TLC272, TLC272A, TLC272B, TLC272Y, TLC277
LinCMOS PRECISION DUAL OPERATIONAL AMPLIFIERS
SLOS091B – OCTOBER 1987 – REVISED AUGUST 1994
30
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
input characteristics
The TLC272 and TLC277 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 TLC272 and TLC277 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 TLC272 and
TLC277 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 TLC272 and TLC277 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.
V
I
–
+
–
+
V
I
(b) INVERTING AMPLIFIER
–
+
(c) UNITY-GAIN AMPLIFIER(a) NONINVERTING AMPLIFIER
V
I
OUT OUT OUT
Figure 40. Guard-Ring Schemes
output characteristics
The output stage of the TLC272 and TLC277 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 TLC272 and TLC277 are measured using a 20-pF load. The devices can
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.
