ADL5920ACPZ-R2 datasheet(18/26 Pages) AD

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ADL5920

Data Sheet

Rev. B | Page 18 of 26

APPLICATIONS INFORMATION

BASIC CONNECTIONS

For ac-coupled operation, the ADL5920 requires a single supply

of 5 V. The supply is connected to the VPOS1, VPOS2, and

VPOS3 supply pins. Decouple each of these pins using two

capacitors with values equal or similar to those shown in Figure 38.

Place these capacitors as close as possible to the VPOS pins.

The RF input and output pins are ac-coupled using broadband

0.01 µF capacitors, which allow operation down to approximately

600 kHz. Larger value capacitors can reduce the minimum input

frequency further.

CHPR± AND CHPF± CAPACITORS

Each rms detector contains an offset compensation loop that

eliminates internal offset voltages and ensures optimal detector

sensitivity. The offset compensation loop works like a high-pass

filter so that all input frequencies (and dc) below a certain

corner frequency are nulled by the servo action of the loop.

An internal 190 pF capacitor and an internal 2 kΩ resistor sets

the nominal corner frequency of this loop. This configuration

results in a high-pass corner frequency of approximately 400 kHz.

For operation at a specific input frequency, the high-pass corner

must be set two to three decades lower than this corner

frequency. For example, for a minimum input frequency of

1 MHz, the high-pass corner frequency must be set to 1 kHz to

ensure that the offset compensation loop does not interfere with

the input signal being measured.

Capacitors connected between CHPF+ and CHPF− and between

CHPR+ and CHPR− can reduce the corner frequency (f3dB) of

the offset compensation loops for each detector. The following

equation sets the corner frequency of the offset compensation

loop:

f3dB = 1/(2π × 2000 × (190 pF + CHPx±))

(5)

For example, setting the CHPx± capacitors values to 0.1 µF

results in a high-pass corner of approximately 800 Hz, ensuring

reliable operation for input frequencies down to 800 kHz.

At input frequencies above 2 GHz, the presence of capacitors or

stray capacitance on the CHPx± nodes adversely affects directivity.

As a result, it is recommended to leave these nodes open with

no stray capacitance present for operation from 2 GHz to 7 GHz.

For broadband operation (for example, from 1 MHz to 7 GHz),

it is recommended to use 0201 size capacitors and to mount the

capacitors as close the pins as possible.

REVERSE

PATH

RMS

DETECTOR

TEMPERATURE

SENSOR

1.4V

VREF

2.5V

FORWARD

PATH

RMS

DETECTOR

BIDIRECTIONAL BRIDGE

VTEMP

VRMSR

VREV

CRMSR

CHPR–

CHPR+

VNEG1

RFIN

TADJI VREF

(SEE TEXT)

C13

(SEE TEXT)

2.43kΩ

1kΩ

R10

(SEE TEXT)

VTGT

VREF

VOCM

DECL

VRMSF

VFWD

EPAD

GND1

GND2

GND3

GND4

GND5

GND6

VPOS1

VPOS2

VPOS3

VDIFF+

C11

100pF

0.1µF

C18

0.1µF

VDIFF

OUTPUT

0.1µF

100pF

C14

100pF

VDIFF–

CRMSF

CHPF–

CHPF+

VNEG2

RFOUT

ADL5920

PWDN/TADJS

VREF

(SEE TEXT)

3.6kΩ

4.7µF

C10

RFOP

0.1µF

C12

0.01µF

C19

0.1µF

RFIP

0.01µF

×1

Figure 38. Basic Connections for Single-Supply AC-Coupled Operation

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