LMV225SD datasheet(22/30 Pages) TI

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R1 =

-1

AdB

· RIN =

-1

· 50 = 1724:

AdB = 20·LOG

RIN

= 31dB

ANTENNA

OUT

VDD

GND

LMV225

10 k:

ENABLE

RFIN/EN

RIN

CIN

100 pF

1.8 k:

LMV225, LMV226, LMV228

SNWS013L – AUGUST 2003 – REVISED MARCH 2013

www.ti.com

APPLICATION NOTES

CONFIGURING A TYPICAL APPLICATION

The LMV225/LMV226/LMV228 are power detectors intended for CDMA and WCDMA applications. Power

applied at its input translates to a DC voltage on the output through a linear-in-dB response. The LMV225

detector is especially suited for power measurements via a high-resistive tap, while the LMV226/LMV228 are

designed to be used in combination with a directional coupler. The LMV226 has an additional output voltage

buffer and therefore a low output impedance. The key features of the devices are shown in .

Table 1. DEVICE CHARACTERISTICS

Input Range (dBm)

Output Buffer

Application

LMV225

−30 / 0

High Resistive Tap

LMV226

−15 / 15

Yes

Directional Coupler

LMV228

−15 / 15

Directional Coupler

In order to match the output power range of the power amplifier (PA) with the range of the LMV225’s input, the

high resistive tap needs to be configured correctly. In case of the LMV226/LMV228 the coupling factor of the

directional coupler needs to be chosen correctly.

HIGH RESISTIVE TAP APPLICATION

The constant input impedance of the device enables the realization of a frequency independent input attenuation

to adjust the LMV225’s range to the range of the PA. Resistor R1 and the 50Ω input resistance (RIN) of the

device realize this attenuation (Figure 66). To minimize insertion loss, resistor R1 needs to be sufficiently large.

The following example demonstrates how to determine the proper value for R1.

Figure 66. Typical LMV225 Application with High Resistive Tap

Suppose the useful output power of the PA ranges up to +31 dBm. As the LMV225 can handle input power

levels up to 0 dBm. R1 should realize a minimum attenuation of 31 - 0 = 31 dB. The attenuation realized by R1

and the effective input resistance RIN of the detector equals:

(1)

Solving this expression for R1, using that RIN = 50Ω, yields:

(2)

In Figure 66, R1 is set to 1800Ω resulting in an attenuation of 31.4 dB.

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Product Folder Links: LMV225 LMV226 LMV228

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