ISL28278 Datasheet, PDF (11/13 Pages) Intersil Corporation – Dual Micropower Single Supply Rail-to-Rail Input and Output (RRIO) Precision Op-Amp

ISL28278, ISL28478
Applications Information
Introduction
The ISL28278 and ISL28478 are Dual and Quad channel
CMOS rail-to-rail input, output (RRIO) micropower precision
operational amplifier with an enable feature. The parts are
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 them suitable for
solar and battery power applications.
Rail-to-Rail Input
The input common-mode voltage range of the ISL28278 and
ISL28478 is from the 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 ISL28278 and ISL28487 achieve input rail-to-rail
operation 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. They
have 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 ISL28278 and ISL28478 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 ISL28278 has 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 ISL28278 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 ISL28278 and ISL28478 are Dual and Quad channel
opamps. If the application only requires one channel when
using the ISL28278 or less than 4 channels when using the
ISL28478, 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 ISL28278
1/4 ISL28478
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, 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,
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FN6145.1
September 28, 2006