Chopper-Stabilized Op Amps

6

_______________________________________________________________________________________

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-

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.

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.

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.

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.

ICL7650

ICL7653

OUTPUT

C

C

0.1

µF

0.1

µF

R2

1M

R1

1M

ICL7650

INTERNAL

BIAS

EXT CLK IN

CLK OUT

NULL

CAP RETURN

B

A

C

P

A

+

-

N

OUTPUT

+IN

-IN

CLAMP

MAIN

+

-

B

C

INT/EXT

A

A

A = CLK OUT

A

EXT CLK IN

B

C

OSC

Figure 1. ICL7650 Test Circuit

Figure 2. Block Diagram