TLC2654, TLC2654A
Advanced LinCMOS
LOW-NOISE CHOPPER-STABILIZED
OPERATIONAL AMPLIFIERS
SLOS020G – NOVEMBER 1988 – REVISED APRIL 2001
21
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
APPLICATION INFORMATION
overload recovery/output clamp (continued)
The clamp is a switch that is automatically activated when the output is approximately 1 V from either supply
rail. When connected to the inverting input (in parallel with the closed-loop feedback resistor), the closed-loop
gain is reduced and the TLC2654 output is prevented from going into saturation. Since the output must source
or sink current through the switch (see Figure 9), the maximum output voltage swing is slightly reduced.
thermoelectric effects
To take advantage of the extremely low offset voltage temperature coefficient of the TLC2654, care must be
taken to compensate for the thermoelectric effects present when two dissimilar metals are brought into contact
with each other (such as device leads being soldered to a printed-circuit board). It is not uncommon for dissimilar
metal junctions to produce thermoelectric voltages in the range of several microvolts per degree Celsius (orders
of magnitude greater than the 0.01
V/°C typical of the TLC2654).
To help minimize thermoelectric effects, pay careful attention to component selection and circuit-board layout.
Avoid the use of nonsoldered connections (such as sockets, relays, switches, etc.) in the input signal path.
Cancel thermoelectric effects by duplicating the number of components and junctions in each device input. The
use of low-thermoelectric-coefficient components, such as wire-wound resistors, is also beneficial.
latch-up avoidance
Because CMOS devices are susceptible to latch-up due to their inherent parasitic thyristors, the TLC2654 inputs
and outputs are designed to withstand – 100-mA surge currents without sustaining latch-up; however,
techniques to reduce the chance of latch-up should be used whenever possible. Internal protection diodes
should not, by design, be forward biased. Applied input and output voltages should not exceed the supply
voltage by more than 300 mV. Care should be exercised when using capacitive coupling on pulse generators.
Supply transients should be stunted by the use of decoupling capacitors (0.1
F typical) located across the
supply rails as close to the device as possible.
The current path established if latch-up occurs is usually between the supply rails and is limited only by the
impedance of the power supply and the forward resistance of the parasitic thyristor. The chance of latch-up
occurring increases with increasing temperature and supply voltage.
electrostatic-discharge protection
The TLC2654 incorporates internal ESD-protection circuits that prevent functional failures at voltages at or
below 2000 V. Care should be exercised in handling these devices, as exposure to ESD may result in
degradation of the device parametric performance.
theory of operation
Chopper-stabilized operational amplifiers offer the best dc performance of any monolithic operational amplifier.
This superior performance is the result of using two operational amplifiers — a main amplifier and a nulling
amplifier – plus oscillator-controlled logic and two external capacitors to create a system that behaves as a
single amplifier. With this approach, the TLC2654 achieves submicrovolt input offset voltage, submicrovolt
noise voltage, and offset voltage variations with temperature in the nV/
°C range.
The TLC2654 on-chip control logic produces two dominant clock phases: a nulling phase and an amplifying
phase. The term chopper-stabilized derives from the process of switching between these two clock phases.
Figure 35 shows a simplified block diagram of the TLC2654. Switches A and B are make-before-break types.