TLC27L4_14 Datasheet, PDF (31/43 Pages) Texas Instruments – LinCMOSE PRECISION QUAD OPERATIONAL AMPLIFIERS

TLC27L4, TLC27L4A, TLC27L4B, TLC27L4Y, TLC27L9
LinCMOS™ PRECISION QUAD OPERATIONAL AMPLIFIERS
SLOS053C – OCTOBER 1987 – REVISED AUGUST 1994
APPLICATION INFORMATION
single-supply operation (continued)
Output
Logic
Logic
Logic
Power
Supply
(a) COMMON SUPPLY RAILS
Output
Logic
Logic
Logic
Power
Supply
(b) SEPARATE BYPASSED SUPPLY RAILS (preferred)
Figure 39. Common Versus Separate Supply Rails
input characteristics
The TLC27L4 and TLC27L9 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 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 TLC27L4 and TLC27L9
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 TLC27L4 and
TLC27L9 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).
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 TLC27L4 and TLC27L9 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 kΩ, since bipolar devices exhibit
greater noise currents.
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