ISL28278_07 Datasheet, PDF (11/13 Pages) Intersil Corporation – Dual and Quad Micropower Single Supply Rail-to-Rail Input and Output (RRIO) Op-Amp

ISL28278, ISL28478
Rail-to-Rail Output
A pair of complementary MOSFET 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. Both parts, with a
100kΩ load, will typically swing to within 4mV of the positive
supply rail and within 3mV of the negative supply rail.
Enable/Disable Feature
The ISL28278 offers two EN pins (EN_A and EN_B) which
disable the op amp when pulled up to at least 2.0V. In the
disabled state (output in a high impedance state), the part
consumes typically 4µA. By disabling the part, multiple parts
can be connected together as a MUX. The outputs are tied
together in parallel and a channel can be selected by the EN
pins. The loading effects of the feedback resistors of the
disabled amplifier must be considered when multiple
amplifier outputs are connected together. 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.
Using Only One Channel
The ISL28278 and ISL28478 are dual and quad channel
op amps. If the application only requires one channel when
using the ISL28278 or less than 4 channels when using the
ISL28478, the user must configure the unused channel(s) to
prevent them from oscillating. The unused channel(s) 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 36).
-
+
1/2 ISL28278
1/4 ISL28478
FIGURE 36. PREVENTING OSCILLATIONS IN UNUSED
CHANNELS
Proper Layout Maximizes Performance
To achieve the maximum performance of the high input
impedance and low offset voltage of the ISL28278 and
ISL28478, 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 37 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, components can be mounted
to the PC board using Teflon standoff insulators.
HIGH IMPEDANCE INPUT
V+
IN
FIGURE 37. GUARD RING EXAMPLE FOR UNITY GAIN
AMPLIFIER
Example Application
Thermocouples are the most popular temperature-sensing
device because of their low cost, interchangeability, and
ability to measure a wide range of temperatures. The
ISL28X78 (Figure 38) is used to convert the differential
thermocouple voltage into single-ended signal with 10X gain.
The ISL28X78's rail-to-rail input characteristic allows the
thermocouple to be biased at ground and the amplifier to run
from a single 5V supply.
.
R₄
R₃ 10kΩ
R₂
K TYPE
THERMOCOUPLE
10kΩ
100kΩ
+ ^V+
ISL28X78
-
V-
+
5V
410µV/°C
R₁
100kΩ
FIGURE 38. THERMOCOUPLE AMPLIFIER
Current Limiting
The ISL28278 and ISL28478 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.
Power Dissipation
It is possible to exceed the +150°C maximum junction
temperatures under certain load and power-supply
conditions. It is therefore important to calculate the
maximum junction temperature (T_JMAX) for all applications
to determine if power supply voltages, load conditions, or
package type need to be modified to remain in the safe
operating area. These parameters are related in Equation 1:
T_JMAX = T_MAX + (θ_JAxPD_MAXTOTAL)
(EQ. 1)
11
FN6145.2
July 11, 2007