11

FN6301.0

June 23, 2006

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 100k

Ω 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.

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.

1/2 ISL28276

IN

V+

FIGURE 39. GUARD RING EXAMPLE FOR UNITY GAIN

AMPLIFIER

HIGH IMPEDANCE INPUT

-

+

5V

+

V+

V-

ISL28276

K TYPE

THERMOCOUPLE

10k

Ω

10k

Ω

100k

Ω

100k

Ω

410µV/°C

FIGURE 40. THERMOCOUPLE AMPLIFIER

ISL28276