ISL28288 Datasheet, PDF (10/12 Pages) Intersil Corporation – Dual Micropower Single Supply Rail-to-Rail Input and Output (RRIO) Precision Op-Amp

ISL28288
Applications Information
Introduction
The ISL28288 is a dual CMOS rail-to-rail input, output
(RRIO) micropower precision operational amplifier with an
enable feature. The part is designed to operate from single
supply (2.4V to 5.0V) or dual supply (±1.2V to ±2.5V) while
drawing only 120µA of supply current. The device has an
input common mode range that extends 10% above the
positive rail and up to 100mV below the negative supply rail.
The output operation can swing within about 4mV of the
supply rails with a 100kΩ load (reference Figures 27 through
30). This combination of low power and precision
performance makes this device suitable for solar and battery
power applications.
Rail-to-Rail Input
The input common-mode voltage range of the ISL28288
goes from negative supply to 10% greater than the positive
supply without introducing additional offset errors or
degrading performance associated with a conventional rail-
to-rail input operational amplifier. 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 ISL28288 achieves 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
100mV below the negative rail and 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
both positive and negative supply rails, limiting the input
voltage to within one diode beyond the supply rails. The
ISL28288 has additional back-to-back diodes across the
input terminals. For applications where the input differential
voltage is expected to exceed 0.5V, external series resistors
must be used to ensure the input currents never exceed
5mA.
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. The ISL28288 with a 100kΩ load will swing to
within 4mV of the positive supply rail and within 3mV of the
negative supply rail.
Enable/Disable Feature
The ISL28288 offers an EN pin that disables the device
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 ISL28288 parts can be
connected together as a MUX. In this configuration, the
outputs are tied together in parallel and a channel can be
selected by the EN pin. The EN pin also has an internal pull
down. If left open, the EN pin will pull to the negative rail and
the device will be enabled by default.
The loading effects of the feedback resistors of the disabled
amplifier must be considered when multiple amplifier outputs
are connected together.
Using Only One Channel
The ISL28288 is a dual opamp. If the application only
requires one channel, the user must configure the unused
channel to prevent it from oscillating. The unused channel
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).
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ISL28288
+
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 ISL28288, 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
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FN6339.0
September 20, 2006