ADRF6516ACPZ-R7 datasheet(19/29 Pages) AD

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Data Sheet

ADRF6516

Rev. C | Page 19 of 29

The exact relationship depends on the programmed fixed gain of

the amplifiers. At minimum gain, only the last VGA contributes

to the −144 dBV/√Hz minimum noise floor, which is equivalent

to 63 nV/√Hz. At lower frequencies within the filter bandwidth

setting, the VGAs translate the filter noise directly to the output

by a factor equal to the gain following the filter.

At low values of VGA gain, the noise at the output is the flat

spectral density contributed by the last VGA. As the gain

increases, more noise from the filter and first VGA appears at

the output. Because the intrinsic filter noise density increases

at lower bandwidth settings, it is more pronounced than it is

at higher bandwidth settings. In either case, the noise density

asymptotically approaches the limit set by the VGAs at the

highest frequencies. For other values of VGA gain and bandwidth

setting, the detailed shape of the noise spectral density changes

according to the relative contributions of the filters and VGAs.

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. A higher sampling rate relative to the maxi-

mum required ADRF6516 corner frequency setting reduces the

order and complexity of this external filter.

DISTORTION CHARACTERISTICS

The distortion performance of the ADRF6516 is similar to its

noise performance. The filters and the VGAs contribute to the

overall distortion and signal handling capabilities. Furthermore,

the front end must also cope with out-of-band signals that can be

larger than the in-band signals. These out-of-band signals are

filtered before reaching the VGA. It is important to understand

the signals presented to the ADRF6516 and to match these

signals with the input and output characteristics of the part.

When the gain is low, the distortion is typically limited by the

input section because the output is not driven to its maximum

capacity. When the gain is high, the distortion is likely limited

by the output section because the input is not driven to its

maximum capacity. An exception to this is when the input is

driven with a small desired signal in combination with a large

out-of-band signal. In this case, the out-of-band signal may

drive the input to distort. As long as the input is not overdriven,

the out-of-band signal is removed by the filter. A high VGA

gain is still needed to raise the small desired signal to a higher

level at the output. The overall distortion introduced by the part

depends on the input drive level, including the out-of-band

signals, and the desired output signal level.

As noted in the Input Buffers section, the input section can

handle a total signal level of 400 mV p-p for a 3 dB preamplifier

gain and 280 mV p-p for a 6 dB preamplifier gain with >70 dBc

harmonic distortion. This includes both in-band and out-of-band

signals.

To distinguish and quantify the distortion performance of the

input section, two different IP3 specifications are presented. The

first is called in-band IP3 and refers to a two-tone test where the

signals are inside the filter bandwidth. This is exactly the same

figure of merit familiar to communications engineers in which

the third-order intermodulation level, IMD3, is measured.

To quantify the effect of out-of-band signals, a new out-of-band

(OOB) IIP3 figure of merit is introduced. This test also involves

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

band so that the lower IMD3 product lands in the middle of the

filter pass band. At the output, only the IMD3 product is visible

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

OOB IIP3 at the input, the IMD3 level is referred to the input

by the overall gain. The OOB 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 IMD3 at a given blocker

level represents a signal-to-distortion limit imposed by the out-

of-band signals.

MAXIMIZING THE DYNAMIC RANGE

The role of the ADRF6516 is to increase the level of a variable

in-band signal while minimizing out-of-band signals. Ideally,

this 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 encoun-

tered in communication systems, the peak-to-average ratio, or

crest factor, must be considered when selecting the peak-to-peak

output. From the selected output signal and the maximum gain

of the ADRF6516, the minimum input level can be defined.

Lower signal levels do not yield the maximum output and suffer

a greater degradation in SNR.

As the input signal level increases, the VGA gain is reduced from

its maximum gain point to maintain the desired fixed output

level. The output noise, initially dominated by the filter, follows

the gain reduction, yielding a progressively better SNR. At some

point, the VGA gain drops sufficiently that the VGA 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 VGA reaches its minimum gain.

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