AD8336-EVALZ Analog Devices Inc, AD8336-EVALZ Datasheet - Page 20

• Manufacturer Part Number: AD8336-EVALZ
• Description: BOARD EVALUATION FOR AD8336
• Manufacturer: Analog Devices Inc
• Series: X-AMP®r

Specifications of AD8336-EVALZ

• Channels Per Ic: 1 - Single
• Amplifier Type: Variable Gain
• Output Type: Single-Ended
• Slew Rate: 550 V/µs
• -3db Bandwidth: 115MHz
• Current - Output / Channel: 20mA
• Operating Temperature: -55°C ~ 125°C
• Current - Supply (main Ic): 28mA
• Voltage - Supply, Single/dual (±): ±3 V ~ 12 V
• Board Type: Fully Populated
• Utilized Ic / Part: AD8336
• Silicon Manufacturer: Analog Devices
• Application Sub Type: Variable Gain Amplifier
• Kit Application Type: Amplifier
• Silicon Core Number: AD8336
• Kit Contents: Board
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD8336
THEORY OF OPERATION
OVERVIEW
The AD8336 is the first VGA designed for operation over
exceptionally broad ranges of temperature and supply voltage.
Its performance has been characterized from temperatures
extending from −55°C to +125°C, and supply voltages from ±3 V
to ±12 V. It is ideal for applications requiring dc coupling, large
output voltage swings, very large gain ranges, extreme temperature
variations, or a combination thereof.
The simplified block diagram is shown in Figure 77. The
AD8336 includes a voltage feedback preamplifier, an amplifier
with a fixed gain of 34 dB, a 60 dB attenuator, and various bias
and interface circuitry. The independent voltage feedback op amp
can be used in noninverting and inverting configurations and
functions as a preamplifier to the variable gain amplifier (VGA).
If desired, the op amp output (PRAO) and VGA input (VGAI)
pins provide for connection of an interstage filter to eliminate
noise and offset. The bandwidth of the AD8336 is dc to 100 MHz
with a gain range of 60 dB (−14 dB to +46 dB).
For applications that require large supply voltages, a reduction
in power is advantageous. The power reduction pin (PWRA)
permits the power and bandwidth to be reduced by about half
in such applications.
*OPTIONAL DEPEAKING CAPACITOR. SEE TEXT.
To maintain low noise, the output stages of both the preamplifier
and the VGA are capable of driving relatively small load resistances.
However, at the largest supply voltages, the signal current may
exceed safe operating limits for the amplifiers and, therefore,
the load current must not exceed 50 mA. With a ±12 V supply
and ±10 V output voltage at the preamplifier or VGA output,
load resistances as low as 200 Ω are acceptable.
For power supply voltages ≥±10 V, the maximum operating
temperature range is derated to +85°C because the power may
exceed safe limits (see the Absolute Maximum Ratings section).
Because harmonic distortion products may increase for various
combinations of low impedance loads and high output voltage
swings, it is recommended that the user determine load and
drive conditions empirically.
R
301Ω
FB2
100Ω
R
FB1
INPP
INPN
*
PWRA
Figure 77. Simplified Block Diagram
+
–
PrA
12dB
VPOS
BIAS
PRAO
VNEG
VGAI
GPOS
GAIN CONTROL
–60dB TO 0dB
ATTENUATOR
INTERFACE
AND
GNEG
VCOM
+
_
34dB
VOUT
4.48kΩ
91.43Ω
Rev. A | Page 20 of 28
PREAMPLIFIER
The gain of the uncommitted voltage feedback preamplifier is
set with external resistors. The combined preamplifier and VGA
gain is specified in two ranges: −14 dB to +46 dB and 0 dB to 60 dB.
Since the VGA gain is fixed at 34 dB (50×), the preamp gain is
adjusted for gains of 12 dB (4×) and 26 dB (200×).
With low preamplifier gains between 2× and 4×, it may be
desirable to reduce the high frequency gain with a shunt
capacitor across R
domain (see Figure 77). To maintain stability, the gain of the
preamplifier must be 6 dB (2×) or greater.
Typical of voltage feedback amplifier configurations, the gain-
bandwidth product of the AD8336 is fixed (at 600); therefore,
the bandwidth decreases as the gain is increased beyond the
nominal gain value of 4×. For example, if the preamp gain is
increased to 20×, the bandwidth reduces by a factor of 5 to about
20 MHz. The −3 dB bandwidth of the preamplifier with a gain
of 4× is about 150 MHz, and for the 20× gain is about 30 MHz.
The preamp gain diminishes for an amplifier configured for
inverting gain, using the same value of feedback resistors as for a
noninverting amplifier, but the bandwidth remains unchanged. For
example, if the noninverting gain is 4×, the inverting gain is −3×,
but the bandwidth stays the same as in the noninverting gain of 4×.
However, because the output-referred noise of the preamplifier
is the same in both cases, the input-referred noise increases as
the ratio of the two gain values increases. For the previous example,
the input-referred noise increases by a factor of 4/3.
VGA
The architecture of the variable gain amplifier (VGA) section
of the AD8336 is based on the Analog Devices, Inc., X-AMP
(exponential amplifier), found in a wide variety of Analog Devices
variable gain amplifiers. This type of VGA combines a ladder
attenuator and interpolator, followed by a fixed-gain amplifier.
The gain control interface is fully differential, permitting positive
or negative gain slopes. Note that the common-mode voltage of
the gain control inputs increases with increasing supply.
The gain slope is 50 dB/V and the intercept is 16.4 dB when the
nominal preamp gain is 4× (12 dB). The intercept changes with
the preamp gain; for example, when the preamp gain is set to
20× (26 dB), the intercept becomes 30.4 dB.
Pin VGAI is connected to the input of the ladder attenuator.
The ladder ratio is R/2R and the nominal resistance is 320 Ω. To
reduce preamp loading and large-signal dissipation, the input
resistance at Pin VGAI is 1.28 kΩ. Safe current density and
power dissipation levels are maintained even when large dc
signals are applied to the ladder.
The tap resistance of the resistors within the R/2R ladder is
640 Ω/3, or 213.3 Ω, and is the Johnson noise source of the
attenuator.
FB2
to ameliorate peaking in the frequency