LTC5582
13
Rev. B
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For a 50Ω input termination, the approximate RF input
power range of the LTC5582 is from –60dBm to 2dBm,
even with high crest factor signals such as a 4-carrier
W-CDMA waveform, and the minimum detectable RF power
level varies as the input RF frequency increases. The linear
dynamic range can also be shifted to suit a particular ap-
plication need. By simply inserting an attenuator in front
of the RF input, the power range is shifted higher by the
amount of the attenuation.
The sensitivity of LTC5582 is dictated by the broadband
input noise power that also determines the output DC
offset voltage. When the inputs are terminated differently,
the DC output voltage may vary slightly. When the input
noise power is minimized, the DC offset voltage is also
reduced to the minimum. And the detector’s sensitivity
and dynamic range will be improved accordingly.
External Filtering (FLTR) Capacitor
This pin is internally biased at VCC – 0.43V via a 1.2k
resistor from the voltage supply, VCC. To assure stable
operation of the LTC5582, an external capacitor C3 with
a value of 8nF or higher is required to connect from the
FLTR Pin to VCC to avoid an abnormal start-up condition.
Don’t connect this filtering capacitor to ground or any
other low voltage reference at any time.
This external capacitor value has a dominant effect on the
output transient response. The lower the capacitance, the
faster the output rise and fall times. For signals with AM
content such as W-CDMA, significant ripple can be ob-
served when the loop bandwidth set by C3 is close to the
modulation bandwidth of the signal. A 4-carrier W-CDMA
RF signal is used as an example in this case. The trade-offs
of the residual ripple vs the output transient times are as
shown in Figure8.
In general, the LTC5582 output ripple remains relatively
constant regardless of the RF input power level for a fixed
C3 and modulation format of the RF signal. Typically, C3
must be selected to smooth out the ripple to achieve the
desired accuracy of RF power measurement.
Output Interface
The output buffer amplifier of the LTC5582 is shown in
Figure9. This Class AB buffer amplifier can source and
APPLICATIONS INFORMATION
Figure8. Residual Ripple, Output Transient Times vs
Filtering Capacitor C3
Figure9. Simplified Schematic of the Output Interface
sink 5mA current to and from the load. The output imped-
ance is determined primarily by the 100Ω series resistor
connected to the output of the buffer amplifier inside the
chip. This will prevent overstress on internal devices in
the event that the output is shorted to ground.
The –3dB small-signal bandwidth of the buffer amplifier is
about 22.4MHz and the full-scale rise/fall time can be as
fast as 80ns, limited by the slew rate of the internal circuit
instead. When the output is resistively terminated or open,
the fastest output transient response is achieved when a
large signal is applied to the RF input. The rise time of the
LTC5582 is about 90ns and the fall time is 5µs, respectively,
for full-scale pulsed RF input power when C3 = 8nF. The
speed of the output transient response is dictated mainly
by the filtering capacitor C3 (at least 8nF) at the FLTR Pin.
See the detailed output transient response in the Typical
Performance Characteristics section. When the RF input
has AM content, residual ripple may be present at the
output depending upon the low frequency content of the
modulated RF signal. This ripple can be reduced with a
100Ω RSS
OUT
INPUT
VCC
V
CLOAD
LTC5582
FILTERNING CAPACITOR C3 (nF)
0
P-P
), FALL TIME (µs)
RISE TIME (µs)
1000200 400 600 800
0
450
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350
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100
50
0
45
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35
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15
10
5
RIPPLE
RISE TIME
FALL TIME