ISL28136_11 Datasheet, PDF (11/14 Pages) Intersil Corporation – 5MHz, Single Precision Rail-to-Rail Input-Output (RRIO) Op Amp

ISL28136
Applications Information
Introduction
The ISL28136 is a single 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. The part 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 causing
drastic changes in input offset voltage and an undesired
change in magnitude and polarity of input offset current.
The ISL28136 achieves input rail-to-rail operation without
sacrificing important precision specifications and degrading
distortion performance. The device’s 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 to 0.25V higher than
the positive rail.
Rail-to-Rail Output
The output stage uses drain-connected N and P-channel
MOSFETs to achieve rail-to-rail output swing. The P-channel
device sources current to swing the output in the positive
direction and the N-channel sinks current to swing the output
in the negative direction. The ISL28136 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 (see “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 38).
-
R_IN
V_IN
+
R_L
V_OUT
FIGURE 38. INPUT CURRENT LIMITING
Enable/Disable Feature
The ISL28136 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. By disabling the part, multiple
ISL28136 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 for more details.
To disable the part, the user needs to supply the 1.5µA
required to pull the EN pin to the V₊ rail. If left open, the EN
pin will pull to the negative rail and the device will be enabled
by default. If the EN function is not required (no need to turn
the part off), as a precaution, it is recommended that the
user tie the EN pin to the V_- pin.
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.
11
FN6153.5
April 5, 2011