ISL28146 Datasheet, PDF (11/15 Pages) Intersil Corporation – 5MHz, Single and Dual Rail-to-Rail Input-Output (RRIO) Op Amps

ISL28146, ISL28246
Applications Information
Introduction
The ISL28146 and ISL28246 are single and dual channel
rail-to-rail input, output (RRIO) micropower precision
operational amplifiers. The parts are designed to operate
from single supply (2.4V to 5.0V) or dual supply (±1.2V to
±2.75V). The parts have 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 causing drastic changes in input offset voltage and an
undesired change in magnitude and polarity of input offset
current.
The ISL28146 and ISL28246 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
an undistorted behavior from typically down to the negative
rail and up to 0.25V higher than the V+ rail.
Rail-to-Rail Output
A pair of complementary MOS 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 ISL28146 and ISL28246 with a 100kΩ load will swing to
within 3mV of the positive supply rail and within 3mV of the
negative supply rail.
Results of Over-Driving the Output
Caution should be used when over-driving the output for long
periods of time. Over-driving 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 (“Pin
Descriptions” on page 10 - 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).
-
R_IN
V_IN
+
R_L
V_OUT
FIGURE 36. INPUT CURRENT LIMITING
Enable/Disable Feature
The ISL28146 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 10µA
at room temperature. The EN pin 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. When not used, the EN pin
should either be left floating or connected directly to the -V
pin.
By disabling the part, multiple ISL28146 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 loading effects of the feedback
resistors of the disabled amplifier must be considered when
multiple amplifier outputs are connected together. Note that
feed through from the IN+ to IN- pins occurs on any Mux
Amp disabled channel where the input differential voltage
exceeds 0.5V (e.g., active channel V_OUT = 1V, while
disabled channel V_IN = GND), so the mux implementation is
best suited for small signal applications. If large signals are
required, use series IN+ resistors, or a large value R_F, to
keep the feed through current low enough to minimize the
impact on the active channel. See “Limitations of the
Differential Input Protection” on page 11.
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
11
FN6321.2
February 11, 2008