ISL28138 Datasheet, PDF (12/16 Pages) Intersil Corporation – 4.5MHz, Single and Dual Precision Rail-to-Rail Input-Output (RRIO) Op Amps with Very Low Input Bias Current

ISL28138, ISL28238
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 ISL28138 and ISL28238 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 11 - 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 42).
-
R_IN
V_IN
+
R_L
V_OUT
FIGURE 42. INPUT CURRENT LIMITING
Enable/Disable Feature
The ISL28138 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
ISL28138 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 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 12 for more
details.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. When not used, the EN pin should
either be left floating or connected directly 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.
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 4.8V/µs, or use
appropriate current limiting resistors.
Large (>2V) differential input voltages can also cause an
increase in disabled I_CC.
Using Only One Channel
If the application only requires one channel of the ISL28238,
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 43).
-
+
FIGURE 43. PREVENTING OSCILLATIONS IN UNUSED
CHANNELS
12
FN6336.2
February 19, 2008