AD605
THEORY OF OPERATION
The AD605 is a dual channel, low noise variable gain amplifier.
Each channel consists of a single-supply X-AMP® (hereafter
called DSX, differential single-supply X-AMP) comprised of:
•
•
•
•
Precision passive attenuator (differential ladder)
Gain control block
VOCM buffer with supply splitting resistors R3 and R4
Active feedback amplifier
(AFA) with gain setting
resistors R1 and R2
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 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 section of the
data sheet for
an example), then VOCM can be specifically set. The input
resistance looking into the VOCM pin is 45 kΩ ± 20%.
1
The linear-in-dB gain response of the AD605 can generally be
described by Equation 1.
G
(dB) = (Gain
Scaling
(dB/V)) × (Gain
Control
(V)) −
(19 dB − (14 dB) × (FB))
(1)
where:
FB
= 0, if FBK-to-OUT are 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 were shorted and
VREF were set to 2.50 V (to establish a gain scaling of 20 dB/V),
the gain equation would simplify to
G
(dB) = (20 (dB/V)) × (VGN (V)) – 19 dB
(2)
To understand the active-feedback amplifier topology, refer to the
data sheet. The AD830 is a practical implementation of the idea.
VREF
VGN
+IN
C1
EXT
175Ω
GAIN
CONTROL
DISTRIBUTED G
M
DIFFERENTIAL
ATTENUATOR
+
G1
+
Ao
175Ω
R2
20Ω
R1
820Ω
00541-036
C2
–IN
VPOS
R3
200kΩ
VOCM
C3
EXT
R4
200kΩ
OUT
3.36kΩ
FBK
G2
+
+
Figure 35. Simplified Block Diagram of a Single Channel of the AD605
Rev. D | Page 13 of 20
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