AD549JHZ datasheet(18/19 Pages) AD

18 / 19 page

Data Sheet

AD549

Rev. K | Page 17 of 18

The pH probe output is ideally 0 V at a pH of 7, independent

of temperature. The slope of the transfer function of the probe,

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

0°C and −74.04 mV/pH at 100°C). By using an AD590 tempera-

ture sensor and an AD534 analog divider, an accurate temperature

compensation network can be added to the basic pH probe ampli-

fier. Table 4 shows voltages at various points, thereby illustrating

the compensation. The AD549 is set for a noninverting gain of

13.51. The output of the AD590 circuitry (Point C) is equal to

10 V at 100°C and decreases by 26.8 mV/°C. The output of the

AD534 analog divider (Point D) is a temperature-compensated

output voltage centered at 0 V for a pH of 7 and has 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).

10 Z2

OUT

11 Z1

AD534

0.1µF

+15V

0.1µF

–15V

OUTPUT

(D)

(B)

(C)

26.6kΩ

–

AD590

IN STAINLESS

STEEL PROBE

OR AC2626

+15V

AD549

0.1µF

PROBE

OUTPUT

(A)

1kΩ

12kΩ

1kΩ

SCALE FACTOR

ADJUST

Figure 46. Temperature Compensated pH Amplifier

Table 4. Illustration of Temperature Compensation

Point

Probe Temperature (°C)

A (Probe Output) (mV)

B (A × 13.51) (V)

C (AD590 Output) (V)

D (10 × (B ÷ C)) (V)

54.20

0.732

7.32

1.00

59.16

0.799

7.99

1.00

61.54

0.831

8.31

1.00

66.10

0.893

8.93

1.00

100

74.04

1.000

10.00

1.00

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