LMV225SD datasheet(23/30 Pages) TI

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f =

2 S (R1 + RIN)

C · CIN

C + CIN

ANTENNA

RFIN/EN

OUT

LMV226/

LMV228

ENABLE

10 k:

VDD

GND

50:

100 pF

LMV225, LMV226, LMV228

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SNWS013L – AUGUST 2003 – REVISED MARCH 2013

DIRECTIONAL COUPLER APPLICATION

The LMV226/LMV228 also has a 50

Ω input resistance. However, its input range differs compared to the

LMV225, i.e.

−15 dBm to +15 dBm. If a typical attenuation of a directional coupler is 20 dB, the LMV226/LMV228

can be directly connected via the directional coupler to the PA without the need of additional external attenuator

(Figure 67). Different PA ranges can be configured using couplers with other coupling factors.

Figure 67. Typical LMV226/LMV228 Application with

Directional Coupler

SHUTDOWN FUNCTIONALITY

The LMV225/LMV226/LMV228 RFIN/EN pins have 2 functions combined:

•

Enable/Shutdown

•

Power input

The capacitor C and the resistor R2 (Figure 66 and Figure 67) separate the DC shutdown functionality from the

AC power measurement. The device is active when Enable = HI, otherwise it is in 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. In case of the LMV225 the corner frequency can be calculated using:

where

•

RIN = 50Ω, CIN = 45 pF typical

(3)

With R1 = 1800Ω and C = 100 pF, this results in a corner frequency of 2.8 MHz. This corner frequency is an

indicative number. The goal is to have a magnitude transfer, which is sufficiently flat in the used frequency range;

capacitor C should be chosen significantly larger than capacitor CIN to assure a proper performance of the high

resistive tap. Capacitor C shouldn’t be chosen excessively large since the RC-time, it introduces in combination

with resistor R2, adds to the turn-on time of the device.

The LMV226/LMV228 do not use a resistor R1 like the LMV225. Though a resistor is seen on the coupler side

(RCOUPLER). Therefore a similar equation holds for the LMV226/LMV228 LF corner frequency, where R1 is

replaced with the coupler output impedance (RCOUPLER).

With RCOUPLER = 50Ω and C = 100 pF, the resulting corner frequency is 50 MHz.

The output voltage is proportional to the logarithm of the input power, often called “linear-in-dB”. Figure 68 shows

the typical output voltage versus PA output power of the LMV225 setup as depicted in Figure 66.

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