10

FN7504.5

March 29, 2007

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

Ω to

12G

Ω. Low loss and expensive Teflon cables are often used

to connect the pH probe to the meter electronics. Figure 28

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 29, 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 27. GUARD RING EXAMPLE FOR UNITY GAIN

AMPLIFIER

HIGH IMPEDANCE INPUT

-

+

3V

+

EL8178

COAX

GENERAL

PURPOSE

COMBINATION

pH PROBE

FIGURE 28. pH PROBE AMPLIFIER

-

+

5V

+

EL8178

K TYPE

THERMOCOUPLE

10k

Ω

10k

Ω

100k

Ω

100k

Ω

410

μV/°C

FIGURE 29. THERMOCOUPLE AMPLIFIER

EL8178