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
+
-
0.1μF
OUTPUT
C
R
C
R
2
R
1
CLAMP
INPUT
0.1μF
(R
1
||R
2
) ≥ 100kΩ
For Full Clamp Effect
7650S
-
+
0.1μF
OUT
CLAMP
IN
0.1μF
741
-
+
-7.5V
+7.5V +15V
-15V
10kΩ
10kΩ
NOTE: R
1
||R
2
indicates the parallel combination of R
1
and R
2
.
FIGURE 3. INVERTING AMPLIFIER WITH (OPTIONAL) CLAMP
FIGURE 4. USING 741 TO BOOST OUTPUT DRIVE CAPACITY
7650S
+
-
0.1μF
V
OUT
C
R
C
CLAMP
V
IN
0.1μF
V
TH
200kΩ - 2MΩ
R
FIGURE 5. LOW OFFSET COMPARATOR
A
1
+
-
+
-
A
2
ICL7650S
+
-
V
OUT
R
1
15.9kΩ
680Ω
R
2
GROUND
R
REF
R
3
Q
1
2kΩ
V+
Q
2
R
5
V
REF
I
REF
GAIN
33kΩ
33kΩ
150pF
C
1
R
0
10kΩ
(LOW T.C.)
I
IN
R
IN
V
IN
715
10
121316
(+15V)
1kΩ
45
2
1
ICL8048
NOTE: For further Applications Assistance, see AN053.
FIGURE 6. ICL8048 OFFSET NULLED BY ICL7650S
