7

FN2920.10

April 12, 2007

Normal logarithmic amplifiers are limited in dynamic range in

the voltage-input mode by their input-offset voltage. The

built-in temperature compensation and convenience

features of the ICL8048 can be extended to a voltage-input

dynamic range of close to 6 decades by using the ICL7650S

to offset-null the ICL8048, as shown in Figure 6. The same

concept can also be used with such devices as the HA2500

or HA2600 families of op amps to add very low offset voltage

capability to their very high slew rates and bandwidths. Note

that these circuits will also have their DC gains, CMRR, and

PSRR enhanced.

7650S

+

-

0.1

μF

OUTPUT

C

R

C

CLAMP

INPUT

0.1

μF

For Full Clamp Effect

7650S

-

+

0.1

μF

OUT

CLAMP

IN

0.1

μF

741

-

+

-7.5V

+7.5V

+15V

-15V

10k

Ω

10k

Ω

FIGURE 3. INVERTING AMPLIFIER WITH (OPTIONAL) CLAMP

FIGURE 4. USING 741 TO BOOST OUTPUT DRIVE CAPACITY

7650S

+

-

0.1

μF

C

R

C

CLAMP

0.1

μF

200k

Ω - 2MΩ

R

FIGURE 5. LOW OFFSET COMPARATOR

+

-

+

-

ICL7650S

+

-

15.9k

Ω

680

Ω

GROUND

2k

Ω

V+

GAIN

33k

Ω

33k

Ω

150pF

10k

Ω

(LOW T.C.)

715

10

12

13

16

(+15V)

1k

Ω

4

5

2

1

ICL8048

NOTE: For further Applications Assistance, see AN053.

FIGURE 6. ICL8048 OFFSET NULLED BY ICL7650S