ISL28286_07 Datasheet, PDF (11/14 Pages) Intersil Corporation – Dual and Quad Micropower Single Supply Dual and Quad Micropower Single Supply Precision Op Amp

ISL28286, ISL28486
Applications Information
Introduction
The ISL28286 and ISL28486 are dual and quad BiCMOS
rail-to-rail input, output (RRIO) micropower precision
operational amplifiers. These devices are designed to
operate from a single supply (2.4V to 5.0V) or dual supplies
(±1.2V to ±2.5V) while drawing only 120μA (ISL28286) of
supply current. This combination of low power and precision
performance makes these devices suitable for solar and
battery power applications.
Rail-to-Rail Input
Many rail-to-rail input stages use two differential input pairs,
a long-tail PNP (or PFET) and an NPN (or NFET). Severe
penalties have to be paid for this circuit topology. As the
input signal moves from one supply rail to another, the
operational amplifier switches from one input pair to the
other causing drastic changes in input offset voltage and an
undesired change in magnitude and polarity of input offset
current.
The ISL28286 and ISL28486 achieve input rail-to-rail without
sacrificing important precision specifications and degrading
distortion performance. The devices’ input offset voltage
exhibits a smooth behavior throughout the entire common-
mode input range. The input bias current versus the
common-mode voltage range gives us an undistorted
behavior from typically down to the negative rail to 10%
higher than the V+ rail (0.5V higher than V+ when V+ equals
5V).
Input Protection
All input terminals have internal ESD protection diodes to
positive and negative supply rails, limiting the input voltage
to within one diode beyond the supply rails. Both parts have
additional back-to-back diodes across the input terminals. If
overdriving the inputs is necessary, the external input current
must never exceed 5mA. External series resistors may be
used as an external protection to limit excessive external
voltage and current from damaging the inputs.
Input Bias Current Compensation
The ISL28286 and ISL28486 contain an input bias
cancellation circuit which reduces the bias currents down to
a typical of 500pA while maintaining an excellent bandwidth
for a micro-power operational amplifier. The input stage
transistors are still biased with adequate current for speed
but the canceling circuit sinks most of the base current,
leaving a small fraction as input bias current.
Rail-to-Rail Output
A pair of complementary MOSFET devices are used to
achieve the rail-to-rail output swing. The NMOS sinks
current to swing the output in the negative direction. The
PMOS sources current to swing the output in the positive
direction. Both parts with a 100kΩ load will swing to within
4mV of the supply rails.
Enable/Disable Feature
The ISL28286 offers two EN pins (EN_A and EN_B) which
disable the op amp when pulled up to at least 2.0V. In the
disabled state (output in a high impedance state), the part
consumes typically 4µA. By disabling the part, multiple parts
can be connected together as a MUX. The outputs are tied
together in parallel and a channel can be selected by the EN
pins. The EN pins also have an internal pull-down. If left
open, the EN pins will pull to the negative rail and the
op amp will be enabled by default.
Using Only One Channel
The ISL28286 and ISL28486 are dual and quad channel
op amps. If the application only requires one channel when
using the ISL28286 or less than 4 channels when using the
ISL28486, the user must configure the unused channel(s) to
prevent them from oscillating. The unused channel(s) will
oscillate if the input and output pins are floating. This will
result in higher than expected supply currents and possible
noise injection into the channel being used. The proper way
to prevent this oscillation is to short the output to the
negative input and ground the positive input (as shown in
Figure 37).
-
+
1/2 ISL28286
1/4 ISL28486
FIGURE 37. PREVENTING OSCILLATIONS IN UNUSED
CHANNELS
Proper Layout Maximizes Performance
To achieve the maximum performance of the high input
impedance and low offset voltage of the ISL28286 and
ISL28486, 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
concern, the use of guard rings around the amplifier inputs
will further reduce leakage currents. Figure 38 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, components can be mounted
to the PC board using Teflon standoff insulators.
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FN6312.1
June 28, 2007