5

FN2920.10

April 12, 2007

CLOCK

The ICL7650S has an internal oscillator, giving a chopping

frequency of 200Hz, available at the CLOCK OUT pin on the 14

pin devices. Provision has also been made for the use of an

external clock in these parts. The INT/EXT pin has an internal

pull-up and may be left open for normal operation, but to utilize

an external clock this pin must be tied to V- to disable the

internal clock. The external clock signal may then be applied to

the EXT CLOCK IN pin. An internal divide-by-two provides the

desired 50% input switching duty cycle. Since the capacitors

are charged only when EXT CLOCK IN is high, a 50% to 80%

positive duty cycle is recommended, especially for higher

frequencies. The external clock can swing between V+ and V-.

The logic threshold will be at about 2.5V below V+. Note also

that a signal of about 400 Hz, with a 70% duty cycle, will be

present at the EXT CLOCK IN pin with INT/EXT high or open.

This is the internal clock signal before being fed to the divider.

In those applications where a strobe signal is available, an

alternate approach to avoid capacitor misbalancing during

overload can be used. If a strobe signal is connected to EXT

CLK IN so that it is low during the time that the overload

signal is applied to the amplifier, neither capacitor will be

charged. Since the leakage at the capacitor pins is quite low

at room temperature, the typical amplifier will drift less than

10

μV/s, and relatively long measurements can be made with

little change in offset.

COMPONENT SELECTION

optimum values depending on the clock or chopping

frequency. For the preset internal clock, the correct value is

0.1

μF, and to maintain the same relationship between the

chopping frequency and the nulling time constant this value

should be scaled approximately in proportion if an external

clock is used. A high quality film type capacitor such as

mylar is preferred, although a ceramic or other lower-grade

capacitor may prove suitable in many applications. For

quickest settling on initial turn-on, low dielectric absorption

capacitors (such as polypropylene) should be used. With

ceramic capacitors, several seconds may be required to

settle to 1

μV.

STATIC PROTECTION

All device pins are static-protected by the use of input diodes.

However, strong static fields and discharges should be avoided,

as they can cause degraded diode junction characteristics,

which may result in increased input-leakage currents.

LATCHUP AVOIDANCE

Junction-isolated CMOS circuits inherently include a parasitic

4-layer (PNPN) structure which has characteristics similar to

an SCR. Under certain circumstances this junction may be

triggered into a low-impedance state, resulting in excessive

supply current. To avoid this condition, no voltage greater than

0.3V beyond the supply rails should be applied to any pin. In

general, the amplifier supplies must be established either at

the same time or before any input signals are applied. If this is

not possible, the drive circuits must limit input current flow to

under 1mA to avoid latchup, even under fault conditions.

OUTPUT STAGE/LOAD DRIVING

The output circuit is a high-impedance type (approximately

18k

Ω), and therefore with loads less than this value, the

chopper amplifier behaves in some ways like a

transconductance amplifier whose open-loop gain is

proportional to load resistance. For example, the open-loop

gain will be 17dB lower with a 1k

Ω load than with a 10kΩ

load. If the amplifier is used strictly for DC, this lower gain is

of little consequence, since the DC gain is typically greater

than 120dB even with a 1k

Ω load. However, for wideband

applications, the best frequency response will be achieved

with a load resistor of 10k

Ω or higher. This will result in a

smooth 6dB/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.

THERMO-ELECTRIC EFFECTS

The ultimate limitations to ultra-high precision DC amplifiers are

the thermo-electric or Peltier effects arising in thermocouple

junctions of dissimilar metals, alloys, silicon, etc. Unless all

junctions are at the same temperature, thermoelectric voltages

typically around 0.1

μV/°C, but up to tens of mV/°C for some

materials, will be generated. In order to realize the extremely

low offset voltages that the chopper amplifier can provide, it is

essential to take special precautions to avoid temperature

gradients. All components should be enclosed to eliminate air

movement, especially that caused by power-dissipating

elements in the system. Low thermoelectric-efficient

connections should be used where possible and power supply

voltages and power dissipation should be kept to a minimum.

High-impedance loads are preferable, and good separation

from surrounding heat-dissipating elements is advisable.

GUARDING

Extra care must be taken in the assembly of printed circuit

boards to take full advantage of the low input currents of the

ICL7650S. Boards must be thoroughly cleaned with TCE or

alcohol and blown dry with compressed air. After cleaning,

the boards should be coated with epoxy or silicone rubber to

prevent contamination.

Even with properly cleaned and coated boards, leakage

currents may cause trouble, particularly since the input pins

are adjacent to pins that are at supply potentials. This

leakage can be significantly reduced by using guarding to

lower the voltage difference between the inputs and adjacent

metal runs. The guard, which is a conductive ring

surrounding the inputs, is connected to a low impedance

point that is at approximately the same voltage as the inputs.

Leakage currents from high-voltage pins are then absorbed

by the guard.