ISL28236_14 Datasheet, PDF (11/15 Pages) Intersil Corporation – 5MHz, Dual Precision Rail-to-Rail Input-Output (RRIO) Op Amps

ISL28236
Applications Information
Introduction
The ISL28236 is a dual channel Bi-CMOS rail-to-rail input, output
(RRIO) micropower precision operational amplifier. The part is
designed to operate from a single supply (2.4V to 5.5V) or a dual
supply (±1.2V to ±2.75V). The ISL28236 has an input common
mode range that extends 0.25V above the positive rail and down
to the negative supply rail. The output operation can swing within
about 3mV of the supply rails with a 100kΩ load.
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. Thus causing drastic
changes in input offset voltage and an undesired change in
magnitude and polarity of input offset current.
The ISL28236 solves this problem using an internal charge pump
to provide a voltage boost to the V+ supply rail driving the input
differential pair. This results in extending the input common
voltage rails to 0.25V beyond the V+ positive rail. The input offset
voltage exhibits a smooth behavior throughout the extended
common-mode input range. The input bias current versus the
common-mode voltage range gives an undistorted behavior from
the negative rail to 0.25V higher than the positive rail.
Power Supply Decoupling
The internal charge pump operates at approximately 27MHz and
oscillator ripple doesn’t show up in the 5MHz bandwidth of the
amplifier. Good power supply decoupling with 0.01µF capacitors at
each device power supply pin, is the most effective way to reduce
oscillator ripple at the amplifier output. Figure 36 shows the
electrical connection of these capacitors using split power
supplies. For single supply operation with V- tied to a ground plane,
only a single 0.01µF capacitor from V+ is needed. When multiple
ISL28236 op amps are used on a single PC board, each op amp
will require a 0.01µF decoupling capacitor at each supply pin.
Rail-to-Rail Output
The rail-to-rail output stage uses CMOS devices that typically
swing to within 3mV of the supply rails with a 100kΩ load. The
NMOS sinks current to swing the output in the negative direction.
The PMOS sources current to swing the output in the positive
direction.
Current Limiting
These devices have no internal current limiting circuitry. If the
output is shorted, it is possible to exceed the absolute maximum
rating for output current or power dissipation, potentially
resulting in the destruction of the device.
Results Of Overdriving The Output
Caution should be used when overdriving the output for long periods
of time. Overdriving the output can occur in two ways.
1. The input voltage times the gain of the amplifier exceeds the
supply voltage by a large value or,
2. The output current required is higher than the output stage can
deliver.
These conditions can result in a shift in the Input Offset Voltage
(V_OS) (as much as 1µV/hr. of exposure) under these conditions.
IN+ and IN- 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 also contain
back-to-back diodes across the input terminals (see “Pin
Descriptions” on page 2 - Circuit 1). For applications where the
input differential voltage is expected to exceed 0.5V, an external
series resistor must be used to ensure the input currents never
exceed 5mA (Figure 36).
V+
0.01µF
DECOUPLING
CAPACITORS
-
R_IN
V_IN
+
R_L
V_OUT
0.01µF
V-
FIGURE 36. LOCAL POWER SUPPLY DECOUPLING AND INPUT
CURRENT LIMITING
Limitations of the Differential Input
Protection
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 non-inverting 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 (R_F) and gain setting (R_G) 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. Used this way, the
IN+ and IN- voltages don’t track, so differentials arise.
2. When the amplifier is disabled but an input signal is still
present. An R_L or R_G 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 V_OUT
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 V_OUT 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 1.9V/µs, or use appropriate
current limiting resistors.
Large (>2V) differential input voltages can also cause an
increase in disabled I_CC.
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FN6921.2
July 24, 2014