REV. C

AD641

–15–

FREQUENCY – MHz

3

2.5

–0.5

1

1000

10

100

2

1.5

1

0.5

0

0dBm

–20dBm

–35dBm

–50dBm

Figure 31. Single AD641 RSSI vs. Frequency

Figure 31 shows a plot of RSSI vs. frequency for various input

signal amplitudes. It can be seen that at higher frequencies the

output drops off as explained in the section “Effect of Fre-

quency on Calibration.’’ If the RSSI circuit is to be operated at

a known frequency with limited bandwidth, the compensation

techniques described in that section can be used to enhance

accuracy.

250 MHz RSSI Converter with 58 dB Dynamic Range

For a larger dynamic range two AD641s can be cascaded, as

shown in Figure 32. The low end usefulness of the circuit will

be set by the noise floor of the overall environment that the circuit

sees. This includes all sources of both radiated and conducted

noise. Proper layout to avoid conducted noise and good shield-

ing to minimize radiated noise are essential for good low signal

operation.

FREQUENCY – MHz

4.5

3.5

0

1

1000

10

100

2

2.5

1

1.5

0.5

0dBm

–20dBm

–50dBm

–80dBm

4

3

Figure 33. Cascaded AD641s RSSI vs. Frequency

Filtering between the devices and input offset nulling techniques

described elsewhere are also useful for extending the dynamic

range of two cascaded devices.

Figure 33 shows a plot of this circuit vs. frequency for various

input amplitudes. The drop off at high frequency can be seen to

be greater than for the single device case due to the compound-

ing effects of the bandwidth limiting of the extra stages.

6

7

4

3

U3

AD846

2

–6V

4.7

LOG

OUTPUT

+50mV/dB

(LO)

+6V

4.7

L1

(SEE

TEXT)

(SEE TEXT)

R5

1.13k

C2

47pF

C1

47pF

+6V

68

R4

100

–6V

18

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

R3

100

NC

NC

R1

R2

SIGNAL

INPUT

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

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

U2 AD641

68

18

Figure 32. Complete 58 dB Dynamic Range Converter for 250 MHz Operation