TLC271 datasheet(59/84 Pages) TI

59 / 84 page

TLC271, TLC271A, TLC271B

LinCMOS

 PROGRAMMABLE LOW-POWER

OPERATIONAL AMPLIFIERS

SLOS090D – NOVEMBER 1987 – REVISED MARCH 2001

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

input characteristics

The TLC271 is specified with a minimum and a maximum input voltage that, if exceeded at either input, could

cause the device to malfunction. Exceeding this specified range is a common problem, especially in

single-supply operation. Note that the lower range limit includes the negative rail, while the upper range limit

is specified at VDD – 1 V at TA = 25°C and at VDD – 1.5 V at all other temperatures.

The use of the polysilicon-gate process and the careful input circuit design gives the TLC271 very good input

offset voltage drift characteristics relative to conventional metal-gate processes. Offset voltage drift in CMOS

devices is highly influenced by threshold voltage shifts caused by polarization of the phosphorus dopant

implanted in the oxide. Placing the phosphorus dopant in a conductor (such as a polysilicon gate) alleviates the

polarization problem, thus reducing threshold voltage shifts by more than an order of magnitude. The offset

voltage drift with time has been calculated to be typically 0.1

µV/month, including the first month of operation.

Because of the extremely high input impedance and resulting low bias current requirements, the TLC271 is well

suited for low-level signal processing; however, leakage currents on printed circuit boards and sockets can

easily exceed bias current requirements and cause a degradation in device performance. It is good practice to

include guard rings around inputs (similar to those of Figure 101 in the Parameter Measurement Information

section). These guards should be driven from a low-impedance source at the same voltage level as the

common-mode input (see Figure 107).

The inputs of any unused amplifiers should be tied to ground to avoid possible oscillation.

noise performance

The noise specifications in operational amplifier circuits are greatly dependent on the current in the first-stage

differential amplifier. The low input bias current requirements of the TLC271 results in a very low noise current,

which is insignificant in most applications. This feature makes the devices especially favorable over bipolar

devices when using values of circuit impedance greater than 50 k

Ω, since bipolar devices exhibit greater noise

currents.

(a) NONINVERTING AMPLIFIER

(b) INVERTING AMPLIFIER

Figure 107. Guard-Ring Schemes

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