EL8176

FN7436 Rev 9.00

Page 13 of 16

January 6, 2015

Input Bias Current Compensation

The input bias currents as low as 500pA are achieved while

maintaining an excellent bandwidth for a micro-power

operational amplifier. Inside the EL8176 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. The input bias current compensation/cancellation is

stable from -40°C to +125°C and operates from typically 10mV

to the positive supply rail.

Rail-to-Rail Output

A pair of complementary MOSFET devices achieves 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 EL8176 with a 100kΩ load will swing to within

3mV of the supply rails.

Enable/Disable Feature

The EL8176 offers an EN pin. The active low EN pin disables the

device when pulled up to at least 2.0V. When disabled, the output

is in a high impedance state and the part consumes typically

3µA. When disabled, the high impedance output allows multiple

parts to be MUXed together. When configured as a MUX, the

outputs are tied together in parallel and a channel can be

selected by pulling the EN pin to 0.8V or lower. The EN pin has an

internal pull-down. If left open or floating, the EN pin will

internally be pulled low, enabling the part by default.

Proper Layout Maximizes Performance

To achieve the maximum performance of the high input

impedance and low offset voltage of the EL8176, 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. The use of guard rings around

the amplifier inputs will further reduce leakage currents.

Figure 46 shows how the guard ring should be configured and

Figure 47 shows the top view of how a surface mount layout can

be arranged. The guard ring does not need to be a specific width,

but it should form a continuous loop around both inputs. By

setting the guard ring voltage equal to the voltage at the

non-inverting input, parasitic capacitance is minimized as well.

For further reduction of leakage currents, components can be

mounted to the PC board using Teflon standoff insulators.

Typical Applications

Thermocouples are the most popular temperature-sensing

device because of their low cost, interchangeability and ability to

measure a wide range of temperatures. The EL8176 is used to

convert the differential thermocouple voltage into single-ended

signal with 10x gain. The EL8176's rail-to-rail input characteristic

allows the thermocouple to be biased at ground and the

converter to run from a single 5V supply.

4

3

1

EL8176

IN

V+

FIGURE 46.

HIGH IMPEDANCE INPUT

6

5

2

FIGURE 47.

-

+

5V

+

V+

V-

EL8176

k TYPE

THERMOCOUPLE

10kΩ

10kΩ

100kΩ

100kΩ

410µV/°C

FIGURE 48. THERMOCOUPLE AMPLIFIER