AD8370 datasheet(20/28 Pages) AD

20 / 28 page

AD8370

Rev. A | Page 20 of 28

SOURCE

LOAD

BALANCED

CONFIGURATION

SINGLE-ENDED

EQUIVALENT

A complete design example is shown in Figure 58. 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 57 at 42 MHz.

Figure 55. Single-Ended-to-Differential Network Conversion

As an example, a second-order, Butterworth, low-pass filter

design is presented where the differential load impedance is

1200 Ω, and the padded source impedance of the AD8370 is

assumed to be 120 Ω. The normalized series inductor value for

the 10-to-1, load-to-source impedance ratio is 0.074 H, and the

normalized shunt capacitor is 14.814 F. For a 70 MHz cutoff

frequency, the single-ended equivalent circuit consists of a

200 nH series inductor followed by a 27 pF capacitor. To realize

the balanced equivalent, simply split the 200 nH inductor in

half to realize the network shown in Figure 56.

–130

–120

–100

–60

–40

–20

–10

–80

–110

–70

–50

–30

–90

10203040506070

FREQUENCY (MHz)

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

= 600

fC = 70MHz

fC = 1Hz

Figure 57. FFT Plot of Two-Tone Intermodulation Distortion at

42 MHz for the Circuit in Figure 58

In Figure 57, 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)

Figure 56. Second-Order, Butterworth, Low-Pass Filter Design Example

Output IP3

+23.5 dBm

P1dB

+12.7 dBm

−3 dB Bandwidth

650 MHz (HG 127)

IMD3

−82 dBc (RL = 1 kΩ)

AD8370 Datasheet(PDF) 20 Page - Analog Devices

AD8370 Datasheet(HTML) 20 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL