AD604
AC COUPLING
The DSX portion of the AD604 is a single-supply circuit and,
therefore, its inputs need to be ac-coupled to accommodate
ground-based signals. External capacitors C1 and C2 in Figure 37
level shift the ground referenced preamplifier output from
ground to the dc value established by VOCM (nominal 2.5 V).
C1 and C2, together with the 175 Ω looking into each of the
DSX inputs (+DSX and −DSX), act as high-pass filters with
corner frequencies depending on the values chosen for C1 and
C2. As an example, for values of 0.1 μF at C1 and C2, combined
with the 175 Ω input resistance at each side of the differential
ladder of the DSX, the −3 dB high-pass corner is 9.1 kHz.
If the AD604 output needs to be ground referenced, another ac
coupling capacitor is required for level shifting. This capacitor
also eliminates any dc offsets contributed by the DSX. With a
nominal load of 500 Ω and a 0.1 μF coupling capacitor, this adds
a high-pass filter with −3 dB corner frequency at about 3.2 kHz.
The choice for all three of these coupling capacitors depends on
the application. They should allow the signals of interest to pass
unattenuated, while at the same time, they can be used to limit
the low frequency noise in the system.
From these equations, it can be seen that all gain curves intercept at
the same −5 dB point; this intercept is +6 dB higher (+1 dB) if
the preamplifier gain is set to +20 dB or +14 dB lower (−19 dB)
if the preamplifier is not used at all. Outside of the central linear
range, the gain starts to deviate from the ideal control law but
still provides another 8.4 dB of range. For a given gain scaling,
V
REF
can be calculated as shown in Equation 7.
VREF
=
2.500 V
×
20 dB/V
Gain Scale
(7)
Usable gain control voltage ranges are 0.1 V to 2.9 V for the
20 dB/V scale and 0.1 V to 1.45 V for the 40 dB/V scale. VGN
voltages of less than 0.1 V are not used for gain control because
below 50 mV the channel (preamplifier and DSX) is powered
down. This can be used to conserve power and, at the same
time, to gate off the signal. The supply current for a powered-
down channel is 1.9 mA; the response time to power the device
on or off is less than 1 μs.
ACTIVE FEEDBACK AMPLIFIER (FIXED GAIN AMP)
To achieve single-supply operation and a fully differential input
to the DSX, an active feedback amplifier (AFA) is used. The
AFA is an op amp with two g
m
stages; one of the active stages is
used in the feedback path (therefore the name), while the other
is used as a differential input. Note that the differential input is
an open-loop g
m
stage that requires it to be highly linear over
the expected input signal range. In this design, the g
m
stage that
senses the voltages on the attenuator is a distributed one; for
example, there are as many g
m
stages as there are taps on the
ladder network. Only a few of them are on at any one time,
depending on the gain-control voltage.
The AFA makes a differential input structure possible because
one of its inputs (G1) is fully differential; this input is made up
of a distributed g
m
stage. The second input (G2) is used for
feedback. The output of G1 is some function of the voltages
sensed on the attenuator taps, which is applied to a high gain
amplifier (A0). Because of negative feedback, the differential
input to the high gain amplifier has to be zero; this in turn
implies that the differential input voltage to G2 times g
m2
(the
transconductance of G2) has to be equal to the differential
input voltage to G1 times g
m1
(the transconductance of G1).
Therefore, the overall gain function of the AFA is
V
OUT
V
ATTEN
where:
g
m1
g
m2
R1
+
R 2
R2
GAIN CONTROL INTERFACE
The gain control interface provides an input resistance of
approximately 2 MΩ at VGN1 and gain scaling factors from
20 dB/V to 40 dB/V for VREF input voltages of 2.5 V to 1.25 V,
respectively. The gain scales linearly-in-dB for the center 40 dB
of gain range, which for VGN is equal to 0.4 V to 2.4 V for the
20 dB/V scale and 0.2 V to 1.2 V for the 40 dB/V scale. Figure 42
shows the ideal gain curves for a nominal preamplifier gain of
14 dB, which are described by the following equations:
G
(20 dB/V) = 20 ×
VGN
– 5,
VREF
= 2.500 V
G
(20 dB/V) = 30 ×
VGN
– 5,
VREF
= 1.666 V
G
(20 dB/V) = 40 ×
VGN
– 5,
VREF
= 1.250 V
50
45
40
35
30
40dB/V
30dB/V
20dB/V
(4)
(5)
(6)
GAIN (dB)
25
20
15
10
5
0.5
LINEAR-IN-dB RANGE
OF AD604 WITH
PREAMPLIFIER
SET TO 14dB
=
×
(8)
–5
1.0
1.5
2.0
GAIN CONTROL VOLTAGE (VGN)
2.5
3.0
Figure 42. Ideal Gain Curves vs. VGN
00540-042
0
V
OUT
is the output voltage.
V
ATTEN
is the effective voltage sensed on the attenuator.
(R1+R2)/R2 = 42
g
m1
/g
m2
= 1.25
The overall gain is thus 52.5 (34.4 dB).
Rev. D | Page 16 of 32
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