9

FN7467.3

February 22, 2007

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 19

details a low-cost alternative solution using the EL8188 and

a low-cost coax cable. The EL8188 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 20, the EL8188 converts the differential thermocouple

voltage into single-ended signal with 10X gain. The

EL8188'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.

-

+

3V

+

EL8188

COAX

GENERAL

PURPOSE

COMBINATION

pH PROBE

FIGURE 19. pH PROBE AMPLIFIER

-

+

5V

+

EL8188

K TYPE

THERMOCOUPLE

10k

Ω

10k

Ω

100k

Ω

100k

Ω

410µV/°C

FIGURE 20. THERMOCOUPLE AMPLIFIER

EL8188