Chopper-Stabilized Op Amps
R2
1M
ICL7650
A
A
B
C
INT/EXT
EXT CLK IN
CLK OUT
OSC
R1
1M
OUTPUT
ICL7650
ICL7653
INTERNAL
BIAS
P
C
+
+IN
C
R
OUTPUT
CLAMP
MAIN
C
-
-IN
A
+
C
N
0.1µF 0.1µF
NULL
A
-
B
Figure 1. ICL7650 Test Circuit
CAP RETURN
C
EXTA
C
EXTB
Detailed Description
Figure 2 shows the major elements of the ICL7650/
ICL7653. Two amplifiers are illustrated, the main amplifi-
er and the nulling amplifier, both of which have offset-
null capability. The main amplifier is connected full time
from the input to the output. The nulling amplifier, under
control of the chopper-frequency oscillator and clock
circuit, alternately nulls itself and the main amplifier. This
nulling arrangement, which is independent of the output
level, operates over the full power-supply and common-
mode ranges. The ICL7650/ICL7653 exhibit an excep-
EXT CLK IN
A = CLK OUT
A
B
C
Figure 2. Block Diagram
tionally high CMRR, PSRR, and A
. Their nulling
VOL
connections, which are MOSFET back gates, have inher-
ently high impedance. Two external capacitors provide
storage for the nulling potentials and the necessary
nulling-loop time constants.
Intermodulation
Intermodulation effects can cause problems in older
chopper-stabilized amplifier modules. Intermodulation
occurs since the amplifier has a finite AC gain, and
therefore will have a small AC signal at the input. In a
chopper-stabilized module, this small AC signal is
detected, chopped, and fed into the offset-correction
circuit. This results in spurious outputs at the sum and
difference frequencies of the chopping and input signal
frequencies. Other intermodulation effects in chopper-
stabilized modules include gain and phase anomalies
near the chopping frequency.
The ICL7650/ICL7653 minimize chopper-frequency
charge injection at the input terminals by carefully bal-
ancing the input switches. Feed-forward injection into
the compensation capacitor, the main cause of output
spikes in this type of circuit, is also minimized.
Output Clamp (ICL7650 Only)
The output clamp reduces the overload recovery time
inherent with chopper-stabilized amplifiers. When tied to
the summing junction or inverting input pin, a current path
between this point and the output occurs just before the
output device saturates. This prevents uncontrolled input
differential and the consequent charge build-up on the
correction-storage capacitors, while causing only a slight
reduction in the output swing.
These effects are substantially reduced in the
ICL7650/ICL7653, which add to the nulling circuit a
dynamic current that compensates for the AC signal on
the inputs. Unlike modules, the ICL7650/ICL7653 can
precisely compensate for the finite AC gain, since both
the AC gain rolloff and the intermodulation compensation
current are controlled by internal matched capacitors.
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