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
10μV/s, and relatively long measurements can be made with
little change in offset.
COMPONENT SELECTION
The two required capacitors, C
EXTA
and C
EXTB
, have
optimum values depending on the clock or chopping
frequency. For the preset internal clock, the correct value is
0.1μF, 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 1μV.
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
18kΩ), 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 1kΩ 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 1kΩ load. However, for wideband
applications, the best frequency response will be achieved
with a load resistor of 10kΩ 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 0.1μV/°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.
