Datasheet

AD549
Rev. H | Page 17 of 20
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).
00511-046
8
14
10
Z
2
12
OUT
7
Y
2
6
Y
1
11
Z
1
1
X
1
2
X
2
AD534
0.1µF
+15
V
0.1µF
–15V
OUTPUT
(D)
(B)
(C)
26.6kΩ
+
–
AD590
IN STAINLESS
STEEL PROBE
OR AC2626
+15V
3
2
6
4
8
7
AD549
0.1µF
0.1µF
pH
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)
0 54.20 0.732 7.32 1.00
25 59.16 0.799 7.99 1.00
37 61.54 0.831 8.31 1.00
60 66.10 0.893 8.93 1.00
100 74.04 1.000 10.00 1.00