DUAL OPERATIONAL AMPLIFIERS
SLOS120A – AUGUST 1993 – REVISED AUGUST 1994
POST OFFICE BOX 655303
• DALLAS, TEXAS 75265
The TLC2810Z 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. The lower range limit includes the negative rail, while the upper range limit is specified
at VDD –1 V at TA = 25°C and at VDD– 1.2 V at all other temperatures.
The use of the polysilicon-gate process and the careful input circuit design give the TLC2810Z very good input
offset voltage drift characteristics relative to conventional metal-gate processes. Offset voltage drift in CMOS
devices is 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 TLC2810Z 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 29 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 33).
Unused amplifiers should be connected as grounded voltage followers to avoid possible oscillation.
(a) NONINVERTING AMPLIFIER
(b) INVERTING AMPLIFIER
(c) UNITY-GAIN AMPLIFIER
Figure 33. Guard-Ring Schemes
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 TLC2810Z 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 k
Ω since bipolar devices exhibit greater