OPA378 Datasheet, PDF (9/14 Pages) Texas Instruments – Low Noise, 900kHz, 50mV, Rail-to-Rail Input/Output Precision OPERATIONAL AMPLIFIER Zerø-Drift Series

OPA378
OPA2378
www.ti.com
APPLICATIONS INFORMATION
SBOS417 – JANUARY 2008
The OPA378 and OPA2378 are unity-gain stable and
free from unexpected output phase reversal. These
devices use a proprietary auto-calibration technique
to provide low offset voltage and very low drift over
time and temperature. For lowest offset voltage and
precision performance, circuit layout and mechanical
conditions should be optimized. Avoid temperature
gradients that create thermoelectric (Seebeck) effects
in the thermocouple junctions formed from connecting
dissimilar conductors. These thermally-generated
potentials can be made to cancel by assuring they
are equal on both input terminals. Other layout and
design considerations include:
• Use low thermoelectric-coefficient conditions
(avoid dissimilar metals).
• Thermally isolate components from power
supplies or other heat sources.
• Shield op amp and input circuitry from air
currents, such as cooling fans.
Following these guidelines reduces the likelihood of
junctions being at different temperatures, which can
cause thermoelectric voltages of 0.1µV/°C or higher,
depending on materials used.
OPERATING VOLTAGE
The OPA378 and OPA2378 op amps operate over a
power-supply range of +1.8V to +5.5V (±0.9V to
±2.75V). Supply voltages higher than +7V (absolute
maximum) can permanently damage the device.
Parameters that vary over supply voltage or
temperature are shown in the Typical Characteristics
section of this data sheet.
INPUT VOLTAGE
The OPA378 and OPA2378 input common-mode
voltage range extends 0.1V beyond the supply rails.
The OPA378 is designed to cover the full
common-mode range without the troublesome
transition region found in some other rail-to-rail
amplifiers.
Normally, input bias current is about 150pA; however,
input voltages exceeding the power supplies can
cause excessive current to flow into or out of the
input pins. Momentary voltages greater than the
power supply can be tolerated if the input current is
limited to 10mA. This limitation is easily accomplished
with an input resistor, as shown in Figure 23.
Current-limiting resistor
required if input voltage
exceeds supply rails by
³ 0.5V.
I_OVERLOAD
10mA max
V_IN
5kW
+5V
OPA378
V_OUT
Figure 23. Input Current Protection
INTERNAL OFFSET CORRECTION
The OPA378 and OPA2378 op amps use an
auto-calibration technique with a time-continuous
350kHz op amp in the signal path. This amplifier is
zero-corrected every 3µs using a proprietary
technique. Upon power-up, the amplifier requires
approximately 100µs to achieve specified V_OS
accuracy. This architecture has no aliasing or flicker
noise.
GENERAL LAYOUT GUIDELINES
Attention to good layout practices is always
recommended. Keep traces short and, when
possible, use a printed circuit board (PCB) ground
plane with surface-mount components placed as
close to the device pins as possible. Place a 0.1µF
capacitor closely across the supply pins. These
guidelines should be applied throughout the analog
circuit to improve performance and provide
benefits such as reducing the electromagnetic
interference (EMI) susceptibility.
Operational amplifiers vary in susceptibility to radio
frequency interference (RFI). RFI can generally be
identified as a variation in offset voltage or dc signal
levels with changes in the interfering RF signal. The
OPA378 has been specifically designed to minimize
susceptibility to RFI and demonstrates remarkably
low sensitivity compared to previous generation
devices. Despite this design, strong RF fields may
cause varying offset levels. If the amplifier cannot be
located away from sources of radiation, shielding may
be needed. Twisting wire input leads makes them
more resistant to RF fields.
Copyright © 2008, Texas Instruments Incorporated
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Product Folder Link(s): OPA378 OPA2378