LMV227SD datasheet(8/12 Pages) NSC

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Application Notes

CONFIGURING A TYPICAL APPLICATION

The LMV227 is a power detector intended for CDMA and

WCDMA applications. Power measured on its input trans-

lates to a DC voltage on the output through a linear-in-dB

response. The detector is especially suited for power mea-

surements via a high-resistive tap, which eliminates the

need for a directional coupler. In order to match the dynamic

output range of the power amplifier (PA) with the dynamic

range of the LMV227’s input, the high resistive tap needs to

be configured correctly.

Input Attenuation

The constant input impedance of the device enables the

realization of a frequency independent input attenuation to

adjust the LMV227’s dynamic range to the dynamic range of

the PA. Resistor R

1 and the 50

Ω input resistance of the

device realize this attenuation (Figure 1). To minimize inser-

tion loss, resistor R

1 needs to be sufficiently large. The

following example demonstrates how to determine the

proper value for R

Suppose the useful output power of the PA ranges up to +31

dBm and the LMV227 can handle input power levels up to 0

dBm. Hence, R

1 should realize a minimum attenuation of 31

- 0 = 31 dB. The attenuation realized by R

1 and the effective

input resistance R

IN of the detector equals:

(1)

Solving this expression for R

1, using that RIN =50

Ω, yields:

(2)

In Figure 1,R

1 is set to 1800

Ω resulting in an attenuation of

31.4 dB

DC and AC Behavior of the RF

IN/EN Pin

The LMV227 RF

IN/EN pin has 2 functions combined:

•

Shutdown functionality

•

Power detection

The capacitor C and the resistor R

2 of Figure 1 separate the

DC shutdown functionality from the AC power measurement.

The device is active when Enable = HI, otherwise it goes into

a low power consumption shutdown mode. During shutdown

the output will be LOW.

Capacitor C should be chosen sufficiently large to ensure a

corner frequency far below the lowest input frequency to be

measured. The corner frequency can be calculated using:

(3)

Where R

IN =50

Ω,C

IN = 45 pF typical.

With R

1 = 1800

Ω and C is 100 pF, this results in a corner

frequency of 2.8 MHz

The output voltage is linear with the logarithm of the input

power, often called "linear-in-dB". Figure 2 shows the typical

output voltage versus PA output power of the LMV227 setup

as depicted in Figure 1.

OUTPUT RIPPLE DUE TO AM MODULATION

A CDMA modulated carrier wave generally contains some

amplitude modulation that might disturb the RF power mea-

surement used for controlling the PA. This section explains

the relation between amplitude modulation in the RF signal

and the ripple on the output of the LMV227. Expressions are

provided to estimate this ripple on the output. The ripple can

be further reduced by connecting an additional capacitor to

the output of the LMV227 to ground.

Estimating Output Ripple

The CDMA modulated RF input signal of Figure 3 can be

described as:

IN(t)=VIN [1 + µ(t)] cos (2 ·

π ·f·t)

(4)

In which the amplitude modulation µ(t) can be between −1

and 1.

20118133

FIGURE 1. Typical Application

20118116

FIGURE 2. Typical Power Detector Response, V

OUT vs.

PA Output Power

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