AD8331 Analog Devices, AD8331 Datasheet - Page 20

• Manufacturer Part Number: AD8331
• Description: Ultralow Noise VGAs with Preamplifier and Programmable RIN
• Manufacturer: Analog Devices

AD8331/AD8332
X-AMP VGA
The input of the VGA is a differential R-2R ladder attenuator
network, with 6 dB steps per stage and a net input impedance of
200 Ω differential. The ladder is driven by a fully differential
input signal from the LNA and is not intended for single-ended
operation. LNA outputs are ac-coupled to reduce offset and
isolate their common-mode voltage. The VGA inputs are biased
through the ladder’s center tap connection to VCM, which is
typically set to 2.5 V and is bypassed externally to provide a
clean ac ground.
The signal level at successive stages in the input attenuator falls
from 0 dB to –48 dB, in 6 dB steps. The input stages of the
X-AMP are distributed along the ladder, and a biasing interpolator,
controlled by the gain interface, determines the input tap point.
With overlapping bias currents, signals from successive taps merge
to provide a smooth attenuation range from 0 dB to –48 dB. This
circuit technique results in excellent, linear-in-dB gain law
conformance and low distortion levels and deviates ±0.2 dB or less
from ideal. The gain slope is monotonic with respect to the control
voltage and is stable with variations in process, temperature, and
supply.
The X-AMP inputs are part of a gain-of-12 feedback amplifier,
which completes the VGA. Its bandwidth is 150 MHz. The input
stage is designed to reduce feedthrough to the output and
ensure excellent frequency response uniformity across gain
setting (see Figure 8 and Figure 9).
Gain Control
Position along the VGA attenuator is controlled by a single-
ended analog control voltage, V
to 1.0 V. The gain control scaling is trimmed to a slope of 50 dB/V
(20 mV/dB). Values of V
minimum or maximum gain values. Both channels of the
AD8332 are controlled from a single gain interface to preserve
matching. Gain can be calculated using Equations 1 and 2.
Gain accuracy is very good since both the scaling factor and
absolute gain are factory trimmed. The overall accuracy relative
to the theoretical gain expression is ±1 dB for variations in
temperature, process, supply voltage, interpolator gain ripple, trim
errors, and tester limits. The gain error relative to a best-fit line for
a given set of conditions is typically ±0.2 dB. Gain matching
between channels is better than 0.1 dB (see Figure 7, which shows
gain errors in the center of the control range). When V
or > 0.95, gain errors are slightly greater.
The gain slope may be inverted, as shown in Figure 58 (avail-
able in most versions). The gain drops with a slope of
–50 dB/V across the gain control range from maximum to
minimum gain. This slope is useful in applications, such as
automatic gain control, where the control voltage is
proportional to the measured output signal amplitude. The
inverse gain mode is selected by setting the MODE pin HI.
GAIN
beyond the control range saturate to
GAIN
, with an input range of 40 mV
GAIN
< 0.1
Rev. C | Page 20 of 32
Gain control response time is less than 750 ns to settle within
10% of the final value for a change from minimum to max-
imum gain.
VGA Noise
In a typical application, a VGA compresses a wide dynamic
range input signal to within the input span of an ADC. While
the input-referred noise of the LNA limits the minimum
resolvable input signal, the output-referred noise, which
depends primarily on the VGA, limits the maximum
instantaneous dynamic range that can be processed at any one
particular gain control voltage. This limit is set in accordance
with the quantization noise floor of the ADC.
Output and input-referred noise as a function of V
plotted in Figure 21 and Figure 23 for the short-circuited input
condition. The input noise voltage is simply equal to the output
noise divided by the measured gain at each point in the control
range.
The output-referred noise is flat over most of the gain range,
since it is dominated by the fixed output-referred noise of the
VGA. Values are 48 nV/√Hz in LO gain mode and 178 nV/√Hz
in HI gain mode. At the high end of the gain control range, the
noise of the LNA and source prevail. The input-referred noise
reaches its minimum value near the maximum gain control
voltage, where the input-referred contribution of the VGA
becomes very small.
At lower gains, the input-referred noise, and thus noise figure,
increases as the gain decreases. The instantaneous dynamic
range of the system is not lost, however, since the input capacity
increases with it. The contribution of the ADC noise floor has
the same dependence as well. The important relationship is
the magnitude of the VGA output noise floor relative to that
of the ADC.
With its low output-referred noise levels, these devices ideally
drive low-voltage ADCs. The converter noise floor drops 12 dB
for every 2 bits of resolution and drops at lower input full-scale
voltages and higher sampling rates. ADC quantization noise is
discussed in the Applications section.
The preceding noise performance discussion applies to a
differential VGA output signal. Although the LNA noise
performance is the same in single-ended and differential
applications, the VGA performance is not. The noise of the
VGA is significantly higher in single-ended usage, since the
contribution of its bias noise is designed to cancel in the
differential signal. A transformer can be used with single-ended
applications when low noise is desired.
GAIN
are