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AD604 Datasheet(PDF) 10 Page - Analog Devices

Part # AD604
Description  Dual, Ultralow Noise Variable Gain Amplifier
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

AD604 Datasheet(HTML) 10 Page - Analog Devices

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AD604
REV. 0
–10–
Since the internal resistors have an absolute tolerance of
±20%,
the gain can be in error by as much as 0.33 dB when REXT is
30
Ω, where it was assumed that R
EXT is exact.
Figure 36 shows how the preamplifier is set to gains of +14,
+17.5 and +20 dB. The gain range of a single channel of the
AD604 is 0 dB to +48 dB when the preamplifier is set to
+14 dB (Figure 36a), 3.5 dB to +51.5 dB for a preamp gain of
+17.5 dB (Figure 36b), and 6 dB to 54 dB for the highest
preamp gain of +20 dB (Figure 36c).
FBK1
PAO1
COM1
R5
32
R7
40
R6
8
PAI1
a. Preamp Gain = 14 dB
R10
40
FBK1
PAO1
COM1
R5
32
R7
40
R6
8
PAI1
b. Preamp Gain = 17.5 dB
FBK1
PAO1
COM1
R5
32
R7
40
R6
8
PAI1
c. Preamp Gain = 20 dB
Figure 36. Preamplifier Gain Programmability
For a preamplifier gain of +14 dB, the preamplifier’s –3 dB
small-signal bandwidth is 130 MHz; when the gain is at the high
end (+20 dB), the bandwidth will be reduced by a factor of two
to 65 MHz. Figure 37 shows the ac responses for the three preamp
gains discussed above; note that the gain for an REXT of 40
should be 17.5 dB, but the mismatch between the internal resis-
tors and the external resistor has caused the actual gain for this
particular preamplifier to be 17.7 dB. The –3 dB small-signal
bandwidth of one complete channel of the AD604 (preamplifier
and DSX) is 40 MHz and is independent of gain.
REXT – Ω
20
19
10
100k
1M
10M
100M
18
17
16
15
14
13
12
11
OPEN
40
SHORT
50
40
REXT
150
8
32
IN
VIN
Figure 37. AC Responses for Preamplifier Gains Shown in
Figure 36.
To achieve its optimum specifications, power and ground man-
agement are critical to the AD604. Large dynamic currents
result because of the low resistances needed for the desired
noise performance. Most of the difficulty is with the very low
gain setting resistors of the preamplifier that allow for a total
input referred noise, including the DSX, as low as 0.8 nV/
√Hz.
The consequently large dynamic currents have to be carefully
handled to maintain performance even at large signal levels.
To accommodate these large dynamic currents as well as a
ground referenced input, the preamplifier is operated from a
dual
±5 V supply. This causes the preamplifiers output to also
be ground referenced, which requires a common-mode level
shift into the single-supply DSX. The two external coupling ca-
pacitors (C1, C2 in Figure 35) connected to nodes PAO1 and
+DSX, and –DSX and ground, respectively, perform this func-
tion (see AC Coupling Section). In addition, they eliminate any
offset that would otherwise be introduced by the preamplifier. It
should be noted that an offset of 1 mV at the input of the DSX
will get amplified by +34.4 dB (
× 52.5) when the gain-control
voltage is at its maximum, this equates to 52.5 mV at the out-
put. AC coupling is consequently required to keep the offset
from degrading the output signal range.
The internal feedback resistors setting the gain of the preampli-
fier are so small (nominally 8
Ω and 32 Ω) that even an addi-
tional 1
Ω in the “ground” connection at pin COM1, which
serves as the input common-mode reference, will seriously
degrade gain accuracy and noise performance. This node is very
sensitive and careful attention is necessary to minimize the
ground impedance. All connections to node COM1 should be
as short as possible.
The preamplifier including the gain setting resistors has a noise
performance of 0.71 nV/
√Hz and 3 pA/√Hz. Note that a signifi-
cant portion of the total input referred voltage noise is due to
the feedback resistors. The equivalent noise resistance presented
by R5 and R6 in parallel is nominally 6.4
Ω, which contributes
0.33 nV/
√Hz to the total input referred voltage noise. The larger
portion of the input referred voltage noise is coming from the
amplifier with 0.63 nV/
√Hz. The current noise is independent of
gain and depends only on the bias current in the input stage of
the preamplifier—it is 3 pA/
√Hz.
The preamplifier can drive 40
Ω (the nominal feedback resis-
tors) and the following 175
Ω ladder load of the DSX with low
distortion. For example, at 10 MHz and
±1 V at the output, the
preamplifier has less than –45 dB of second and third harmonic
distortion when driven from a low (25
Ω) source resistance.
In some cases one may need more than 48 dB of gain range, in
which case two AD604 channels could be cascaded. Since the
preamplifier has limited input signal range, consumes over half
(120 mW) of the total power (220 mW), and its ultralow noise
is not necessary after the first AD604 channel, a shutdown
mechanism that disables only the preamplifier is built in. All
that is required to shut down the preamplifier is to tie the
COM1 and/or COM2 pin to the positive supply. The DSX will
be unaffected and can be used as before (see APPLICATIONS
section for further details).


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