ADRF6520ACPZ-R7 datasheet(22/29 Pages) AD

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ADRF6520

Data Sheet

Rev. 0 | Page 22 of 29

increases gradually with higher gain. This behavior is apparent

in the noise floor variation at different VGA gain settings.

At low values of the VGA2 gain, the noise at the output is the

flat spectral density contributed by VGA2. As the VGA2 gain

increases, more of the filter and VGA1 noise is gained up by

VGA2, and the noise of the filter and VGA1 appears at the

output.

Because the noise spectral density outside the filter bandwidth

is limited by the VGA output noise, it may be necessary to use

an external, fixed frequency, passive filter prior to analog-to-

digital conversion to prevent noise aliasing from degrading the

signal-to-noise ratio (SNR). A higher sampling rate, relative to

the maximum required ADRF6520 corner frequency setting,

reduces the order and complexity of this external filter.

DISTORTION CHARACTERISTICS

To maintain low distortion through the cascaded VGAs and

filter of the ADRF6520, consider the distortion limits of each

stage. The first VGA has higher signal handling capability and

slightly more bandwidth than the 6 dB amplifier and VGA2,

because it must cope with out of band signals that can be larger

than the in-band signals. In filter mode, these out of band

signals are filtered before reaching the 6 dB amplifier and

VGA2. It is important to understand the signals presented to

the ADRF6520 and to match these signals with the input and

output characteristics of the device. It is useful to partition the

ADRF6520 into the front end (composed of VGA1 and the filter)

and the back end (composed of the 6 dB amplifier and VGA2).

VGA1 can handle a 4 V p-p signal at a maximum analog

attenuation setting (VGN1 = 0 V) without experiencing

appreciable distortion at the input. In most applications, VGA1

gain must be adjusted such that the maximum signal presented

at the filter inputs (or the input of the 6 dB amplifier in filter

bypass mode) is <1.5 V p-p. At this level, the front end does not

limit the distortion performance. The rms detector output,

VRMS, can be used as an indicator of the signal level present at

this critical interface. Choose the second VGA gain such that its

output levels do not exceed 1.5 V p-p if the user wants to

achieve better than 55 dBc HD2/HD3 linearity.

For these signal level considerations, it is recommended that the

out of band signal, if larger than the desired in-band signal, be

addressed. In filter mode, such an out of band signal only

affects the VGA1 operation, because it is filtered out by the

filter and does not affect the following stages. In this case, a

high VGA2 gain may be needed to raise the small desired signal

to a higher level at the output. In filter bypass mode, such out of

band signals may need to be filtered prior to the ADRF6520.

The overall distortion introduced by the device depends on the

input drive level, including the out of band signals, and the desired

output signal level. To achieve best distortion performance and

the desired overall gain, keep in mind the maximum signal levels

indicated previously in this section when selecting different

VGA gains.

To distinguish and quantify the distortion performance of the

input section, two different IP2 and IP3 specifications are

presented. The first is called in-band IP2/IP3 and refers to a

two-tone test where the signals are inside the filter bandwidth.

This specification is exactly the same figure of merit familiar

to communications engineers in which the second-order and

third-order intermodulation levels, IMD2 and IMD3

respectively, are measured.

To quantify the effect of out of band signals, an out of band IIP2

and IIP3 figure of merits are introduced. These tests also involve

two-tone stimulus; however, the two tones are placed out of

band so that the lower IMD product falls in the middle of the

filter pass band. At the output, only the IMD product is visible

because the original two tones are filtered out. To calculate the

out of band IIP2/IIP3 at the input, the IMD2/IMD3 level is

referred to the input by the overall gain. The out of band

IIP2/IIP3 allows the user to predict the impact of out of band

blockers or interferers at an arbitrary signal level on the in-band

performance. The ratio of the desired input signal level to the

input referred IMD2/IMD3 at a given blocker level represents a

signal-to-distortion limit imposed by the out of band signals.

MAXIMIZING THE DYNAMIC RANGE

When used in filter mode, the role of the ADRF6520 is to increase

the level of a variable in-band signal while minimizing out of

band signals. Ideally, this increase is achieved without degrading

the SNR of the incoming signal or introducing distortion to the

incoming signal.

The first goal is to maximize the output signal swing, which can

be defined by the ADC input range or the input signal capacity

of the next analog stage. For the complex waveforms often

encountered in communication systems, the peak to average

ratio, or crest factor, must be considered when choosing the

peak-to-peak output. From the chosen output signal and the

maximum gain of the ADRF6520, the minimum input level can

be defined.

As the input signal level increases, the VGA2 gain is reduced

from its maximum gain point to maintain the desired fixed

output level. VGA1 can then be adjusted as the input signal

level keeps increasing. This sequencing of the gain maintains

the best NF for the cascaded chain. The output noise, initially

dominated by the filter and VGA1 combination, follows the

gain reduction, yielding a progressively better SNR. At some

point, the VGA2 gains drop sufficiently so that their noise

becomes dominant, resulting in a slower reduction in SNR from

that point. From the perspective of SNR alone, the maximum

input level is reached when the VGA1 reaches its minimum gain.

Distortion must also be considered when maximizing the

dynamic range. At low and moderate signal levels, the output

distortion is constant and assumed to be adequate for the

selected output level. At some point, the input signal becomes

large enough that distortion at the input limits the system. This

distortion can be kept in check by monitoring the rms detector

voltage, VRMS.

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