AD8362
An approximate schematic of the signal input section of the
AD8362 is shown in Figure 52. The ladder attenuator is
composed of 11 sections (12 taps), each of which progressively
attenuates the input signal by 6.33 dB. Each tap is connected to
a variable transconductance cell whose bias current determines
the signal weighting given to that tap. The interpolator
determines which stages are active by generating a discrete set
of bias currents, each having a Gaussian profile. These are
arranged to move from left to right, thereby determining the
attenuation applied to the input signal as the gain is
progressively lowered over the 69.3 dB range under control of
the VSET input. The detailed manner in which the
transconductance of adjacent stages varies as the virtual tap
point slides along the attenuator accounts for the ripple
observed in the conformance curves. Its magnitude is slightly
temperature dependent and also varies with frequency (see
Figure 10 to Figure 12). Notice that the system’s responses to
signal inputs at INHI and INLO are not completely
independent; these pins do not constitute a fully floating
differential input.
AD8362
THIS INPUT
IS DRIVEN
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
COMM ACOM
NC
CHPF
DECL
INHI
VREF
VTGT
VPOS
VOUT
VSET
1nF
RF INPUT
Z = 50Ω
R
SH
INLO
DECL
1nF
PWDN ACOM
COMM CLPF
DECL AND
INLO ARE
NOT DRIVEN
Figure 53. Input Coupling from a Single-Ended Source
Figure 53 illustrates one of many ways of coupling the signal
source to the AD8362. Because the input pins are biased to
about 3.6 V (for VS = 5 V) dc-blocking capacitors are required
when driving from a grounded source. For signal frequencies
>5 MHz, a value of 1 nF is adequate. While either INHI or
INLO may be used, INHI is chosen here, and INLO is
connected to the low side of the source. The resistor RSH is not
needed if a 100 Ω termination is acceptable. The corresponding
intercept is still −67 dBV, that is, 447 µV rms. However, specified
in power terms at 100 Ω, the PZ is now at 2 nW or −57 dBm.
ATTENUATION
GUASSIAN INTERPOLATOR
CONTROL
For a source termination of 50 Ω, the internal 100 Ω from INHI
to DECL must be shunted by a chip resistor of 100 Ω. At high
frequencies, a low attenuation pad at the input improves the
VSWR. For example, with a resistor of RSH = 33 Ω and an added
resistor of 25 Ω from the source to INHI, a termination of 50 Ω
is provided, with 6 dB of attenuation, raising the intercept to
−48 dBm.
TO FIXED
GAIN STAGE
gm
gm
gm
gm
INHI
DECL
USING A NARROW-BAND INPUT MATCH
INLO
While transformers offer the simplest method for providing
single-sided to balanced conversion, a good alternative is
using a specially designed narrow-band LC network, shown in
Figure 54, which also provides an input match. Using this basic
formulation, the match is to 50 Ω, with a voltage gain of 1.5
(3.56 dB) from the input connector to the AD8362. This
network is specially designed to provide a high degree of
amplitude balance at INHI and INLO as well as an exact phase
inversion. The narrow-band match provides a useful degree of
frequency selectivity, and the capacitors also serve to provide
the required dc blocking.
STAGE 1
6.33dB
STAGE 11
6.33dB
STAGE 2
6.33dB
Figure 52. Simplified Input Circuit
ALTERNATIVE INPUT COUPLING MEANS
The input stages of the AD8362 are optimally driven from a
fully balanced source, which should be provided wherever
possible. The ac low sides of both halves of the attenuator
internally connect to the DECL pin, which is therefore the RF
signal low terminal for both INHI and INLO. In many cases,
unbalanced sources can be applied directly to one or the other
of these two pins. The chief disadvantage of this driving method
is a reduction in dynamic range, particularly at very high
frequencies.
AD8362
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
COMM
CHPF
DECL
INHI
ACOM
VREF
VTGT
VPOS
VOUT
VSET
ACOM
CLPF
NC
C1
RF INPUT
Z = 50Ω
100Ω
L
INLO
DECL
PWDN
COMM
C2
THESE INPUTS ARE
EQUAL IN AMPLITUDE
AND OF OPPOSITE SIGN
Figure 54. Narrow-Band Reactive Input Coupling
Rev. B | Page 23 of 36
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