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.