TC911A Datasheet, PDF (4/5 Pages) TelCom Semiconductor, Inc – AUTO-ZEROED OPERATIONAL AMPLIFIERS

TC911A
TC911B
AUTO-ZEROED MONOLITHIC
OPERATIONAL AMPLIFIERS
Pin Compatibility
The CMOS TC911 is pin compatible with the industry
standard ICL7650 chopper-stabilized amplifier. The ICL7650
must use external 0.1µF capacitors connected at pins 1 and
8. With the TC911, external offset voltage error cancel-
ing capacitors are not required. On the TC911 pins 1, 8
and 5 are not connected internally. The ICL7650 uses pin 5
as an optional output clamp connection. External chopper
capacitors and clamp connections are not necessary with
the TC911. External circuits connected to pins 1, 8 and 5 will
have no effect. The TC911 can be quickly evaluated in
existing ICL7650 designs. Since external capacitors are not
required, system part count, assembly time, and total sys-
tem cost are reduced. Reliability is increased and PC board
layout eased by having the error storage capacitors inte-
grated on the TC911 chip.
The TC911 pinout matches many existing op amps:
741, LM101, LM108, OP05–OP08, OP-20, OP-21, ICL7650
and ICL7652. In many applications operating from +5V
supplies the TC911 offers superior electrical performance
and can be a functional pin-compatible replacement. Offset
voltage correction potentiometers, compensation capaci-
tors, and chopper-stabilization capacitors can be removed
when retrofitting existing equipment designs.
Thermocouple Errors
Heating one joint of a loop made from two different
metallic wires causes current flow. This is known as the
Seebeck effect. By breaking the loop, an open circuit voltage
J3 = J4
J2 = J5
J1 = J6
NO TEMPERATURE DIFFERENTIAL
AND SAME METALLIC CONNECTION
J2
J1
J3
PACKAGE
PIN
J4
J6
(Seebeck voltage) can be measured. Junction temperature
and metal type determine the magnitude. Typical values are
0.1µV/°C to 10µV/°C. Thermal-induced voltages can be
many times larger than the TC911 offset voltage drift. Unless
unwanted thermocouple potentials can be controlled, sys-
tem performance will be less than optimum.
Unwanted thermocouple junctions are created when
leads are soldered or sockets/connectors are used. Low
thermo-electric coefficient solder can reduce errors. A 60%
Sn/36% Pb solder has 1/10 the thermal voltage of common
64% Sn/36% Pb solder at a copper junction.
The number and type of dissimilar metallic junctions in
the input circuit loop should be balanced. If the junctions are
kept at the same temperature, their summation will add to
zero-canceling errors (Figure 1).
Shielding precision analog circuits from air currents —
especially those caused by power dissipating components
and fans — will minimize temperature gradients and ther-
mocouple-induced errors.
Avoiding Latch-Up
Junction-isolated CMOS circuits inherently contain a
parasitic p-n-p-n transistor circuit. Voltages exceeding the
supplies by 0.3V should not be applied to the device pins.
Larger voltages can turn the p-n-p-n device on, causing
excessive device power supply current and excessive power
dissipation. TC911 power supplies should be established at
the same time or before input signals are applied. If this is not
possible input current should be limited to 0.1mA to avoid
triggering the p-n-p-n structure.
Overload Recovery
The TC911 recovers quickly from the output saturation.
Typical recovery time from positive output saturation is
20msec. Negative output saturation recovery time is typi-
cally 5msec.
J5
–
J3 V3
+
+
J4 V4
–
J2
+–
V2
J1
+–
V1
VT = V1 + V2 + V3 – V4 – V5 – V6 = 0
+ ^V5 –
J5
+ ^V6 –
J6
VT = 0
Figure 1. Unwanted Thermocouple Errors Eliminated by
Reducing Thermal Gradients and Balancing Junctions
3-266
TELCOM SEMICONDUCTOR, INC.