AD549

REV. A

–12–

The pH probe output is ideally zero volts at a pH of 7 indepen-

dent of temperature. The slope of the probe’s transfer function,

though predictable, is temperature dependent (–54.2 mV/pH at

0 and –74.04 mV/pH at 100

°C). By using an AD590 tempera-

ture sensor and an AD535 analog divider, an accurate tempera-

ture compensation network can be added to the basic pH probe

amplifier. The table in Figure 47 shows voltages at various points

and illustrates the compensation. The AD549 is set for a nonin-

verting gain of 13.51. The output of the AD590 circuitry (point

C) will be equal to 10 V at 100

°C and decrease by 26.8 mV/°C.

The output of the AD535 analog divider (point D) will be a

temperature compensated output voltage centered at zero volts

for a pH of 7, and having a transfer function of –1.00 V/pH

unit. The output range spans from –7.00 V (pH = 14) to +7.00 V

(pH = 0).

A

B

C

D

TEMP

(PROBE OUTPUT)

(A

13.51)

(590 OUTPUT)

(10 B/C)

0

54.20 mV

0.732 V

7.32 V

1.00 V

25 C

59.16 mV

0.799 V

7.99 V

1.00 V

37 C

61.54 mV

0.831 V

8.31 V

1.00 V

60 C

66.10 mV

0.893 V

8.93 V

1.00 V

100 C

74.04 mV

1.000 V

10.00 V

1.00 V

Figure 47. Table Illustrating Temperature Compensation

TEMPERATURE COMPENSATED pH PROBE

AMPLIFIER

A pH probe can be modeled as a mV-level voltage source with a

series source resistance dependent upon the electrode’s compo-

sition and configuration. The glass bulb resistance of a typical

pH electrode pair falls between 10

Ω. It is therefore

important to select an amplifier with low enough input currents

such that the voltage drop produced by the amplifier’s input

bias current and the electrode resistance does not become an

appreciable percentage of a pH unit.

The circuit in Figure 46 illustrates the use of the AD549 as a

pH probe amplifier. As with other electrometer applications, the

use of guarding, shielding, Teflon standoffs, etc., is a must in

order to capitalize on the AD549’s low input current. If an

AD549L (60 fA max input current) is used, the error contrib-

uted by input current will be held below 60

µV for pH electrode

Ω. Input offset voltage (which can

be trimmed) will be below 0.5 mV.

Figure 46. Temperature Compensated pH Probe Amplifier

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

TO-99 (H) Package

0.045 (1.1)

0.020 (0.51)

0.034 (0.86)

0.028 (0.41)

1

3

5

0.2

(5.1)

TYP

2

4

6

7

8

45° EQUALLY

SPACED

BOTTOM VIEW

SEATING

PLANE

0.019 (0.48)

0.016 (0.41)

0.335 (8.50)

0.305 (7.75)

0.370 (9.40)

0.335 (8.50)

0.500

(12.70)

MIN

0.040

(1.0)

MAX

0.185 (4.70)

0.165 (4.19)

8 LEADS

DIA

INSULATION

0.05 (1.27) MAX

REFFERENCE

PLANE