ADL5505 datasheet(13/20 Pages) AD

13 / 20 page

ADL5505

Rev. 0 | Page 13 of 20

CIRCUIT DESCRIPTION

The ADL5505 employs two-stage detection. The critical aspect

of this technical approach is the concept of first stripping the

carrier to reveal the envelope and then performing the required

analog computation of rms.

RMS CIRCUIT DESCRIPTION AND FILTERING

The rms processing is executed using a proprietary translinear

technique. This method is a mathematically accurate rms

computing approach and allows for achieving unprecedented

rms accuracies for complex modulation signals irrespective of

the crest factor of the input signal. An integrating filter capaci-

tor performs the square-domain averaging. The VRMS output

can be expressed as

VRMS

−

∫ 2

where A is a scaling parameter that is determined by the on-chip

resistor ratio.

There are no other scaling parameters involved in this computa-

tion, which means that the rms output is inherently free from

any sources of error due to temperature, supply, and process

variations.

FILTERING

An important aspect of rms-dc conversion is the need for

averaging (the function is root-mean-square). The on-chip

averaging in the square domain has a corner frequency of

approximately 180 kHz. and is sufficient for common modu-

lation signals, such as CDMA-, CDMA2000-, W-CDMA-, and

QPSK-/QAM-based OFDM (for example, LTE, WLAN, and

WiMAX).

Adequate filtering ensures the accuracy of the rms measure-

ment; however, some ripple or ac residual can still be present on

the dc output. To reduce this ripple, an external shunt capacitor

can be used at the output to form a low-pass filter with the on-

chip, 100 Ω resistance (see the Selecting the Output Low-Pass

Filter section).

OUTPUT BUFFER

A buffer takes the internal rms signal and amplifies it accor-

dingly before it is output on the VRMS pin. The output stage

of the rms buffer is a common source PMOS with a resistive

load to provide a rail-to-rail output. The buffer has a 100 Ω

on-chip series resistance on the output, allowing for easy low-

pass filtering.

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