AD640

REV.

D

–15–

diminishes the risk of instability due to poor grounding. Never-

theless, it must be remembered that at high frequencies even

very small lengths of wire, including the leads to capacitors,

have significant impedance. The ground plane itself can also

generate small but troublesome voltages due to circulating cur-

rents in a poor layout. A printed circuit evaluation board is

available from Analog Devices (Part Number ADEB640) to

facilitate the prototyping of an application using one or two

AD640s, plus various external 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 AD640s 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.

50 MHz–150 MHz Converter with 70 dB Dynamic Range

Figure 30 shows a logarithmic converter using two AD640s

which can provide at least 70 dB of dynamic range, limited

mostly by first stage noise. In this application, an rf choke (L1)

prevents the transmission of dc offset from the first to the sec-

ond AD640. One or two turns in a ferrite core will generally

suffice for operation at frequencies above 30 MHz. For ex-

ample, one complete loop of 20 gauge wire through the two

holes in a Fair-Rite type 2873002302 core provides an inductance

of 5

µH, which presents an impedance of 1.57 kΩ at 50 MHz.

The shunting effect across the 150

Ω differential impedance at

the signal interface is thus fairly slight.

The signal source is optionally terminated by R1. To minimize

the input offset voltage R2 should be chosen to match the dc

resistance of the terminated source. (However, the offset voltage

is not a critical consideration in this ac-coupled application.)

Note that all unused inputs are grounded; this improves the

isolation from the outputs back to the inputs.

A transimpedance op amp (U3, AD844) converts the summed

logarithmic output currents of U1 and U2 to a ground referenced

voltage scaled 1 V per decade. The resistor R5 is nominally 1 k

Ω

but is increased slightly to compensate for the slope deficit at the

operating frequency, which can be determined from Figure 12.

The inverting input of U3 forms a virtual ground, so that each

logarithmic output of U1 and U2 is loaded by 100

Ω (R3 or

R4). These resistors in conjunction with capacitors C1 and C2

form independent low-pass filters with a time constant of about

INPUT LEVEL – dBm IN 50

0

–60

–50

–40

–30

–20

–10

0

+1

–1

4

1

0

–70

2

3

Figure 31. Logarithmic Output and Nonlinearity for Circuit

of Figure 30, for a Sine Wave Input at f = 80 MHz

5 ns. These capacitors should be connected directly across Pins

13 and 14, as shown, to prevent high frequency output currents

from circulating in the ground plane. A second 5 ns time con-

stant is formed by feedback resistor R5 in conjunction with the

transcapacitance of U3.

This filtering is adequate for input frequencies of 50 MHz or

above; more elaborate filtering can be devised for pulse

applications requiring a faster rise time. In applications where

only a long term measure of the input is needed, C1 and C2 can

5k

0.1 F

0.1 F

+15V

–15V

TO U3

AND U4

15

13

14

16

19

18

17

11

12

20

6

8

7

5

3

4

9

1

2

10

SIG

+IN

ATN

OUT

CKT

COM

RG1 RG0 RG2 LOG

OUT

LOG

COM

+VS SIG

+OUT

SIG

–IN

ATN

LO

ATN

COM

BL1

BL2

ITC

–VS

SIG

–OUT

1k

1k

ATN

COM

ATN

IN

U2 AD640

C6

0.1 F

68

+6V

68

R7

3.3M

1/2

AD712

DENOTES A CONNECTION TO THE GROUND PLANE;

OBSERVE COMMON CONNECTIONS WHERE SHOWN.

ALL UNMARKED CAPACITORS ARE 0.1 F CERAMIC.

OFFSET

NULLING

FEEDBACK

U4a

5k

SIGNAL INPUT

15

13

14

16

19

18

17

11

12

20

SIG

+IN

ATN

OUT

CKT

COM

RG1 RG0 RG2 LOG

OUT

LOG

COM

+VS SIG

+OUT

SIG

–IN

ATN

LO

ATN

COM

BL1

BL2

ITC

–VS

SIG

–OUT

1k

1k

ATN

COM

ATN

IN

U1 AD640

6

8

7

5

3

49

12

10

C1

(SEE

TEXT)

C2

(SEE

TEXT)

C7

4.7 F

C8

4.7 F

1

2

3

B

OFFSET

NULLING

FEEDBACK

U4b

7

6

5

A

1/2

AD712

1/2

AD712

U3a

1

2

3

NC

R2

50k

R3

50k

U3b

1/2

AD712

7

6

5

R4

200k

R5

200k

LOG

OUTPUT

+100mV/dB

C4

4.7 F

R1

49.9

C3

100 F

C5

0.1 F

A

B

+15V

–15V

TO U1

AND U2

TO U3

AND U4

9.1V

9.1V

+6V

–6V

18

–6V

18

NC = NO CONNECT

R6

3.3M

Figure 32. Complete 95 dB Dynamic Range Converter