ISL70218SEH
Applications Information
Functional Description
R
F
V+
The ISL70218SEH is a dual, 3.2MHz, single or dual supply,
rail-to-rail output amplifier with a common mode input voltage
range extending to a range of 0.5V below the V- rail. The input
stage is optimized for precision sensing of ground-referenced
signals in single-supply applications. The input stage is able to
handle large input differential voltages without phase inversion,
making this amplifier suitable for high-voltage comparator
applications. The bipolar design features high open loop gain and
excellent DC input and output temperature stability. This op amp
features very low quiescent current of 850µA, and low
temperature drift. The device is fabricated in a new precision 40V
complementary bipolar DI process and is immune from latch-up
for up to a 36V supply range.
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IN
-
V
-
IN
R
+
IN
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V
+
R
IN
L
R
G
V-
FIGURE 49. INPUT ESD DIODE CURRENT LIMITING
Output Drive Capability
The bipolar rail-to-rail output stage features low saturation levels
that enable an output voltage swing to less than 15mV when the
total output load (including feedback resistance) is held below
50µA (Figures 21 and 22). With ±15V supplies, this can be
achieved by using feedback resistor values >300kΩ.
Operating Voltage Range
The op amp is designed to operate over a single supply range of 3V
to 36V or a split supply voltage range of +1.8V/-1.2V to ±18V. The
device is fully characterized at 30V (±15V). Both DC and AC
performance remain virtually unchanged over the complete
operating voltage range. Parameter variation with operating
voltage is shown in the “Typical Performance Curves” beginning on
page 8.
The output stage is internally current limited. Output current limit
over temperature is shown in Figures 23 and 24. The amplifiers
can withstand a short circuit to either rail as long as the power
dissipation limits are not exceeded. This applies to only one
amplifier at a time for the dual op amp. Continuous operation
under these conditions may degrade long-term reliability.
The input common mode voltage to the V+ rail (V+ - 1.8V over the
full temperature range) may limit amplifier operation when
operating from split V+ and V- supplies. Figure 4 shows the
common mode input voltage range variation over temperature.
The amplifiers perform well when driving capacitive loads
(Figures 45 and 46). The unity gain, voltage follower (buffer)
configuration provides the highest bandwidth but is also the
most sensitive to ringing produced by load capacitance found in
BNC cables. Unity gain overshoot is limited to 35% at
capacitance values to 0.33nF. At gains of 10 and higher, the
device is capable of driving more than 10nF without significant
overshoot.
Input Stage Performance
The ISL70218SEH PNP input stage has a common mode input
range extending up to 0.5V below ground at +25°C. Full amplifier
performance is guaranteed for input voltage down to ground (V-)
over the -55°C to +125°C temperature range. For common mode
voltages down to -0.5V below ground (V-), the amplifiers are fully
functional, but performance degrades slightly over the full
temperature range. This feature provides excellent CMRR, AC
performance, and DC accuracy when amplifying low-level,
ground-referenced signals.
Output Phase Reversal
Output phase reversal is a change of polarity in the amplifier
transfer function when the input voltage exceeds the supply
voltage. The ISL70218SEH is immune to output phase reversal
out to 0.5V beyond the rail (VABS MAX) limit (Figure 38).
The input stage has a maximum input differential voltage equal
to a diode drop greater than the supply voltage and does not
contain the back-to-back input protection diodes found on many
similar amplifiers. This feature enables the device to function as
a precision comparator by maintaining very high input
impedance for high-voltage differential input comparator
voltages. The high differential input impedance also enables the
device to operate reliably in large signal pulse applications,
without the need for anti-parallel clamp diodes required on
MOSFET and most bipolar input stage op amps. Thus, input
signal distortion caused by nonlinear clamps under high slew
rate conditions is avoided.
Single Channel Usage
The ISL70218SEH is a dual op amp. If the application requires
only one channel, the user must configure the unused channel to
prevent it from oscillating. The unused channel oscillates if the
input and output pins are floating. This results in
higher-than-expected supply currents and possible noise
injection into the channel being used. The proper way to prevent
oscillation is to short the output to the inverting input, and
ground the positive input (Figure 50).
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In applications in which one or both amplifier input terminals is
at risk of exposure to voltages beyond the supply rails,
current-limiting resistors may be needed at each input terminal
(see Figure 49, RIN+, RIN-) to limit current through the
power-supply ESD diodes to 20mA.
+
FIGURE 50. PREVENTING OSCILLATIONS IN UNUSED CHANNELS
FN7957.1
August 24, 2012
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INTERSIL [ Intersil ]
