TLC27L2CPE4 Datasheet, PDF (27/49 Pages) Texas Instruments – LinCMOS PRECISION DUAL OPERATIONAL AMP LIFIERS

TLC27L2, TLC27L2A, TLC27L2B, TLC27L7
LinCMOS PRECISION DUAL OPERATIONAL AMPLIFIERS
SLOS052D − OCTOBER 1987 − REVISED OCTOBER 2005
APPLICATION INFORMATION
input characteristics
The TLC27L2 and TLC27L7 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 TLC27L2 and TLC27L7
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 TLC27L2 and
TLC27L7 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 TLC27L2 and TLC27L7 result in a 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.
VI
VO
VI
VO
VO
VI
(a) NONINVERTING AMPLIFIER
(b) INVERTING AMPLIFIER
(c) UNITY-GAIN AMPLIFIER
Figure 40. Guard-Ring Schemes
output characteristics
The output stage of the TLC27L2 and TLC27L7 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 TLC27L2 and TLC27L7 were measured using a 20-pF load. The devices
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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