REV. C

AD641

–14–

PRACTICAL APPLICATIONS

We show here two applications, using AD641s to achieve a wide

dynamic range. As already mentioned, the use of a differential

signal path and differential logarithmic outputs diminishes the

risk of instability due to poor grounding. Nevertheless, it must

be remembered that at high frequencies even very small lengths

of wire, including the leads to capacitors, have significant im-

pedance. The ground plane itself can also generate small but

troublesome voltages due to circulating currents in a poor lay-

out. A printed circuit evaluation board is available from Analog

Devices (Part Number AD641-EB) to facilitate the prototyping

of an application using one or two AD641s, plus various exter-

nal components.

At very low signal levels various effects can cause significant

deviation from the ideal response, apart from the inherent non-

linearities of the transfer function already discussed. Note that

any spurious signal presented to the AD641s is demodulated and

added to the output. Thus, in the absence of thorough shielding,

emissions from any radio transmitters or RFI from equipment

operating in the locality will cause the output to appear too

high. The only cure for this type of error is the use of very care-

ful grounding and shielding techniques.

RSSI APPLICATIONS

The AD641 can be used to perform an RSSI (Received Signal

Strength Indicator) function. This is a commonly used function

in radio receivers, but can be used in other instrumentation such

as photomultiplier tubes. The signal strength indicator on FM

radios is one example of an RSSI application. It is this signal

that is monitored to determine where to stop during seek or

scan operations.

The AD641 is used to measure the strength of the incoming RF

signal and outputs a current that is proportional to the loga-

rithm of its ac amplitude. In this manner signal amplitudes with

a wide dynamic range and wide bandwidth can be measured.

250 MHz RSSI Converter with 44 dB Dynamic Range

Figure 30 shows the schematic for an RSSI circuit that uses a

single AD641. The dynamic range for this circuit using a single

AD641 is 44 dB. The AD641 amplifies and full wave rectifies

(detects) the input and outputs a current. The AD846 is used to

convert the current to a ground referenced voltage. With a 1 k

Ω

feedback resistor, the output varies by 1 V/decade or 50 mV/dB.

6

7

4

3

–6V

4.7

U3

AD846

RSSI

OUTPUT

+50mV/dB

(LO)

+6V

2

C1

47pF

4.7

NC

DENOTES A CONNECTION TO THE

GROUND PLANE; OBSERVE COMMON

CONNECTIONS WHERE SHOWN.

ALL UNMARKED CAPACITORS ARE

0.1 F CERAMIC. FOR VALUES OF

NUMBERED COMPONENTS SEE TEXT

R1

R2

SIGNAL

INPUT

18

68

1.0k

9

8

7

6

10

5

3

2

14

12

13

14

15

11

16

18

19

20

17

LOG

OUT

LOG

COM

SIG

+OUT

RG2

SIG

–OUT

RG0

RG1

CKT

COM

ATN

OUT

SIG

+IN

ITC

BL1

ATN

IN

ATN

COM

ATN

COM

ATN

LO

SIG

–IN

BL2

1k

1k

U1 AD641

–6V

+6V

R3

100

Figure 30. RSSI Using Single AD641