS

REFSENSE

AD9201

DVSS

THREE–STATE

INB-I

REFT-I

REFB-I

REFB-Q

REFT-Q

INB-Q

INA-Q

D8

5V

DATA OUT

0.1 F

22

10pF

22

10 F

0.1 F

5V

0.1 F

10 F

0.1 F

22

22

5V

22

1k

50

1k

10pF

10pF

0.1 F

10 F

0.1 F

0.1 F

10 F

0.1 F

1k

0.1 F

10 F

0.01 F

0.33 F

Figure 44. AD8051 Driving an AD9201, a 10-Bit 20 MSPS A/D Converter

applied to the noninverting input of the AD8051. The amplifier

output is 2 V p-p, which is the maximum input range of the

AD9201. The 22

Ω series resistor limits the maximum current

that flows and helps to lower the distortion of the A/D.

The AD9201 has differential inputs for each channel. These are

designated the A and B inputs. The B inputs of each channel are

connected to VREF (Pin 8) which supplies a positive reference

of 2.5 V. Each of the B inputs has a small low pass filter that

also helps to reduce distortion.

The output of the op amp is ac coupled into INA-I (Pin 2) via

two parallel capacitors to provide good high frequency and low

frequency coupling. The 1 k

Ω resistor references the signal to

VREF that is applied to INB-I. Thus, INA-I will swing both

positive and negative with respect to the bias voltage applied to

INB-I.

With the sampling clock running at 20 MSPS, the A/D output

was analyzed with a digital analyzer. Two input frequencies

were used, 1 MHz and 9.5 MHz, which is just short of the

Nyquist frequency. These signals were well filtered to minimize

any harmonics.

Figure 45 shows the FFT response of the A/D for the case of

1 MHz analog input. The SFDR is 71.66 dB and the A/D is

producing 8.8 ENOB (effective number of bits). When the

analog frequency was raised to 9.5 MHz, the SFDR was re-

duced to 60.18 dB and the A/D operated with 8.46 ENOBs as

shown in Figure 46. The inclusion of the AD8051 in the circuit

had no worsening of the distortion performance of the AD9201.

PART#

0

FCLK

FUND

VIN

THD

SNR

SINAD

ENOB

SFDR

2ND

3RD

4TH

5TH

6TH

7TH

8TH

9TH

FFTSIZE 8192

20.0E 6

998.5E 3

0.51dB

68.13

54.97

54.76

8.80

71.66

74.53

76.06

76.35

79.05

80.36

75.08

88.12

77.87

10.0

5.0

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

55.0

60.0

65.0

70.0

75.0

80.0

85.0

90.0

95.0

100.0

105.0

110.0

115.0

120.0

0.0E 0

1.0E 6

2.0E 6

3.0E 6

4.0E 6

5.0E 6

6.0E 6

7.0E 6

8.0E 6

9.0E 6

10.0E 6

2ND

3RD

4TH

5TH

6TH

7TH

8TH

9TH

FUND

Figure 45. FFT Plot for AD8051 Driving the AD9201 at

1 MHz

10.0

5.0

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

55.0

60.0

65.0

70.0

75.0

80.0

85.0

90.0

95.0

100.0

105.0

110.0

115.0

120.0

0.0E 0

1.0E 6

2.0E 6

3.0E 6

4.0E 6

5.0E 6

6.0E 6

7.0E 6

8.0E 6

9.0E 6

10.0E 6

PART#

0

FCLK

FUND

VIN

THD

SNR

SINAD

ENOB

SFDR

2ND

3RD

4TH

5TH

6TH

7TH

8TH

9TH

FFTSIZE 8192

20.0E 6

9.5E 6

0.44dB

57.08

54.65

52.69

8.46

60.18

60.18

60.23

82.01

78.83

81.28

77.28

84.54

92.78

FUND

3RD

4TH

6TH

7TH

8TH

2ND

Figure 46. FFT Plot for AD8051 Driving the AD9201 at

9.5 MHz