ISL28168_11 Datasheet, PDF (13/17 Pages) Intersil Corporation – Low Input Bias Current Op Amps

ISL28168, ISL28268
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 (see Figure 41).
-
R_IN
V_IN
+
R_L
V_OUT
FIGURE 41. INPUT CURRENT LIMITING
Enable/Disable Feature
The ISL28168 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
ISL28168 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.
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.
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 ISL28268 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 42).
-
+
FIGURE 42. PREVENTING OSCILLATIONS IN UNUSED
CHANNELS
Proper Layout Maximizes Performance
To achieve the maximum performance of the high input
impedance and low offset voltage, care should be taken in
the circuit board layout. The PC board surface must remain
clean and free of moisture to avoid leakage currents
between adjacent traces. Surface coating of the circuit board
will reduce surface moisture and provide a humidity barrier,
reducing parasitic resistance on the board. When input
leakage current is a concern, the use of guard rings around
the amplifier inputs will further reduce leakage currents.
Figure 43 shows a guard ring example for a unity gain
amplifier that uses the low impedance amplifier output at the
same voltage as the high impedance input to eliminate
surface leakage. The guard ring does not need to be a
specific width, but it should form a continuous loop around
both inputs. For further reduction of leakage currents,
13
FN6378.4
July 25, 2011