AD8370 datasheet(20/28 Pages) AD

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AD8370

Rev. 0 | Page 20 of 28

27pF

RS = 120Ω

RL= 1200Ω

200nH

100nH

27pF

BALANCED

CONFIGURATION

DE-NORMALIZED

SINGLE-ENDED

EQUIVALENT

RS =

= 0.1

RL= 1Ω

LN = 0.074H

14.814F

NORMALIZED

SINGLE-ENDED

EQUIVALENT

= 60

Ω

= 60

Ω

= 600

Ω

= 600

Ω

fC = 70MHz

fC = 1Hz

Figure 54. Second-Order Butterworth Low-Pass Filter Design Example

A complete design example is shown in Figure 56. The AD8370

is configured for single-ended-to-differential conversion with

the input terminated down to present a single-ended 75 Ω input.

A sixth-order Chebyshev differential filter is used to interface

the output of the AD8370 to the input of the AD9430 170 MSPS

12-bit ADC. The filter minimizes aliasing effects and improves

harmonic distortion performance.

The input of the AD9430 is terminated with a 1.5 kΩ resistor so

that the overall load presented to the filter network is ~1 kΩ.

The variable gain of the AD8370 extends the useable dynamic

range of the ADC. The measured intermodulation distortion of

the combination is presented in Figure 55 at 42 MHz.

–130

–120

–100

–60

–40

–20

–10

–80

–110

–70

–50

–30

–90

10203040

506070

FREQUENCY (MHz)

Figure 55. FFT Plot of Two-Tone Intermodulation Distortion at

42 MHz for the Circuit in Figure 56

In Figure 55, the intermodulation products are comparable to

the noise floor of the ADC. The spurious-free dynamic range of

the combination is better than 66 dB for a 70 MHz measurement

bandwidth.

3 V OPERATION

It is possible to operate the AD8370 at voltages as low as 3 V

with only minor performance degradation. Table 6 gives typical

specifications for operation at 3 V.

Table 6.

Parameter

Typical (70 MHz, RL = 100 Ω)

Ouptut IP3

+23.5 dBm

P1dB

+12.7 dBm

−3 dB Bandwidth

650 MHz (HG 127)

IMD3

−82 dBc (RL = 1 kΩ)

AD8370

SERIAL CONTROL INTERFACE

FROM 75

Ω

Tx-LINE

0.1

µF

1nF

0.1

µF

+VS

27pF

68nH

180nH

220nH

39pF

27pF

1.5k

Ω

68nH

180nH

220nH

Ω

CAC

100nF

CAC

100nF

CAC

100nF

RS 120Ω

VINB

AD9430

VINA

Figure 56. ADC Interface Example

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