AD605-EB Analog Devices Inc, AD605-EB Datasheet - Page 13

• Manufacturer Part Number: AD605-EB
• Description: BOARD EVAL/DEMO ADJ GAIN AD605
• Manufacturer: Analog Devices Inc
• Type: Amplifiers: OP Ampsr

Specifications of AD605-EB

• Rohs Status: RoHS non-compliant
• Contents: Evaluation Board
• For Use With/related Products: AD605

THEORY OF OPERATION
The AD605 is a dual-channel, low noise VGA. Figure 35 shows
the simplified block diagram of one channel. Each channel consists
of a single-supply X-AMP® (hereafter called DSX, differential
single-supply X-AMP) comprising the following:
• Precision passive attenuator (differential ladder)
• Gain control block
• VOCM buffer with supply splitting resistors R3 and R4
• Active feedback amplifier
The linear-in-dB gain response of the AD605 can generally be
described by Equation 1.
where:
FB = 0, if FBK to OUT is shorted.
FB = 1, if FBK to OUT is open.
Each channel provides between −14 dB to +34.4 dB through
0 dB to +48.4 dB of gain, depending on the value of the resistance
connected between Pin FBK and Pin OUT. The center 40 dB of
gain is exactly linear-in-dB while the gain error increases at the top
and bottom of the range. The gain is set by the gain control voltage
(VGN). The VREF input establishes the gain scaling. The useful
gain scaling range is between 20 dB/V and 40 dB/V for a VREF
voltage of 2.5 V and 1.25 V, respectively. For example, if FBK to
OUT is shorted and VREF is set to 2.50 V (to establish a gain
scaling of 20 dB/V), the gain equation simplifies to
R1 and R2
G (dB) = (Gain Scaling (dB/V)) × (Gain Control (V)) −
(19 dB − (14 dB) × (FB))
G (dB) = (20 (dB/V)) × (VGN (V)) – 19 dB
VOCM
VREF
VGN
+IN
–IN
EXT
1
(AFA) with gain setting resistors
C3
VPOS
Figure 35. Simplified Block Diagram of a Single Channel of the AD605
R3
200kΩ
R4
200kΩ
C1
C2
+
EXT
Rev. F | Page 13 of 24
175Ω
175Ω
(1)
(2)
DIFFERENTIAL
ATTENUATOR
CONTROL
GAIN
The desired gain can then be achieved by setting the unipolar
gain control (VGN) to a voltage within its nominal operating
range of 0.25 V to 2.65 V (for 20 dB/V gain scaling). The gain is
monotonic for a complete gain control range of 0.1 V to 2.9 V.
Maximum gain can be achieved at a VGN of 2.9 V.
Because the two channels are identical, only Channel 1 is used
to describe their operation. VREF and VOCM are the only inputs
that are shared by the two channels, and because they are normally
ac grounds, crosstalk between the two channels is minimized.
For the highest gain scaling accuracy, VREF should have an
external low impedance voltage source. For low accuracy 20 dB/V
applications, the VREF input can be decoupled with a capacitor
to ground. In this mode, the gain scaling is determined by the
midpoint between +VCC and GND; therefore, care should be
taken to control the supply voltage to 5 V. The input resistance
looking into the VREF pin is 10 kΩ ± 20%.
The AD605 is a single-supply circuit, and the VOCM pin is used
to establish the dc level of the midpoint of this portion of the
circuit. VOCM needs only an external decoupling capacitor to
ground to center the midpoint between the supply voltages (5 V,
GND). However, if the dc level of the output is important to the
user (see the Applications Information section of the
data sheet for an example), VOCM can be specifically set. The
input resistance looking into the VOCM pin is 45 kΩ ± 20%.
1
20Ω
R2
To understand the active-feedback amplifier topology, refer to the
data sheet. The AD830 is a practical implementation of the idea.
+
+
G1
G2
DISTRIBUTED g
820Ω
R1
+
Ao
m
3.36kΩ
OUT
FBK
AD9050
AD605
AD830