EL8178FWZ-T7A 查看數據表（PDF） - Intersil
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EL8178FWZ-T7A Datasheet PDF : 15 Pages
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EL8178
effects of the disabled amplifiers’ feedback networks must be
considered when evaluating the active amplifier’s
performance in Mux Amp configurations.
Note that feed through from the IN+ to IN- pins occurs on
any Mux Amp disabled channel where the input differential
voltage exceeds 0.5V (e.g., active channel VOUT = 1V, while
disabled channel VIN = GND), so the mux implementation is
best suited for small signal applications. In any application
where two or more amplifier outputs are muxed, use series
IN+ resistors, or large value RFs in each amplifier to keep
the feed through current low enough to minimize the impact
on the active channel. See “Usage Implications” on page 11
for more details.
IN+ and IN- Input Protection
In addition to ESD protection diodes to each supply rail, the
EL8178 has additional back-to-back protection diodes across
the differential input terminals. If the magnitude of the
differential input voltage exceeds the diode’s VF, then one of
these diodes will conduct. For elevated temperatures, the
leakage of the protection diodes (see Circuit 1 in “Pin
Descriptions” on page 10) increases, resulting in the increase
in IBIAS, as seen in Figures 21 and 22.
USAGE IMPLICATIONS
If the input differential voltage is expected to exceed 0.5V, an
external current limiting resistor must be used to ensure the
input current never exceeds 5mA. For noninverting unity gain
applications, the current limiting can be via a series IN+ resistor,
or via a feedback resistor of appropriate value. For other gain
configurations, the series IN+ resistor is the best choice, unless
the feedback (RF) and gain setting (RG) resistors are both
sufficiently large to limit the input current to 5mA.
Large differential input voltages can arise from several
sources:
1. During open loop (comparator) operation. The IN+ and
IN- input voltages don’t track.
2. When the amplifier is disabled but an input signal is still
present. An RL or RG to GND keeps the IN- at GND, while
the varying IN+ signal creates a differential voltage. Mux
Amp applications are similar, except that the active
channel VOUT determines the voltage on the IN- terminal.
3. When the slew rate of the input pulse is considerably
faster than the op amp’s slew rate. If the VOUT can’t keep
up with the IN+ signal, a differential voltage results, and
visible distortion occurs on the input and output signals.
To avoid this issue, keep the input slew rate below
0.2V/µs, or use appropriate current limiting resistors.
Large (>2V) differential input voltages can also cause an
increase in disabled ICC.
EN Input Protection
The EN input has internal ESD protection diodes to both the
positive and negative supply rails, limiting the input voltage
range to within one diode beyond the supply rails
(see “Circuit 2” diagram on page 9). If the input voltage is
expected to exceed V+ or V-, then an external series resistor
should be added to limit the current to 5mA.
Output Current Limiting
The EL8178 has no internal current-limiting circuitry. If the
output is shorted, it is possible to exceed the “Absolute
Maximum Rating” for “operating junction temperature”,
potentially resulting in the destruction of the device.
Power Dissipation
It is possible to exceed the +150°C maximum junction
temperature (TJMAX) under certain load and power-supply
conditions. It is therefore important to calculate TJMAX for all
applications to determine if power supply voltages, load
conditions, or package type need to be modified to remain in
the safe operating area. These parameters are related in
Equation 1:
TJMAX = TMAX + (θJAxPDMAX)
(EQ. 1)
where PDMAX is calculated using Equation 2:
PDMAX
=
VS × ISMAX + (VS
-
VO
U
T
M
A
X
)
×
V-----O----U----T----M-----A----X--
RL
(EQ. 2)
where:
• TMAX = Maximum ambient temperature
• θJA = Thermal resistance of the package
• PDMAX = Maximum power dissipation of the amplifier
• VS = Supply voltage
• IMAX = Maximum supply current of the amplifier
• VOUTMAX = Maximum output voltage swing of the
application
• RL = Load resistance
Proper Layout Maximizes Precision
To achieve the optimum levels of high input impedance
(i.e., low input currents) and low offset voltage, care should
be taken in the circuit board layout. The PC board surface
must remain clean and free of moisture to avoid leakage
currents between adjacent traces. Surface coating of the
circuit board will reduce surface moisture and provide a
humidity barrier, reducing parasitic resistance on the board.
When input leakage current is a paramount concern, the use
of guard rings around the amplifier inputs will further reduce
leakage currents. Figure 31 shows a guard ring example for
a unity gain amplifier that uses the low impedance amplifier
output at the same voltage as the high impedance input to
eliminate surface leakage. The guard ring does not need to
be a specific width, but it should form a continuous loop
around both inputs. For further reduction of leakage
currents, mount components to the PC board using Teflon
standoffs.
11
FN7504.6
March 17, 2008
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