AD8337BCPZ Analog Devices Inc, AD8337BCPZ Datasheet - Page 19

• Manufacturer Part Number: AD8337BCPZ
• Description: VGA, DC COUPLED, SMD, LFCSP-8, 8337
• Manufacturer: Analog Devices Inc

Specifications of AD8337BCPZ

• No. Of Amplifiers: 1
• Bandwidth: 280MHz
• Gain Accuracy: 3.5dB
• No. Of Channels: 1
• Supply Voltage Range: 4.5V To 10V
• Amplifier Case Style: LFCSP
• No. Of Pins: 8
• Operating Temperature Range: -40°C To
• 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 to make sure that the GAIN input does not inadver-
tently 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
preamplifier 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 section).
ATTENUATOR
The input resistance of the VGA attenuator is nominally 265 Ω.
Assuming the default preamplifier feedback network R
is 200 Ω, the effective preamplifier load is about 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 Pin VCOM must provide the
dynamic current to the preamplifier feedback network and the
VGA attenuator. For many single 5 V applications, a reference,
such as the ADR391, and a good op amp provide an adequate
VCOM source if a 2.5 V supply is unavailable.
n e
−
out
=
(
R
S
×
t A
)
2
+
(e
n
−
PrA
×
t A
2 )
+
(i
n
−
PrA
×
FB1
R
+ R
S
2 )
FB2
Rev. B | Page 19 of 24
+
(e
n
−
R
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 about 1.55 nV/√Hz.
Equation 2 shows the calculation that determines the output-
referred noise at maximum gain (24 dB or 16×).
where:
•
•
•
•
•
•
•
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
change. However, 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 − out
R
R
FB2
FB1
A
R
e
i
e
e
e
gain, output-referred noise divided by a fixed gain of 8×).
n − PrA
=
n − PrA
n −
n −
n − VGA
S
t
FB1
is the total gain from preamp input to VGA output.
is the source resistance.
×
R
R
A
FB
FB
(1.75
is the current noise of the preamp at the INPP pin.
and R
is the input-referred voltage noise of the preamp.
VGA
1
2
is the input-referred voltage noise of the VGA (low
S
is the voltage noise of R
is the voltage noise of R
= 0 Ω, R
×
2 )
FB2
16)
+
= 100 Ω as previously noted, e
2
(e
+
FB1
n
2(1.29
−
= R
R
FB2
FB2
×
×
A
= 100 Ω, A
8)
VGA
2
+
FB1
FB2
(1.9
.
.
2 )
+
×
(e
t
8)
= 16×, and A
n
2
−
=
VGA
35
n − out
nV
AD8337
×
does not
A
VGA
VGA
Hz
(2)
=
(1)
2 )