TLC2654, TLC2654A

Advanced LinCMOS

™ LOW-NOISE CHOPPER-STABILIZED

OPERATIONAL AMPLIFIERS

SLOS020F – NOVEMBER 1988 – REVISED JULY 1999

21

POST OFFICE BOX 655303

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

µV/°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

µF 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.