AD8331 Datasheet, PDF (18/32 Pages) Analog Devices – Ultralow Noise VGAs with Preamplifier and Programmable RIN

AD8331/AD8332
A simplified schematic of the LNA is shown in Figure 59. INH
is capacitively coupled to the source. An on-chip bias generator
centers the output dc levels at 2.5 V and the input voltages at
3.25 V. A capacitor CLMD of the same value as the input coupling
capacitor CINH is connected from the LMD pin to ground.
C_FB
R_FB
LOP
VPOS
LON
C_INH
R_S
INH
C_SH
I₀
I₀
Q1
Q2
I₀
I₀
LMD
C_LMD
Figure 59. Simplified LNA Schematic
The LNA supports differential output voltages as high as
5 V p-p with positive and negative excursions of ±1.25 V, about
a common-mode voltage of 2.5 V. Since the differential gain
magnitude is 9, the maximum input signal before saturation is
± 275 mV or 550 mV p-p. Overload protection ensures quick
recovery time from large input voltages. Since the inputs are
capacitively coupled to a bias voltage near midsupply, very large
inputs can be handled without interacting with the ESD
protection.
Low value feedback resistors and the current-driving capability
of the output stage allow the LNA to achieve a low input-
referred voltage noise of 0.74 nV/√Hz. This is achieved with a
modest current consumption of 10 mA per channel (50 mW).
On-chip resistor matching results in precise gains of 4.5 per side
(9 differential), critical for accurate impedance control. The use
of a fully differential topology and negative feedback minimizes
distortion. Low HD2 is particularly important in second
harmonic ultrasound imaging applications. Differential
signaling enables smaller swings at each output, further
reducing third order distortion.
Active Impedance Matching
The LNA supports active impedance matching through an
external shunt feedback resistor from Pin LON to Pin INH. The
input resistance RIN is given by Equation 5, where A is the
single-ended gain of 4.5, and 6 kΩ is the unterminated input
impedance.
R _IN
=
R_FB
1+ A
6 kΩ = ^{6 kΩ × RFB}
33 kΩ + R_FB
(5)
CFB is needed in series with RFB, since the dc levels at Pins LON
and INH are unequal. Expressions for choosing RFB in terms of
RIN and for choosing CFB are found in the Applications section.
CSH and the ferrite bead enhance stability at higher frequencies
where the loop gain declines and prevents peaking. Frequency
response plots of the LNA are shown in Figure 19 and Figure 20.
The bandwidth is approximately 130 MHz for matched input
impedances of 50 Ω to 200 Ω and declines at higher source
impedances. The unterminated bandwidth (RFB = ∞) is
approximately 80 MHz.
Each output can drive external loads as low as 100 Ω in addition
to the 100 Ω input impedance of the VGA (200 Ω differential).
Capacitive loading up to 10 pF is permissible. All loads should
be ac-coupled. Typically, Pin LOP output is used as a single-
ended driver for auxiliary circuits, such as those used for
Doppler mode ultrasound imaging, and Pin LON drives RFB.
Alternatively, a differential external circuit can be driven from
the two outputs, in addition to the active feedback termination.
In both cases, important stability considerations discussed in
the Applications section should be carefully observed.
The impedance at each LNA output is 5 Ω. A 0.4 dB reduction
in open-circuit gain results when driving the VGA, and 0.8 dB
with an additional 100 Ω load at the output. The differential
gain of the LNA is 6 dB higher. If the load is less than 200 Ω on
either side, a compensating load is recommended on the
opposite output.
LNA Noise
The input-referred voltage noise sets an important limit on
system performance. The short-circuit input voltage noise of the
LNA is 0.74 nV/√Hz or 0.82 nV/√Hz (at maximum gain),
including the VGA noise. The open-circuit current noise is
2.5 pA/√Hz. These measurements, taken without a feedback
resistor, provide the basis for calculating the input noise and
noise figure performance of the configurations in Figure 60.
Figure 61 and Figure 62 are simulations extracted from these
results, and the 4.1 dB NF measurement with the input actively
matched to a 50 Ω source. Unterminated (RFB = ∞) operation
exhibits the lowest equivalent input noise and noise figure.
Figure 61 shows the noise figure versus source resistance, rising
at low RS, where the LNA voltage noise is large compared to the
source noise, and again at high RS due to current noise. The
VGA’s input-referred voltage noise of 2.7 nV/√Hz is included in
all of the curves.
Rev. C | Page 18 of 32