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AD8370

Rev. A | Page 16 of 28

CHOOSING BETWEEN GAIN RANGES

There is some overlap between the two gain ranges; users can

choose which one is most appropriate for their needs. When

deciding which preamp to use, consider resolution, noise,

linearity, and spurious-free dynamic range (SFDR). The most

important points to keep in mind are

•

The low gain range has better gain resolution.

•

The high gain range has a better noise figure.

•

The high gain range has better linearity and SFDR at

higher gains.

•

Conversely, the low gain range has higher SFDR at lower

gains.

Figure 45 provides a summary of noise, OIP3, IIP3, and SFDR

as a function of device power gain. SFDR is defined as

()

IIP3

SFDR

−

where:

IIP3 is the input third-order intercept point, the output

intercept point in dBm minus the gain in dB.

NF is the noise figure in dB.

NS is source resistor noise, –174 dBm for a 1 Hz bandwidth at

300°K (27°C).

In general, NS = 10 log10(kTB), where k = 1.374 ×10−23 , T is the

temperature in degrees Kelvin, and B is the noise bandwidth in

Hertz.

–30

–20

–10

100

110

120

130

140

150

160

170

180

–30

–20

–10

POWER GAIN (dB)

SFDR HIGH GAIN

SFDR LOW GAIN

OIP3 HIGH GAIN

OIP3 LOW GAIN

IIP3 LOW GAIN

IIP3 HIGH GAIN

NF HIGH GAIN

NF LOW GAIN

Figure 45. OIP3, IIP3, NF, and SFDR Variation with Gain

As the gain increases, the input amplitude required to deliver

the same output amplitude is reduced. This results in less

distortion at the input stage, and therefore the OIP3 increases.

At some point, the distortion of the input stage becomes small

enough such that the nonlinearity of the output stage becomes

dominant. The OIP3 does not improve significantly because the

gain is increased beyond this point, which explains the knee in

the OIP3 curve. The IIP3 curve has a knee for the same reason;

however, as the gain is increased beyond the knee, the IIP3

starts to decrease rather than increase. This is because in this

region OIP3 is constant, therefore the higher the gain, the lower

the IIP3. The two gain ranges have equal SFDR at

approximately 13 dB power gain.

LAYOUT AND OPERATING CONSIDERATIONS

Each input and output pin of the AD8370 presents either a

100 Ω or 50 Ω impedance relative to their respective ac grounds.

To ensure that signal integrity is not seriously impaired by the

printed circuit board, the relevant connection traces should

provide an appropriate characteristic impedance to the ground

plane. This can be achieved through proper layout.

When laying out an RF trace with a controlled impedance,

consider the following:

•

Space the ground plane to either side of the signal trace at

least three line-widths away to ensure that a microstrip

(vertical dielectric) line is formed, rather than a coplanar

(lateral dielectric) waveguide.

•

Ensure that the width of the microstrip line is constant and

that there are as few discontinuities as possible, such as

component pads, along the length of the line. Width

variations cause impedance discontinuities in the line and

may result in unwanted reflections.

•

Do not use silkscreen over the signal line because it alters

the line impedance.

Keep the length of the input and output connection lines as

short as possible.

Figure 46 shows the cross section of a PC board, and Table 5

show the dimensions that provide a 100 Ω line impedance for

FR-4 board material with εr = 4.6.

Table 5.

100 Ω

50 Ω

22 mils

13 mils

53 mils

8 mils

2 mils

Figure 46. Cross-Sectional View of a PC Board

It possible to approximate a 100 Ω trace on a board designed

with the 50 Ω dimensions above by removing the ground plane

within 3 line-widths of the area directly below the trace.

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