Analog Devices AD604 - Manual (Page 15)

AD604
The larger portion of the input referred voltage noise comes
from the amplifier with 0.63 nV/VHz. The current noise is
independent of gain and depends only on the bias current in
the input stage of the preamplifier, which is 3 pA/VHz.
The preamplifier can drive 40 O (the nominal feedback resistors)
and the following 175 O 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 O) source resistance.
In applications that require more than 48 dB of gain range, two
AD604 channels can be cascaded. Because the preamplifier has
a limited input signal range and 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 provided. To shut down the
preamplifier, connect the COM1 pin and/or COM2 pin to the
positive supply; the DSX is unaffected. For additional details,
refer to the Applications Information section.
---------- o-------------
E -DSX1 VGN1 24]
E
+DSX1 VREF 23 l
E
PAO1 OUT1 22l
E FBK1 GND1 21]
E PAI1 VPOS 20 PI
>-E
COM1 AD604 V NEG
19l
1—1 7 COM2 VNEG 18 1
\ m~ 8 " VPOS
\y V 9 "
F ¥ K2 A
b G n D2 ie l 1
QO PAO2 OUT2 jU
nr +DSX2 VOCM 14 l o
E -DSX2 VGN2 ^ i
Figure 40. Shutdown of Preamplifiers Only
DIFFERENTIAL LADDER (ATTENUATOR)
The attenuator before the fixed-gain amplifier of the DSX is
realized by a differential 7-stage R-1.5R resistive ladder network
with an untrimmed input resistance of 175 O single-ended or
350 O differential. The signal applied at the input of the ladder
network is attenuated by 6.908 dB per tap; thus, the attenuation
at the first tap is 0 dB, at the second, 13.816 dB, and so on, all
the way to the last tap where the attenuation is 48.356 dB
(see Figure 41).
A unique circuit technique is used to interpolate continuously
among the tap points, thereby providing continuous attenuation
from 0 dB to -48.36 dB. The ladder network, together with the
interpolation mechanism, can be considered a voltage-controlled
potentiometer.
Because the DSX circuit uses a single voltage power supply, the
input biasing is provided by the VOCM buffer driving the MID
node (see Figure 41). Without internal biasing, the user would
have to dc bias the inputs externally. If not done carefully, the
biasing network can introduce additional noise and offsets. By
providing internal biasing, the user is relieved of this task and
only needs to ac-couple the signal into the DSX. Note that the
input to the DSX is still fully differential if driven differentially;
that is, Pin +DSXx and Pin -DSXx see the same signal but with
opposite polarity (see the Ultralow Noise, Differential Input­
Differential Output VGA section).
What changes is the load seen by the driver; it is 175 O when
each input is driven single-ended but 350 O when driven
differentially. This is easily explained by thinking of the ladder
network as two 175 O resistors connected back-to-back with
the middle node, MID, being biased by the VOCM buffer. A
differential signal applied between the +DSXx and -DSXx
nodes results in zero current into the MID node, but a single-
ended signal applied to either input, +DSXx or -DSXx, while
the other input is ac-grounded causes the current delivered by
the souice/oflpwinto th#VOCM butfer^ia the MID node.
• The^adder^resistlrJLaiue of 175 Ol^rivides tHb-optimum
balance between the load driving capability of the preamplifier
and the noise contribution of the resistors. An advantage of the
X-AMP architecture is that the output referred noise is constant
vs. gain over most of the gain range. Figure 41 shows that the
tap resistance is equal for all taps after only a few taps away
from the inputs. The resistance seen looking into each tap is
54.4 O, which makes 0.95 nV/VHz of Johnson noise spectral
density. Because there are two attenuators, the overall noise
contribution of the ladder network is V2 times 0.95 nV/VHz
or 1.34 nV/VHz, a large fraction of the total DSX noise. The
balance of the DSX circuit components contributes another
1.2 nV/VHz, which together with the attenuator produces
1.8 nV/VHz of total DSX input referred noise.
R -6.908dB R -13.82dB R -20.72dB R -27.63dB R -34.54dB R -41.45dB R -48.36dB
-----------( I---------- wv (
i 1 . 5R :
1 (
i 1 . 5R :
1 -------------------------(
i 1 . 5R :
1 W» 1
i 1 . 5R :
1----------Wv—(
■1.5R ■
1--------- WV—(
■1.5R : ■1.5R :
R
VW 1
:1.5R •
R
:1.5R •
R
1 VW (
:1.5R :
R
1 VA (
:1.5R :
R
1 WV 1
:1.5R ■
R
1 W» I
•1.5R :
R
i vw <
•1.5R :
NOTES
1. R = 960
2. 1.5R = 1440
Figure 41. R-1.5R Dual Ladder Network
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