ADL5336-EVALZ AD [Analog Devices], ADL5336-EVALZ Datasheet - Page 24

• Manufacturer Part Number: ADL5336-EVALZ
• Description: Cascadable IF VGAs
• Manufacturer: AD [Analog Devices]

ADL5336
As the setpoint of VGA1 increases, the total output noise decreases.
Linearity limits how high the setpoint of VGA1 for a given
system can be programmed. For two equal sinusoidal tones,
353 mV rms corresponds to 1.4 V p-p, whereas 707 mV rms
corresponds to 2.8 V p-p. For a 1.4 V p-p composite output,
IMD3 is approximately −65 dBc; however, for a 2.8 V p-p
composite output, IMD3 is theoretically 12 dB worse at −53 dBc.
For each VGA, total RTO noise increases at higher maximum-
gain settings; therefore, the overall combination of maximum
gain should be minimized while still satisfying all system
requirements with adequate margin.
In linear terms, the noise figure of the cascaded amplifiers can
be given by
Because both VGAs are X-AMPs, the noise figure of each VGA
degrades dB-for-dB as the gain of each VGA decreases. This is
due to the attenuation ladder on the input that attenuates the
signal before the signal is gained up. If only the gain of the second
VGA is changing, the cascaded noise figure does not change
appreciably because the noise figure of the second VGA is being
divided by the constant gain of the first VGA. When the gain of
VGA2 drops to the minimum and the input signal level is still
decreasing, VGA1 takes over and its gain starts to change. The
cascaded noise figure increases dB-for-dB while the gain of VGA1
decreases.
While cascading the VGAs, keeping intermodulation distortion
components low is at direct odds with keeping noise figure and
output noise density low. It can be shown that the third-order
intercept of a cascaded system in linear terms is
where P
each VGA in watts. Thus, when the overall IP3 is the largest
(distortion is the smallest), the gain of VGA2 is at its maximum.
Vice-versa, when the gain of VGA2 is at its minimum, the
overall IP3 is the smallest, and distortion is at its maximum.
Table 10 provides conditions for optimization for the output
noise density, noise figure, and distortion parameters.
Table 10. Optimized Conditions
Output Noise
Noise Figure
IMD/IP3
1
2
Having the gain of VGA2 at maximum does not change the overall noise
figure much due to the noise figure contribution of VGA2 being divided by
the gain of VGA1.
IMD levels do not change much over the X-Amp gain range, but best IMD
levels are achieved at high gains.
NF
P3 = 1/(1/(G
CAS
3_VGA1
= NF
and P
VGA1
VGA2
3_VGA2
+ (NF
P
VGA1 Gain
Minimum
Maximum
Maximum
3_VGA1
are the third-order intercept points of
VGA2
) + 1/P
− 1)/G
2
3_VGA2
VGA1
)
VGA2 Gain
Minimum
Maximum
Maximum
1
Rev. B | Page 24 of 32
When starting from a very small input power, such that neither
VGA has reached their respective setpoints, and the analog gain
of both VGAs is forced to its maximum, the cascaded OIP3 is at
its maximum, while the cascaded noise figure is at its minimum.
As the input power is increased, each VGA keeps its gain at
maximum until its respective setpoint is reached, at which point
the gain of the VGA (whose setpoint has been reached) decreases
to accomodate the increaced input power and thus changes the
cascaded OIP3 and noise figure.
Figure 68 shows how the OIP3 changes while input power is
varied in AGC mode, which consequently changes the analog
gains of the VGAs. The setpoint of VGA2 is fixed to 100 (or
250 mV rms), and the setpoint of VGA1 is changed from 001
(88 mV rms) to 100 (250 mV rms), and finally, to 111
(707 mV rms).
Figure 69 shows how the NF changes while the input power is
varied in AGC, which again, consequently changes the analog
gains of the VGAs. The setpoint of VGA2 is still fixed to 100
(250 mV rms), and the changes made to the setpoint of VGA1
is the same as before.
Figure 69. Noise Figure vs. Overall Voltage Gain over Several Setpoints;
35
30
25
20
15
10
Figure 68. OIP3 vs. Overall Voltage Gain over Several Setpoints;
60
50
40
30
20
10
–20
0
–20
–15
VGA1 Gain Code = 11 and VGA2 Gain Code = 00
VGA1 Gain Code = 11 and VGA2 Gain Code = 00
–10
–10
OVERALL VOLTAGE GAIN (dB)
OVERALL VOLTAGE GAIN (dB)
–5
0
0
5
LOW TONE, SETPOINT = 001
HIGH TONE, SETPOINT = 001
LOW TONE, SETPOINT = 100
HIGH TONE, SETPOINT = 100
LOW TONE, SETPOINT = 111
HIGH TONE, SETPOINT = 111
10
10
15
SETPOINT = 001
SETPOINT = 100
SETPOINT = 111
20
Data Sheet
20
25
30
30
25
20
15
10
5
0