Datasheet

CORE
-In
+In
Clamp
1.5kW
1.5kW
1k
FilterResponse(dB)
Frequency(Hz)
100k10k 1M 10M 100M 1G
0
10
20
30
40
-
-
-
-
f =25MHzwithParasitics
OverTemperature
29dBat800MHz-
C
OPA378
OPA2378
www.ti.com SBOS417D –JANUARY 2008–REVISED OCTOBER 2009
INPUT DIFFERENTIAL VOLTAGE OPA378 operational amplifier family incorporates an
internal input low-pass filter that reduces the amplifier
The typical input bias current of the OPA378 during
response to EMI. Both common-mode and
normal operation is approximately 150pA. In
differential-mode filtering are provided by the input
over-driven conditions, the bias current can increase
filter. The filter is designed for a cutoff frequency of
significantly (see Figure 24). The most common
approximately 25MHz (–3dB), with a roll-off of 20dB
cause of an over-driven condition occurs when the op
per decade. Figure 29 shows the EMI filter.
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 28.
Figure 28. Equivalent Input Circuit
Figure 29. EMI Filter
INTERNAL OFFSET CORRECTION
GENERAL LAYOUT GUIDELINES
The OPA378 and OPA2378 family of op amps use an
auto-calibration technique with a time-continuous
Attention to good layout practices is always
350kHz op amp in the signal path. This amplifier is
recommended. Keep traces short and, when
zero-corrected every 3μs using a proprietary
possible, use a printed circuit board (PCB) ground
technique. Upon power-up, the amplifier requires
plane with surface-mount components placed as
approximately 100μs to achieve specified V
OS
close to the device pins as possible. Place a 0.1μF
accuracy. This architecture has no aliasing or flicker
capacitor closely across the supply pins. These
noise.
guidelines should be applied throughout the analog
circuit to improve performance.
NOISE
For lowest offset voltage and precision performance,
The OPA378 series of op amps have excellent circuit layout and mechanical conditions should be
distortion characteristics. Total harmonic distortion + optimized. Avoid temperature gradients that create
noise is below 0.003% (G = +1, V
O
= 3V
RMS
, and f = thermoelectric (Seebeck) effects in the thermocouple
1kHz, with a 10kΩ load). Design of low-noise op amp junctions formed from connecting dissimilar
circuits requires careful consideration of a variety of conductors. These thermally-generated potentials can
possible noise contributors: noise from the signal be made to cancel by assuring they are equal on
source, noise generated in the op amp, and noise both input terminals. Other layout and design
from the feedback network resistors. The total noise considerations include:
of the circuit is the root-sum-square combination of all
• Use low thermoelectric-coefficient conditions
the noise components.
(avoid dissimilar metals).
• Thermally isolate components from power
EMI SUSCEPTIBILITY AND INPUT FILTERING
supplies or other heat sources.
Operational amplifiers vary in their susceptibility to
• Shield op amp and input circuitry from air
electromagnetic interference (EMI). If conducted EMI
currents, such as cooling fans.
enters the operational amplifier, the dc offset
Following these guidelines reduces the likelihood of
observed at the amplifier output may shift from its
junctions being at different temperatures, which can
nominal value while the EMI is present. This shift is a
cause thermoelectric voltages of 0.1μV/°C or higher,
result of signal rectification associated with the
depending on materials used.
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
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Product Folder Link(s): OPA378 OPA2378