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TELCOM SEMICONDUCTOR, INC.

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DUAL AUTO-ZEROED

OPERATIONAL AMPLIFIERS

TC913A

TC913B

Theory of Operation

Each of the TC913's two op-amps actually consists of

two amplifiers. A main amplifier is always connected from

the input to the output. A separate nulling amplifier alter-

nately nulls its own offset and then the offset of the amplifier.

Since each amplifier is continuously being nulled, offset

voltage drift with time, temperature, and power supply

variations is greatly reduced.

All nulling circuitry is internal and the nulling operation is

transparent to the user. Offset nulling voltages are stored on

two internal capacitors. An internal oscillator and control

logic, shared by the TC913's two amplifiers, control the

nulling process.

Pin Compatibility

The TC913 pinout is compatible with OP-14, LM358,

MC1458, LT1013, TLC322, and similar dual op amps. In

many circuits operating from single or

±5V supplies, the

TC913 is a drop-in replacement offering DC performance

rivaling that of the best single op amps.

The TC913's amplifiers include a low-impedance class

AB output buffer. Some previous CMOS chopper amplifiers

used a high-impedance output stage which made open-loop

gain dependent on load resistance. The TC913's open-loop

gain is not dependent on load resistance.

Overload Recovery

The TC913 recovers quickly from output saturation.

Typical recovery time from positive output saturation is

20msec. Negative output saturation recovery time is typi-

cally 5msec.

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 power dissipa-

tion. The TC913's 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.