Circuit Note

CN-0150

Devices Connected/Referenced

AD8318

1 MHz to 8 GHz, 70 dB, Logarithmic

Detector/Controller

AD7887

2.7 V to 5.25 V, Micropower, 2-Channel,

125 kSPS, 12-Bit ADC in 8-Lead MSOP

Circuits from the Lab™ reference circuits are engineered and

tested for quick and easy system integration to help solve today’s

analog, mixed-signal, and RF design challenges. For more

ADR421

Precision, Low Noise, 2.5 V Reference

Software-Calibrated, 1 MHz to 8 GHz, 60 dB RF Power

Measurement System Using a Logarithmic Detector

Rev. C

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EVALUATION AND DESIGN SUPPORT

Circuit Evaluation Boards

CN-0150 Circuit Evaluation Board (EVAL-CN0150A-SDPZ)

System Demonstration Platform (EVAL-SDP-CB1Z)

Design and Integration Files

Schematics, Layout Files, Bill of Materials

CIRCUIT FUNCTION AND BENEFITS

This circuit measures RF power at any frequency from

1 MHz to 8 GHz over a range of approximately 60 dB. The

measurement result is provided as a digital code at the output of

a 12-bit ADC with serial interface and integrated reference. The

output of the RF detector has a glueless interface to the ADC and

uses most of the ADC’s input range without further adjustment.

A simple two-point system calibration is performed in the

digital domain.

The AD8318 maintains accurate log conformance for signals of

1 MHz to 6 GHz and provides useful operation to 8 GHz.

The device provides a typical output voltage temperature

stability of ±0.5 dB.

The AD7887 ADC can be configured for either dual or single

channel operation via the on-chip control register. There is a

default single-channel mode that allows the AD7887 to be operated

as a read-only ADC, thereby simplifying the control logic.

Typical data is shown for the two devices operating over a

−40°C to +85°C temperature range.

12

CMIP

11

CMIP

10

TADJ

9

VPSO

1

CMIP

2

CMIP

3

VPSI

4

VPSI

13

TEMP

8

CMOP

14

INHI

7

VSET

15

INLO

6

VOUT

16

ENBL

5

CLPF

AD8318

C7

100pF

C9

0.1µF

C8

0.1µF

C1 1nF

C2 1nF

R4

499Ω

C6

100pF

C5

0.1µF

+5V

R1

52.3Ω

DOUT

DIN

SCLK

CS

AIN0

AIN1/

GND

0.1µF

10µF

SERIAL

INTERFACE

AD7887

µC/µP

0.1µF

SEE

TEXT

PULSED RF

INPUT

Figure 1. Software-Calibrated RF Measurement System (Simplified Schematic: All Connections Not Shown)