ADRF6510ACPZ-R7 datasheet(17/28 Pages) AD

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ADRF6510

Rev. 0 | Page 17 of 28

–100

–135

0.5

1.0

1.5

2.0

2.5

3.0

FREQUENCY (MHz)

–130

–125

–120

–115

–110

–105

BANDWIDTH = 1MHz

GAIN = 40dB

GAIN = 20dB

GAIN = 0dB

Figure 47. Total Output Noise with a 1 MHz Corner Frequency

for Three Different Gain Settings

Note that the noise spectral density outside the filter bandwidth

is limited by the fixed VGA output noise. It may be necessary to

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

to-digital conversion to prevent noise aliasing from degrading

the signal-to-noise ratio. The higher the sampling rate relative

to the maximum ADRF6510 corner frequency setting to be used,

the lower the order of the external filter.

DISTORTION CHARACTERISTICS

The distortion performance of the ADRF6510 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 ADRF6510 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 1 V p-p for a 6 dB preamplifier and

500 mV p-p for a 12 dB preamplifier with >50 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, IM3, 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 IM3 product lands in the middle of the

filter pass band. At the output, only the IM3 product is visible

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

OOB IP3 at the input, the IM3 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 IM3 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 ADRF6510 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 choosing the peak-to-peak

output. From the chosen output signal and the maximum gain

of the ADRF6510, 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 constant VGA

noise becomes dominant, resulting in a constant SNR from that

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

level is reached when the VGA 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. The maximum tolerable

input signal depends on whether the input distortion becomes

unacceptably large or the minimum gain is reached.

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