ISL28158_08 Datasheet, PDF (13/17 Pages) Intersil Corporation – Micro-power Single and Dual Precision Rail-to-Rail Input-Output (RRIO) Low Input Bias Current Op Amps

ISL28158, ISL28258
external series resistor must be used to ensure the input
currents never exceed 5mA (Figure 47).
-
R_IN
V_IN
+
R_L
V_OUT
FIGURE 47. INPUT CURRENT LIMITING
Enable/Disable Feature
The ISL28158 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
ISL28158 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 13 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 0.1V/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
The ISL28258 is a dual op amp. If the application only
requires one channel, 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 48).
-
+
FIGURE 48. PREVENTING OSCILLATIONS IN UNUSED
CHANNELS
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.
13
FN6377.3
August 29, 2008