12

LTC1540

sn1540 1540fas

LT/TP 0403 1K REV A • PRINTED IN USA

© LINEAR TECHNOLOGY CORPORATION 1997

RF Field Detector

Figure 10 shows the complete circuit for a field detector

which was tested at 445MHz. A transmission line is used

to match the detector diode (1N5712) to a quarter-wave

whip antenna. The 0.23λ wavelength transmission line

section transforms the 1pF (350Ω) diode junction capaci-

tance to a virtual short at the base of the antenna. At the

same time it converts the received antenna current to a

voltage loop at the diode, giving excellent sensitivity.

Figure 10. Nanopower Field Detector

–

+

LTC1540

2V TO 11V

2

1540 F10

10nF

0.23λ

1N5712

10nF

12M

180k

λ /4

10k

FB

7

6

5

8

3

4

1

Q

Q

CMOS

ONE SHOT

(CD4047)

PART NUMBER

DESCRIPTION

COMMENTS

LT

Dual/Quad 17µA Precision Single Supply Op Amps

LT1351

Single 250µA, 3MHz, 200V/µs Op Amp with Shutdown

C-Load

LT1352/LT1353

Dual/Quad 250µA, 3MHz, 200V/µs Op Amps

C-Load Op Amps Stable Driving Any Capacitive Load

LTC1440

Micropower Comparator with 1% Reference

1.182V ±1% Reference, ±10mV (Max) Input Offset

LTC1443/LTC1444/LTC1445

Micropower Quad Comparators with 1% Reference

LTC1443 Has 1.182V Reference, LTC1444/LTC1445 Have

1.221V Reference and Adjustable Hysteresis

LTC1474

Low Quiescent Current High Efficiency

10µA Standby Current, 92% Efficiency, Space Saving 8-Pin

Step-Down Converter

MSOP Package

LT1495

1.5µA Max, Dual Precision Rail-to-Rail

Input and Output Op Amp

LT1521

300mA Low Dropout Regulator with Micropower

0.5V Dropout Voltage, 12µA Quiescent Current, Adjustable

Quiescent Current and Shutdown

Output 3V, 3.3V and 5V Fixed

LT1634

Micropower Precision Shunt Voltage Reference

1.25V, 2.5V, 4.096V, 5V Outputs, 10µA Operating Current,

0.05% Initial Accuracy 25ppm/°C Max Drift, SO-8, MSOP and

TO-92 Packages

C-Load is a trademark of Linear Technology Corporation.

RELATED PARTS

The rectified output is monitored by the LTC1540 com-

parator. The internal reference is used to set up a threshold

of about 18mV at the inverting input. A rising edge at the

comparator output triggers a one shot that temporarily

enables answer back and any other pulsed functions.

The total supply current is 400nA. Among other monolithic

one shots, the CD4047 draws the least amount of transient

current.

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417