ADL5336ACPZ-R7 datasheet(24/30 Pages) AD

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ADL5336

Data Sheet

Rev. C | Page 24 of 30

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

NFCAS = NFVGA1 + (NFVGA2 − 1)/GVGA1

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

P3 = 1/(1/(GVGA2P3_VGA1) + 1/P3_VGA2)

where P3_VGA1 and P3_VGA2 are the third-order intercept points of

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

VGA1 Gain

VGA2 Gain

Output Noise

Minimum

Noise Figure

Maximum

Maximum1

IMD/IP3

Maximum2

Maximum

1 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.

2 IMD levels do not change much over the X-Amp gain range, but best IMD

levels are achieved at high gains.

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 increased input power and 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).

–20

–15

–10

–5

1015202530

OVERALL VOLTAGE GAIN (dB)

LOW TONE, SETPOINT = 001

HIGH TONE, SETPOINT = 001

LOW TONE, SETPOINT = 100

HIGH TONE, SETPOINT = 100

LOW TONE, SETPOINT = 111

HIGH TONE, SETPOINT = 111

Figure 68. OIP3 vs. Overall Voltage Gain over Several Setpoints;

VGA1 Gain Code = 11 and VGA2 Gain Code = 00

–20

–10

OVERALL VOLTAGE GAIN (dB)

SETPOINT = 001

SETPOINT = 100

SETPOINT = 111

Figure 69. Noise Figure vs. Overall Voltage Gain over Several Setpoints;

VGA1 Gain Code = 11 and VGA2 Gain Code = 00

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.

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