LTC5582

13

Rev. D

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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-CDMAwaveform,andtheminimumdetectableRFpower

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 Figure 8.

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

Figure 9. This Class AB buffer amplifier can source and

APPLICATIONS INFORMATION

Figure 8. Residual Ripple, Output Transient Times vs

Filtering Capacitor C3

Figure 9. 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

LTC5582isabout90nsandthefalltimeis5µs,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

100

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5582 F09

LTC5582

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5582 F08

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