AD8370
Data Sheet
Rev. B | 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
(
)
S
N
NF
IIP3
SFDR
=
3
2
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 ×1023, T is the
temperature in degrees Kelvin, and B is the noise bandwidth in
Hertz.
–30
–20
–10
0
10
20
NOISE
FIGURE
(dB);
OIP3
AND
IIP3
(dBm)
30
40
50
100
110
120
130
140
150
SFDR
(dB)
160
170
180
–30
–20
–10
0
10
20
30
40
POWER GAIN (dB)
03692-043
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 or 50 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.
show the dimensions that provide a 100 line impedance for
FR-4 board material with εr = 4.6.
Table 5.
100
50
W
22 mils
13 mils
H
53 mils
8 mils
T
2 mils
W
3W
ER
3W
H
T
03692-
044
Figure 46. Cross-Sectional View of a PC Board
It possible to approximate a 100 trace on a board designed
with the 50 dimensions above by removing the ground plane
within 3 line-widths of the area directly below the trace.