AD8364-EVAL Analog Devices, AD8364-EVAL Datasheet - Page 20

• Manufacturer Part Number: AD8364-EVAL
• Description: LF to 2.7GHz, Dual 60dB TruPwr Detector
• Manufacturer: Analog Devices

PRELIMINARY TECHNICAL DATA
CHOOSING THE RIGHT VALUE FOR CHPF AND CLPF
The AD8364’s variable gain amplifier includes an offset cancellation
loop, which introduces a high-pass filter effect in its transfer
function. The corner frequency, f
of the lowest input signal in the desired measurement bandwidth
frequency to properly measure the amplitude of the input signal. The
required value of the external capacitor is given by
Thus, for operation at frequencies down to 100 kHz, CHP[A,B]
should be 318 pF.
In the standard connections for the measurement mode, the
VST[A,B] pin is tied to OUT[A,B]. For small changes in input
amplitude (a few decibels), the time-domain response of this loop is
essentially linear with a 3 dB low-pass corner frequency of
nominally f
around this local loop set the minimum recommended value of this
capacitor to about 300 pF, making f
For operation at lower signal frequencies, or whenever the averaging
time needs to be longer, use
When the input signal exhibits large crest factors, such as a WCDMA
signal, CLP[A,B] must be much larger than might at
first seem necessary. This is due to the presence of significant low
frequency components in the complex, pseudo-random modulation,
which generates fluctuations in the output of the AD8364.
CHP
CLP
PWDN
VPSA
COMR
VPSB
Figure 35. Scheme Providing 100 mV/dB Slope for Operation
INHA
INHB
INLA
INLB
[
[
LP
A
2
2
2
2
3
3
A
25
3
6
7
8
9
0
2
1
= 1/(2×π×CLP[A,B] × 1.1 kΩ). Internal time delays
,
,
B
CHPB
B
CHPA
]
24
1
]
TruPwr™
TruPwr™
DECB
Channel B
DECA
Channel A
over a 3 mV to 300 mV Input Range
=
23
2
=
COMB ADJB ADJA VREF VLVL CLPB
COMA VPSR ACMB T EMP ACMA CLPA
200
900
22
3
OUTA
OUTB
BIAS
21
4
µF
µF
20
5
V2I
TEMP
2
19
6
2
HP
×
×
, of this filter must be below that
18
7
LP
V2I
π
π
= 482 KHz.
17
8
×
×
f
f
16
15
14
13
12
11
10
9
HP
LP
OUTA
OUTP
OUTN
OUTB
VSTA
FBKA
FBKB
VSTB
(
(
f
f
HP
LP
in
in
R1
4.02K Ohms
R2
4.32K Ohms
R3
2K Ohms
Vout
Hz
Hz
)
)
(19)
(18)
Rev. PrC Ι Page 20 of 23
AD8364