AD8362
modulation, which generates fluctuations in the output of the
AD8362. Increasing CLPF also increases the step response of
the AD8362 to a change at its input.
CHOOSING A VALUE FOR CHPF
The 3.5 GHz VGA of the AD8362 includes an offset cancel-
lation loop, which introduces a high-pass filter effect in its
transfer function. To properly measure the amplitude of the
input signal, the corner frequency (fHP) of this filter must be
well below that of the lowest input signal in the desired
measurement bandwidth frequency. The required value
of the external capacitor is given by
Table 4 shows recommended values of CLPF for popular
modulation schemes. In each case, CLPF is increased until
residual output noise falls below 50 mV. A 10% to 90% step
response to an input step is also listed. Where the increased
response time is unacceptably high, CLPF must be reduced.
If the output of the AD8362 is sampled by an ADC, averaging
in the digital domain can further reduce the residual noise.
CHPF = 200 ꢁF/2(π)fHP (fHP in Hz)
(13)
For operation at frequencies as low as 100 kHz, set fHP to
Figure 52 shows how residual ripple and rise/fall time vary with
filter capacitance when the AD8362 is driven by a single carrier
W-CDMA signal (Test Model 1-64) at 2140 MHz.
approximately 25 kHz (CHPF = 8 nF). For frequencies above
approximately 2 MHz, no external capacitance is required
because there is adequate internal capacitance on this node.
180
170
160
150
18
17
16
15
14
13
12
11
10
9
CHOOSING A VALUE FOR CLPF
In the standard connections for the measurement mode, the
VSET pin is tied to VOUT. For small changes in input ampli-
tude such as a few decibels, the time-domain response of this
loop is essentially linear with a 3 dB low-pass corner frequency
of nominally fLP = 1/(CLPF × 1.1 kΩ). Internal time delays
around this local loop set the minimum recommended value
of this capacitor to about 300 pF, making fLP = 3 MHz.
140 RESIDUAL RIPPLE (mV p-p)
130
120
110
100
90
FALL TIME (ms)
80
8
70
7
60
6
50
5
40
30
20
10
4
3
2
1
For operation at lower signal frequencies, or whenever the
averaging time needs to be longer, use
RISE TIME (ms)
CLPF = 900 ꢁF/2(π)fLP (fLP in Hz)
(14)
0
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9 1.0
FILTER CAPACITANCE (µF)
When the input signal exhibits large crest factors, such as a
CDMA or W-CDMA signal, CLPF must be much larger than
might seem necessary. This is due to the presence of significant
low frequency components in the complex, pseudorandom
Figure 52. Residual Ripple, Rise and Fall Time vs. Filter Capacitance,
Single Carrier W-CDMA Input Signal, Test Model 1-64
Table 4. Recommended CLPF Values for Various Modulation Schemes
Response Time (Rise/Fall)
10% to 90%
Modulation Scheme/Standard
Crest Factor
12.0 dB
11.0 dB
9.1 dB
11.0 dB
14.0 dB
CLPF
Residual Ripple
W-CDMA , Single-Carrier, Test Model 1-64
W-CDMA 4-Carrier, Test Model 1-64
CDMA2000, Single-Carrier, 9CH Test Model
CDMA2000, 3-Carrier, 9CH Test Model
0.1 μF 28 mV p-p
0.1 μF 20 mV p-p
0.1 μF 38 mV p-p
0.1 μF 29 mV p-p
0.1 μF 30 mV p-p
171 μs/1.57 ms
162 μs/1.55 ms
179 μs /1.55 ms
171 μs/1.55 ms
157 μs/1.47 ms
WiMAX 802.16 (64QAM, 256 Subcarriers, 10 MHz Bandwidth)
Rev. D | Page 21 of 32
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