ISL28276 Datasheet, PDF (11/12 Pages) Intersil Corporation – Dual Precision Micropower Single Supply Rail-to-Rail Input and Output Precision Op-Amps

ISL28276
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. The
ISL28276 has additional back-to-back diodes across the
input terminals. If overdriving the inputs is necessary, the
external input current must never exceed 5mA. External
series resistors may be used as an external protection to
limit excessive external voltage and current from damaging
the inputs.
Input Bias Current Compensation
The input bias currents of the ISL28276 are decimated down
to a typical of 500pA while maintaining an excellent
bandwidth for a micro-power operational amplifier. Inside the
ISL28276 is an input bias canceling circuit. The input stage
transistors are still biased with an adequate current for
speed but the canceling circuit sinks most of the base
current, leaving a small fraction as input bias current.
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. The ISL28276 with a 100kΩ load will swing to
within 3mV of the supply rails.
Enable/Disable Feature
The ISL28276 offers an EN pin that disables the device
when pulled up to at least 2.2V. In the disabled state (output
in a high impedance state), the part consumes typically 4µA.
By disabling the part, multiple ISL28276 parts can be
connected together as a MUX. The outputs are tied together
in parallel and a channel can be selected by the EN pin. 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.
Proper Layout Maximizes Performance
To achieve the maximum performance of the high input
impedance and low offset voltage of the ISL28276, 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 39 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
IN
V+
1/2 ISL28276
FIGURE 39. 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
ISL28276 (Figure 40) is used to convert the differential
thermocouple voltage into single-ended signal with 10X gain.
The ISL28276's rail-to-rail input characteristic allows the
thermocouple to be biased at ground and the converter to
run from a single 5V supply.
R₄
R₃ 10kΩ
R₂
K TYPE
THERMOCOUPLE
10kΩ
100kΩ
+ ^V+
ISL28276
-
V-
+
5V
410µV/°C
R₁
100kΩ
FIGURE 40. THERMOCOUPLE AMPLIFIER
11
FN6301.0
June 23, 2006