EL8178

FN7504 Rev 7.00

Page 11 of 13

May 14, 2008

Proper Layout Maximizes Precision

To achieve the optimum levels of high input impedance

(i.e., low input currents) 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 paramount concern, the use of guard rings

around the amplifier inputs will further reduce leakage currents.

Figure 28 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, mount components to the

PC board using Teflon standoffs..

Typical Applications

A general-purpose combination pH probe has extremely high

output impedance typically in the range of 10G

 to 12G. Low

loss and expensive Teflon cables are often used to connect the

pH probe to the meter electronics. Figure 29 details a low-cost

alternative solution using the EL8178 and a low-cost coax

cable. The EL8178 PMOS high impedance input senses the

pH probe output signal and buffers it to drive the coax cable. Its

rail-to-rail input nature also eliminates the need for a bias

resistor network required by other amplifiers in the same

application.

Thermocouples are the most popular temperature sensing

devices because of their low cost, interchangeability, and

ability to measure a wide range of temperatures. In Figure 30,

the EL8178 converts the differential thermocouple voltage into

single-ended signal with 10x gain. The EL8178's rail-to-rail

input characteristic allows the thermocouple to be biased at

ground and permits the op amp to operate from a single 5V

supply.

IN

V+

FIGURE 28. GUARD RING EXAMPLE FOR UNITY GAIN

AMPLIFIER

HIGH IMPEDANCE INPUT

-

+

3V

+

V+

V-

EL8178

COAX

GENERAL

PURPOSE

COMBINATION

pH PROBE

FIGURE 29. pH PROBE AMPLIFIER

-

+

5V

+

V+

V-

EL8178

K TYPE

THERMOCOUPLE

10k



10k



100k



100k



410

V/°C

FIGURE 30. THERMOCOUPLE AMPLIFIER