TLC27M4, TLC27M4A, TLC27M4B, TLC27M4Y, TLC27M9

LinCMOS

™ PRECISION QUAD OPERATIONAL AMPLIFIERS

SLOS093C – OCTOBER 1987 – REVISED MAY 1999

31

POST OFFICE BOX 655303

APPLICATION INFORMATION

single-supply operation (continued)

Logic

Logic

Logic

(a) COMMON SUPPLY RAILS

(b) SEPARATE BYPASSED SUPPLY RAILS (preferred)

Logic

Logic

Logic

Power

Supply

Power

Supply

Output

Output

Figure 39. Common Versus Separate Supply Rails

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

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.