OPA379AIDBVR Datasheet, PDF (9/29 Pages) Texas Instruments – 1.8V, 2.9mA, 90kHz, Rail-to-Rail I/O PERATIONAL AMPLIFIERS

OPA379
OPA2379
OPA4379
www.ti.com ................................................................................................................................................... SBOS347D – NOVEMBER 2005 – REVISED MAY 2008
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 (±0.9V to ±2.75V). Parameters
that will vary 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 vs Temperature (Figure 8).
PROTECTING INPUTS FROM
OVER-VOLTAGE
Normally, input currents are 5pA. However, a large
voltage input (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 20.
I_OVERLOAD
10mA max
V_IN
5kW
V_S
OPA379
V_OUT
Figure 20. 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, Figure 17) 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 21.
This resistor significantly reduces ringing while
maintaining direct current (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 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_S
R_S
OPA379
V_OUT
V_IN
10W to
20W
R_L
C_L
Figure 21. Series Resistor in Unity-Gain Buffer
Configuration Improves Capacitive Load Drive
Copyright © 2005–2008, Texas Instruments Incorporated
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Product Folder Link(s): OPA379 OPA2379 OPA4379