OPA378_1 Datasheet, PDF (10/24 Pages) Texas Instruments – Low-Noise, 900kHz, RRIO, Precision OPERATIONAL AMPLIFIER Zerø-Drift Series

OPA378
OPA2378
SBOS417C – JANUARY 2008 – REVISED JUNE 2009 ..................................................................................................................................................... www.ti.com
INPUT DIFFERENTIAL VOLTAGE
The typical input bias current of the OPA378 during
normal operation is approximately 150pA. In
over-driven conditions, the bias current can increase
significantly (see Figure 24). The most common
cause of an over-driven condition occurs when the op
amp is outside of the linear range of operation. When
the output of the op amp is driven to one of the
supply rails the feedback loop requirements cannot
be satisfied and a differential input voltage develops
across the input pins. This differential input voltage
results in activation of parasitic diodes inside the front
end input chopping switches that combine with 1.5kΩ
EMI filter resistors to create the equivalent circuit
shown in Figure 27.
OPA378 operational amplifier family incorporates an
internal input low-pass filter that reduces the amplifier
response to EMI. Both common-mode and
differential-mode filtering are provided by the input
filter. The filter is designed for a cutoff frequency of
approximately 25MHz (–3dB), with a roll-off of 20dB
per decade. Figure 28 shows the EMI filter.
0
-10
-20
1.5kW _Clamp
+In
CORE
-In
1.5kW
Figure 27. Equivalent Input Circuit
INTERNAL OFFSET CORRECTION
The OPA378 and OPA2378 family of 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.
NOISE
The OPA378 series of op amps have excellent
distortion characteristics. Total harmonic distortion +
noise is below 0.003% (G = +1, V_O = 3V_RMS, and f =
1kHz, with a 10kΩ load). Design of low-noise op amp
circuits requires careful consideration of a variety of
possible noise contributors: noise from the signal
source, noise generated in the op amp, and noise
from the feedback network resistors. The total noise
of the circuit is the root-sum-square combination of all
the noise components.
EMI SUSCEPTIBILITY AND INPUT FILTERING
Operational amplifiers vary in their susceptibility to
electromagnetic interference (EMI). If conducted EMI
enters the operational amplifier, the dc offset
observed at the amplifier output may shift from its
nominal value while the EMI is present. This shift is a
result of signal rectification associated with the
internal semiconductor junctions. While all operational
amplifier pin functions can be affected by EMI, the
input pins are likely to be the most susceptible. The
-30
-40
1k
f_C = 25MHz with Parasitics
Over Temperature
-29dB at 800MHz
10k 100k
1M
10M 100M 1G
Frequency (Hz)
Figure 28. EMI Filter
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.
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.
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