AD8362
OPERATION IN RF MEASUREMENT MODE
The balun outputs must be ac-coupled to the input of the
AD8362. The balun used in this example (M/A-COM ETC
1.6-4-2-3) is specified for operation from 0.5 GHz to 2.5 GHz.
BASIC CONNECTIONS
Basic connections for operating the AD8362 in measurement
mode are shown in Figure 47. While the AD8362 requires a
single supply of nominally 5 V, its performance is essentially
unaffected by variations of up to 10%.
If a center-tapped, flux-coupled transformer is used, connect
the center tap to the DECL pins, which are biased to the same
potential as the inputs (~3.6 V).
The supply is connected to the VPOS pin using the decoupling
network also displayed in Figure 47. The capacitors used in this
network must provide a low impedance over the full frequency
range of the input and should be placed as close as possible to
the VPOS pin. Two different capacitors are used in parallel to
reduce the overall impedance because these have different reso-
nant frequencies. The measurement accuracy is not critically
dependent on supply decoupling because the high frequency
signal path is confined to the relevant input pins. Lead lengths
from both DECL pins to ground and from INHI/INLO to the
input coupling capacitors should be as short as possible. All
COMM pins should also connect directly to the ground plane.
At lower frequencies where impedance matching is not neces-
sary, the AD8362 can be driven from a low impedance differential
source, remembering the inputs must be ac-coupled.
Choosing Input Coupling Capacitors
As noted, the inputs must be ac-coupled. The input coupling
capacitors combine with the 200 ꢀ input impedance to create
an input high pass corner frequency equal to
f
HP = 1/(200 × π × CC)
(12)
Typically, fHP should be set to at least one tenth the lowest input
frequency of interest.
To place the device in measurement mode, connect VOUT to
VSET and connect VTGT directly to VREF.
Single-Ended Input Drive
As previously noted, the input stages of the AD8362 are optimally
driven from a fully balanced source, which should be provided
wherever possible. In many cases, unbalanced sources can be
applied directly to one or the other of the two input pins. The
chief disadvantage of this driving method is a 10 dB to 15 dB
reduction in dynamic range at frequencies above 500 MHz.
DEVICE DISABLE
The AD8362 is disabled by a logic high on the PWDN pin,
which can be directly grounded for continuous operation.
When enabled, the supply current is nominally 20 mA and
essentially independent of supply voltage and input signal
strength. When powered down by a logic low on PWDN,
the supply current is reduced to 230 μA.
Figure 48 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 can be used, INHI is chosen here.
RECOMMENDED INPUT COUPLING
The full dynamic range of the AD8362, particularly at very
high frequencies (above 500 MHz), is realized only when the
input is presented to it in differential (balanced) form. In Figure 47,
a transmission line balun is used at the input. Having a 1:4
impedance ratio (1:2 turns ratio), the 200 Ω differential input
resistance of the AD8362 becomes 50 Ω at the input to the balun.
AD8362
1
2
3
4
5
6
7
8
COMM
CHPF
DECL
INHI
16
15
14
13
12
11
10
9
ACOM
VREF
VTGT
VPOS
VOUT
VSET
ACOM
CLPF
0.01µF
1nF
1nF
1nF
V
S
RF INPUT
100Ω
5V @ 24mA
INLO
AD8362
DECL
PWDN
COMM
1:4 Z-RATIO
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
1nF
C1
C8
0.1µF
1000pF
C10
1000pF
C4
1nF
C6
SIGNAL
INPUT
Z = 50Ω
100pF
C2
1nF
INLO
V
OUT
Figure 48. Input Coupling from a Single-Ended 50 Ω Source
C7
1nF
C5
DECL
PWDN
COMM
100pF
T1
ETC1.6-4-2-3
C3
0.1µF
Figure 47. Basic Connections for RF Power Measurement
Rev. D | Page 19 of 32
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