4

FN2920.10

April 12, 2007

Test Circuit

Application Information

Detailed Description

AMPLIFIER

The functional diagram shows the major elements of the

ICL7650S. There are two amplifiers, the main amplifier, and the

nulling amplifier. Both have offset-null capability. The main

amplifier is connected continuously from the input to the output,

while the nulling amplifier, under the control of the chopping

oscillator and clock circuit, alternately nulls itself and the main

amplifier. The nulling connections, which are MOSFET gates,

are inherently high impedance, and two external capacitors

provide the required storage of the nulling potentials and the

necessary nulling-loop time constants. The nulling arrangement

operates over the full common-mode and power-supply

ranges, and is also independent of the output level, thus giving

Careful balancing of the input switches, and the inherent

balance of the input circuit, minimizes chopper frequency

charge injection at the input terminals, and also the feed

forward-type injection into the compensation capacitor, which

is the main cause of output spikes in this type of circuit.

INTERMODULATION

Previous chopper-stabilized amplifiers have suffered from

intermodulation effects between the chopper frequency and

input signals. These arise because the finite AC gain of the

amplifier necessitates a small AC signal at the input. This is

seen by the zeroing circuit as an error signal, which is

chopped and fed back, thus injecting sum and difference

frequencies and causing disturbances to the gain and phase

vs frequency characteristics near the chopping frequency.

These effects are substantially reduced in the ICL7650S by

feeding the nulling circuit with a dynamic current,

corresponding to the compensation capacitor current, in such

a way as to cancel that portion of the input signal due to finite

AC gain. Since that is the major error contribution to the

ICL7650S, the intermodulation and gain/phase disturbances

are held to very low values, and can generally be ignored.

CAPACITOR CONNECTION

The null/storage capacitors should be connected to the

CEXTA and CEXTB pins, with a common connection to the

CRETN pin. This connection should be made directly by

either a separate wire or PC trace to avoid injecting load

current IR drops into the capacitive circuitry. The outside foil,

where available, should be connected to CRETN.

OUTPUT CLAMP

The OUTPUT CLAMP pin allows reduction of the overload

recovery time inherent with chopper-stabilized amplifiers.

When tied to the inverting input pin, or summing junction, a

current path between this point and the OUTPUT pin occurs

just before the device output saturates. Thus uncontrolled

input differentials are avoided, together with the consequent

charge buildup on the correction-storage capacitors. The

output swing is slightly reduced.

Output Source Current

+25

2.9

4.5

-

mA

0 to +70

2.3

-

-

mA

Output Sink Current

+2525

30-

mA

0 to +70

20

-

-

mA

Internal Chopping Frequency

Pins 13 and 14 Open

+25

120

250

375

Hz

Clamp ON Current (Note 5)

+2525

70-

μA

Clamp OFF Current (Note 5)

-4V

+25

-

0.001

5

nA

0 to +70

-

-

10

nA

NOTES:

3. These parameters are guaranteed by design and characterization, but not tested at temperature extremes because thermocouple effects prevent

precise measurement of these voltages in automatic test equipment.

4. OUTPUT CLAMP not connected. See typical characteristic curves for output swing vs clamp current characteristics.

5. See OUTPUT CLAMP under detailed description.

6. All significant improvements over the industry-standard ICL7650 are highlighted in bold italics.

Electrical Specifications

PARAMETER

SYMBOL

TEST CONDITIONS

TEMP.

(°C)

MIN

TYP

MAX

UNITS

ICL7650S

+

-

OUTPUT

1M

Ω

C

0.1

μF0.1μF

C

1M

Ω