Chopper-Stabilized Op Amps

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7

Nulling Capacitor Connection

the ICL7650. If you do not need the clamp feature,

and will produce slightly lower noise and improved AC

common-mode rejection. If you need to use the clamp

feature, order the ICL7650 and connect the external

capacitors to V-. To prevent load-current IR drops and

other extraneous signals from being injected into the

capacitors, use a separate PC board trace to connect

the capacitor commons directly to the V- pin. The out-

side foil of the capacitors should be connected to the

low-impedance side of the null storage circuit, V- or

Clock Operation

The ICL7650’s internal oscillator generates a 200Hz fre-

quency, which is available at the CLK OUT pin. The

device can also be operated with an external clock, if

desired. An internal pull-up permits the INT/EXT pin to

be left open for normal operation. However, the internal

clock must be disabled and INT/EXT must be tied to V-

if an external clock is used. An external clock signal

may then be applied to the EXT CLK IN pin. The duty

cycle of the external clock is not critical at low frequen-

cies. However, a 50% to 80% positive duty cycle is pre-

ferred for frequencies above 500Hz, since the

capacitors are charged only when EXT CLK IN is high.

This ensures that any transients have time to settle

before the capacitors are turned off. The external clock

should swing between ground and V+ for power sup-

plies up to ±6V, and between V+ and (V+ - 6V) for

higher supply voltages.

To avoid a capacitor imbalance during overload, use a

strobe signal. Neither capacitor will be charged if a

strobe signal is connected to EXT CLK IN so that it is

low while the overload signal is being applied to the

amplifier. A typical amplifier will drift less than 10µVs

since the leakage of the capacitor pins is quite low at

room temperature. Relatively long measurements may

be made with little change in offset.

Applications Information

Device Selection

In applications that require lowest noise, Maxim’s

ICL7652 may be preferred over the ICL7650/ICL7653.

The ICL7650/ICL7653 offer a higher gain-bandwidth

product and lower input bias currents, while the

ICL7652 reduces noise by using larger input FETs.

These larger FETs, however, increase the leakage at

the ICL7652’s external null pins. Therefore, the

ICL7650/ICL7653 can operate to a higher temperature

with 0.1µF capacitors before the clock ripple (due to

leakage at the null capacitor pins) becomes excessive

and 1µF external capacitors are required.

Output Stage/Load Driving

The ICL7650/ICL7653 somewhat resemble a transcon-

ductance amplifier whose open-loop gain is proportional

to load resistance. This behavior is apparent when loads

are less than the high-impedance stage (approximately

18k

Ω for one output circuit). The open-loop gain, for

example, will be 17dB lower with a 1k

Ω load than with a

10k

Ω load. This lower gain is of little consequence if the

amplifier is used strictly for DC since the DC gain is typi-

cally greater than 120dB, even with a 1k

Ω load. For

wideband applications, however, the best frequency

response will be achieved with a load resistor of 10k

Ω or

higher. The result will be a smooth 6dB per octave

response from 0.1Hz to 2MHz, with phase shifts of less

than 10° in the transition region where the main amplifier

takes over from the null amplifier.

Component Selection

optimum values depending on the clock or chopping

frequency. The correct value is 0.1µF for the preset

internal clock. When using an external clock, scale this

component value in proportion to the relationship

between the chopping frequency and the nulling time

constant. A low-leakage ceramic capacitor may prove

suitable for many applications; however, a high-quality

film-type capacitor (such as mylar) is preferred. For

lowest settling time at initial turn-on, use capacitors with

low dielectric absorption (such as polypropylene

types). With low-dielectric-absorption capacitors, the

ICL7650/ICL7653 will settle to 1µV offset in 100ms, but

several seconds may be required if ceramic capacitors

are used.

Thermoelectric Effects

Thermoelectric effects developed in thermocouple

junctions of dissimilar materials (metals, alloys, silicon,

etc.) ultimately limit precision DC measurements.

Unless all junctions are at the same temperature, ther-

moelectric voltages (typically around 10µV/°C, but up

to hundreds of µV/°C for some materials) will be gener-

ated. In order to realize the extremely low offset volt-

ages that the chopper amplifier can provide, take

special precautions to avoid temperature gradients. To

eliminate air movement, enclose all components (par-

ticularly those caused by power-dissipating elements in

the system). Minimize power-supply voltages and

power dissipation, and use low-thermoelectric-coeffi-

cient connections where possible. It is advisable to

separate the device surrounding heat-dissipating ele-

ments, and to use high-impedance loads.