TLC27M4CPWR datasheet(31/48 Pages) TI

31 / 48 page

(a) NONINVERTING AMPLIFIER

(b) INVERTING AMPLIFIER

TLC27M4, TLC27M4A, TLC27M4B, TLC27M4Y, TLC27M9

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SLOS093D – OCTOBER 1987 – REVISED OCTOBER 2012

Input Characteristics

The TLC27M4 and TLC27M9 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 TLC27M4 and TLC27M9

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 TLC27M4 and

TLC27M9 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 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 TLC27M4 and TLC27M9 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.

Figure 40. Guard-Ring Schemes

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TLC27M4CPWR Datasheet(PDF) 31 Page - Texas Instruments