OPA379 Datasheet, PDF (7/12 Pages) Burr-Brown (TI) – 1.8V, 2.9uA, 90kHz, Rail-to-Rail I/O OPERATIONAL AMPLIFIERS

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APPLICATION INFORMATION
The OPA379 family of operational amplifiers minimizes
power consumption without compromising bandwidth or
noise. Power-supply rejection ratio (PSRR),
common-mode rejection ratio (CMRR), and open-loop
gain (A_OL) typical values are 100dB or better.
When designing for ultra-low power, choose system
components carefully. To minimize current consumption,
select large-value resistors. Any resistors will react with
stray capacitance in the circuit and the input capacitance
of the operational amplifier. These parasitic RC
combinations can affect the stability of the overall system.
A feedback capacitor may be required to assure stability
and limit overshoot or gain peaking.
Good layout practice mandates the use of a 0.1µF bypass
capacitor placed closely across the supply pins.
OPERATING VOLTAGE
OPA379 series op amps are fully specified and tested from
+1.8V to +5.5V. Parameters that vary significantly with
supply voltage are shown in the Typical Characteristics
curves.
INPUT COMMON-MODE VOLTAGE RANGE
The input common-mode voltage range of the OPA379
family typically extends 100mV beyond each supply rail.
This rail-to-rail input is achieved using a complementary
input stage. CMRR is specified from the negative rail to 1V
below the positive rail. Between (V+) − 1V and (V+) + 0.1V,
the amplifier operates with higher offset voltage because
of the transition region of the input stage. See the typical
characteristic, Offset Voltage vs Common-Mode Voltage.
PROTECTING INPUTS FROM
OVER-VOLTAGE
Normally, input currents are 5pA. However, large inputs
(greater than 500mV beyond the supply rails) can cause
excessive current to flow in or out of the input pins.
Therefore, as well as keeping the input voltage below the
maximum rating, it is also important to limit the input
current to less than 10mA. This limiting is easily
accomplished with an input voltage resistor, as shown in
Figure 1.
OPA379
OPA2379
OPA4379
SBOS347A − NOVEMBER 2005 − REVISED DECEMBER 2005
I_OVERLOAD
10mA max
V_IN
5kΩ
+5V
OPA379
V_OUT
Figure 1. Input Current Protection for Voltages
Exceeding the Supply Voltage
NOISE
Although micropower amplifiers frequently have high
wideband noise, the OPA379 series offer excellent noise
performance. Resistors should be chosen carefully
because the OPA379 has only 2.8µV_PP of 0.1Hz to 10Hz
noise, and 80nV/√Hz of wideband noise; otherwise, they
can become the dominant source of noise.
CAPACITIVE LOAD AND STABILITY
Follower configurations with load capacitance in excess of
30pF can produce extra overshoot (see typical
characteristic, Small-Signal Overshoot vs Capacitive
Load) and ringing in the output signal. Increasing the gain
enhances the ability of the amplifier to drive greater
capacitive loads. In unity-gain configurations, capacitive
load drive can be improved by inserting a small (10Ω to
20Ω) resistor, R_S, in series with the output, as shown in
Figure 2. This resistor significantly reduces ringing while
maintaining DC performance for purely capacitive loads.
However, if there is a resistive load in parallel with the
capacitive load, a voltage divider is created, introducing a
Direct Current (DC) error at the output and slightly
reducing the output swing. The error introduced is
proportional to the ratio R_S/R_L, and is generally negligible.
V+
R_S
OPA379
V_OUT
V_IN
10Ω to
20Ω
R_L
C_L
Figure 2. Series Resistor in Unity-Gain Buffer
Configuration Improves Capacitive Load Drive
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