TLC271_01 Datasheet, PDF (59/82 Pages) Texas Instruments – LinCMOSE PROGRAMMABLE LOW-POWER OPERATIONAL AMPLIFIERS

TLC271, TLC271A, TLC271B
LinCMOS PROGRAMMABLE LOW-POWER
OPERATIONAL AMPLIFIERS
SLOS090D – NOVEMBER 1987 – REVISED MARCH 2001
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 V_DD – 1 V at T_A = 25°C and at V_DD – 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 kΩ, since bipolar devices exhibit greater noise
currents.
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VI
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(a) NONINVERTING AMPLIFIER
(b) INVERTING AMPLIFIER
Figure 107. Guard-Ring Schemes
(c) UNITY-GAIN AMPLIFIER
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