TLC272CPWR datasheet(30/53 Pages) TI1

30 / 53 page

TLC272, TLC272A, TLC272B, TLC272Y, TLC277

LinCMOS

 PRECISION DUAL OPERATIONAL AMPLIFIERS

SLOS091E – OCTOBER 1987 – REVISED FEBRUARY 2002

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

input characteristics

The TLC272 and TLC277 are 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 TLC272 and TLC277 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 TLC272 and

TLC277 are 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 4 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 40).

Unused amplifiers should be connected as grounded unity-gain followers 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 TLC272 and TLC277 result 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.

(b) INVERTING AMPLIFIER

(a) NONINVERTING AMPLIFIER

OUT

Figure 40. Guard-Ring Schemes

output characteristics

The output stage of the TLC272 and TLC277 is designed to sink and source relatively high amounts of current

(see typical characteristics). If the output is subjected to a short-circuit condition, this high current capability can

cause device damage under certain conditions. Output current capability increases with supply voltage.

All operating characteristics of the TLC272 and TLC277 are measured using a 20-pF load. The devices can

drive higher capacitive loads; however, as output load capacitance increases, the resulting response pole

occurs at lower frequencies, thereby causing ringing, peaking, or even oscillation (see Figure 41). In many

cases, adding a small amount of resistance in series with the load capacitance alleviates the problem.

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