ADL5904ACPZN-R7 datasheet(19/28 Pages) AD

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ADL5904

Data Sheet

Rev. 0 | Page 18 of 27

THEORY OF OPERATION

The ADL5904 is a true rms detector with a 45 dB measurement

range at 1.9 GHz with useable range up to 6 GHz. It features no

error ripple over its range, low temperature drift, and very low

power consumption. Temperature stability of the rms output

measurements provides ≤±0.5 dB error (typical) over the

temperature range of −40°C to +85°C at up to 3.5 GHz. The

measurement output voltage scales linearly in decibels with a

slope of typically 36.9 mV/dB at 1.9 GHz.

The core rms processing of the ADL5904 uses a proprietary

multistage technique that provides accuracy for complex

modulation signals irrespective of the crest factor of the input

signal. An integrating filter capacitor at the CRMS pin performs

the square domain averaging.

ENVELOPE

DETECTOR

RMS

–

ADL5904

VPOS

GND

VRMS

RST

RFIN

VIN–

DECL

ENBL

CRMS

DNC

13 Q

VCAL

Figure 43. Functional Block Diagram

The output of the first signal processing stage (the envelope

detector), also drives the noninverting input of a threshold

detecting comparator. The inverting input of this comparator is

typically driven by a fixed external dc voltage. When the output

of the envelope detector exceeds the voltage on the inverting input

of the comparator, the comparator goes high. This excursion is

then captured and held by an SR flip flop. The state of this flip

flop is then held until the level sensitive RST pin is taken high.

BASIC CONNECTIONS FOR RMS MEASUREMENT

The ADL5904 requires a single supply of 3.3 V. The supply is

connected to the VPOS supply pins. Decouple these pins using

two capacitors with values equal or similar to those shown in

the Figure 44. Place these capacitors as close to Pin 5 as possible.

Connect Pin 11 (GND) and the exposed pad to a ground plane

with low electrical and thermal impedance.

A single-ended input at the RFIN pin drives the ADL5904. Because

the input is dc-coupled, an external ac coupling capacitor must

be used. A 470 nF capacitor is recommended for applications

that require frequency coverage from 6 GHz down to tens of

kilohertz. For applications that do not need such low frequency

coverage, a larger value of capacitance can be used.

In addition to the ac-coupling capacitor, an external 82.5 Ω

shunt resistor is required to provide a wideband input match.

Figure 13 shows a comparison of the input return loss, with and

without the external shunt resistor.

The DECL pin provides a bypass capacitor connection for an

on-chip regulator. The DECL pin is connected to ground with

a 4.02 Ω resistor and a 0.1 µF capacitor. The CRMS pin is the

averaging node for the rms computation. Place an rms averaging

capacitor between the CRMS and DECL pins. For information on

choosing the CRMS capacitor, see the Choosing a Value for

CRMS section. Using smaller values for CRMS allows quicker

response times to a pulsed waveform. Higher values of CRMS are

required for correct rms computation as the peak to average

ratio of modulated signals increases and the bandwidth of the

modulated signals decreases.

If the threshold detection circuitry is not used, the VIN− pin can be

tied to VPOS or left open (this pin has an internal pull-up to VPOS).

The ENBL pin configures the device enable interface. Connecting

the ENBL pin to a logic high signal (2 V to 3.3 V) enables the

device, and connecting the pin to a logic low signal (0 V to 0.6 V)

disables the device. The exposed pad is internally connected to

GND and must be soldered to a low impedance ground plane.

4.02Ω

100nF

100pF

VPOS

3.3V

0.1µF

ENABLE/

DISABLE

RFIN

ENVELOPE

DETECTOR

RMS

–

ADL5904

VPOS

GND

VRMS

RST

RFIN

470nF

VIN–

DECL

82.5Ω

ENBL

100nF

CRMS

DNC

VCAL

Figure 44. Basic Connections for RMS Power Measurement

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