AD8337BCPZ-WP Analog Devices Inc, AD8337BCPZ-WP Datasheet - Page 19

• Manufacturer Part Number: AD8337BCPZ-WP
• Description: IC AMP VGA DC-COUPLED GP 8-LFCSP
• Manufacturer: Analog Devices Inc
• Series: X-AMP®r
• Type: Variable Gain Amplifierr

Specifications of AD8337BCPZ-WP

• Applications: Signal Processing
• Mounting Type: Surface Mount
• Package / Case: 8-LFCSP
• For Use With: AD8337-EVALZ-SS - BOARD EVALUATION FOR AD8337 SSAD8337-EVALZ-INV - BOARD EVALUATION FOR AD8337 INSAD8337-EVALZ - BOARD EVALUATION FOR AD8337
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The gain scaling factor (gain slope) is designed for 20 dB/V. This
relatively low slope ensures that noise on the GAIN input is not
unduly amplified. Because a VGA functions as a multiplier, it is
important that the GAIN input does not inadvertently modulate
the output signal with unwanted noise. Because of its high input
impedance, a simple low-pass filter can be added to the GAIN
input to filter unwanted noise.
OUTPUT STAGE
The output stage is a Class AB, voltage-feedback, complementary
emitter-follower with a fixed gain of 18 dB, similar to the pream-
plifier in speed and bandwidth. Because of the ac-beta roll-off
of the output devices and the inherent reduction in feedback
beyond the −3 dB bandwidth, the impedance looking into the
output pin of the preamp and output stages appears to be inductive
(increasing impedance with increasing frequency). The high
speed output amplifier used in the AD8337 can drive large
currents, but its stability is susceptible to capacitive loading.
A small series resistor mitigates the effects of capacitive loading
(see the Applications Information section).
ATTENUATOR
The input resistance of the VGA attenuator is nominally 265 Ω.
For example, if the default preamplifier feedback network R
R
114 Ω. The attenuator is composed of eight 3.01 dB sections for
a total attenuation range of −24.08 dB. Following the attenuator
is a fixed gain amplifier with 8× (18.06 dB) gain. Because of this
relatively low gain, the output offset is kept well below 20 mV
over temperature; the offset is largest at maximum gain when
the preamplifier offset is amplified. The VCOM pin defines the
common-mode reference for the output, as shown in Figure 65.
SINGLE-SUPPLY OPERATION AND AC COUPLING
If the AD8337 is to be operated from a single 5 V supply, the
bias supply for VCOM must be a very low impedance 2.5 V
reference, especially if dc coupling is used. If the device is dc-
coupled, the VCOM source must be able to handle the preamplifier
and VGA dynamic load currents in addition to the bias currents.
When ac coupling the preamplifier input, a bias network and
bypass capacitor must be connected to the opposite polarity
input pin. The bias generator for the VCOM pin must provide
the dynamic current to the preamplifier feedback network and
the VGA attenuator. For many single 5 V applications, a refer-
ence, such as the ADR391, and a good op amp provide an
adequate VCOM source if a 2.5 V supply is unavailable.
n e
FB2
−
is 200 Ω, the effective preamplifier load is approximately
out
=
(
n e
R
S
×
t A
)
2
+
(e
n
−
PrA
×
t A
2 )
+
(i
n
−
PrA
×
R
S
2 )
+
FB1
(e
Rev. C | Page 19 of 32
n
+
−
R
FB1
×
R
R
FB2
FB1
NOISE
The total input-referred voltage and current noise of the positive
input of the preamplifier are about 2.2 nV/√Hz and 4.8 pA/√Hz.
The VGA output-referred noise is about 21 nV/√Hz at low gains.
This result is divided by the VGA fixed gain amplifier gain of 8×
and results in a voltage noise density of 2.6 nV/√Hz referred to
the VGA input. This value includes the noise of the VGA gain
setting resistors as well. If this voltage is again divided by the
preamp gain of 2, the VGA noise referred all the way to the
preamp input is about 1.3 nV/√Hz. From this, it is determined
that the preamplifier, including the 100 Ω gain setting resistors,
contributes about 1.8 nV/√Hz. The two 100 Ω resistors contribute
1.29 nV/√Hz each at the output of the preamp. With the gain
resistor noise subtracted, the preamplifier noise is approximately
1.55 nV/√Hz.
Equation 2 shows the calculation that determines the output-
referred noise at maximum gain (24 dB or 16×).
where:
A
R
e
i
e
e
e
output-referred noise divided by a fixed gain of 8×).
Assuming R
8×, the noise simplifies to
e
Dividing the result by 16 gives the total input-referred noise with
a short-circuited input as 2.2 nV/√Hz. When the preamplifier is
used in the inverting configuration with the same R
100 Ω as previously noted, e
because the gain dropped by 6 dB, the input-referred noise
increases by a factor of 2 to about 4.4 nV/√Hz. The reason for this
increase is that the noise gain to the output of the noise generators
stays the same, yet the preamp in the inverting configuration has a
gain of −1 compared to the +2 in the noninverting configuration;
this increases the input-referred noise by 2.
n − PrA
n − PrA
n −
n −
n − VGA
n − out
S
t
is the total gain from preamp input to VGA output.
is the source resistance.
×
R
R
FB
FB
A
is the current noise of the preamp at the INPP pin.
=
is the input-referred voltage noise of the preamp.
1
2
VGA
is the input-referred voltage noise of the VGA (low gain,
is the voltage noise of R
is the voltage noise of R
(1.75
2 )
S
= 0 Ω, R
+
×
(e
16)
n
−
2
+
R
FB1
FB2
2(1.29
= R
×
A
VGA
n − out
FB2
×
FB1
FB2
8)
= 100 Ω, A
.
.
does not change. However,
2
2 )
+
+
(1.9
(e
n
×
−
t
8)
VGA
= 16×, and A
2
×
=
A
35
FB1
VGA
nV
and R
AD8337
2 )
Hz
VGA
FB2
=
=
(1)
(2)