AD645 AD [Analog Devices], AD645 Datasheet - Page 8

• Manufacturer Part Number: AD645
• Description: Low Noise, Low Drift FET Op Amp
• Manufacturer: AD [Analog Devices]

AD645
A pH Probe Buffer Amplifier
A typical pH probe requires a buffer amplifier to isolate its 10
to 10
amplifier is shown in Figure 35. The low input current of the
AD645 allows the voltage error produced by the bias current
and electrode resistance to be minimal. The use of guarding,
shielding, high insulation resistance standoffs, and other such
standard methods used to minimize leakage are all needed to
maintain the accuracy of this circuit.
The slope of the pH probe transfer function, 50 mV per pH unit
at room temperature, has a +3300 ppm/ C temperature coeffi-
cient. The buffer of Figure 35 provides an output voltage equal
to 1 volt/pH unit. Temperature compensation is provided by
resistor RT which is a special temperature compensation resis-
tor, part number Q81, 1 k , 1%, +3500 ppm/ C, available from
Tel Labs Inc.
Circuit Board Notes
The AD645 is designed for through hole mount into PC boards.
Maintaining picoampere level resolution in that environment
requires a lot of care. Since both the printed circuit board and
the amplifier’s package have a finite resistance, the voltage dif-
ference between the amplifier’s input pin and other pins (or
traces on the PC board) will cause parasitic currents to flow into
(or out of) the signal path. These currents can easily exceed the
1.5 pA input current level of the AD645 unless special precau-
tions are taken. Two successful methods for minimizing leakage
are: guarding the AD645’s input lines and maintaining adequate
insulation resistance.
PHOTODIODE
pH
PROBE
Figure 34. A Photodiode Preamp Employing a “T”
Network for Added Gain
9
source resistance from external circuitry. Just such an
Figure 35. A pH Probe Amplifier
GUARD
3
2
V
AD645
OS
AD645
1
8
100k
ADJUST
10pF
10
4
Rf
8
–
+
7
V
V
S
S
5
6
–V
+V
OUTPUT
19.6k
RT
1k
+3500ppm/ C
S
S
V
1VOLT/pH UNIT
OUT
1.1k
R
i
= I R (1
10k
R
D
G
0.1 F
0.1 F
f
+
+15V
V
R
R
COM
–15V
OUT
G
i
6
)
–8–
addition, the input trace should be guarded along both of its
edges, along its entire length.
Contaminants such as solder flux, on the board’s surface and on
the amplifier’s package, can greatly reduce the insulation resis-
tance and also increase the sensitivity to atmospheric humidity.
Both the package and the board must be kept clean and dry. An
effective cleaning procedure is to: first, swab the surface with
high grade isopropyl alcohol, then rinse it with deionized water,
and finally, bake it at 80 C for 1 hour. Note that if either poly-
styrene or polypropylene capacitors are used on the printed cir-
cuit board that a baking temperature of 70 C is safer, since both
Guarding the input lines by completely surrounding them with a
metal conductor biased near the input lines’ potential has two
major benefits. First, parasitic leakage from the signal line is
reduced, since the voltage between the input line and the guard
is very low. Second, stray capacitance at the input terminal is
minimized which in turn increases signal bandwidth. In the
header or can package, the case of the AD645 is connected to
Pin 8 so that it may be tied to the input potential (when operat-
ing as a follower) or tied to ground (when operating as an in-
verter). The AD645’s positive input (Pin 3) is located next to
the negative supply voltage pin (Pin 4). The negative input (Pin
2) is next to the balance adjust pin (Pin 1) which is biased at a
potential close to that of the negative supply voltage. Note that
any guard traces should be placed on both sides of the board. In
of these plastic compounds begin to melt at approximately
+85 C.
0.370 (9.40)
0.335 (8.51)
0.335 (8.51)
0.305 (7.75)
0.160 (4.06)
0.115 (2.93)
0.040 (1.02) MAX
(5.33)
0.210
MAX
PIN 1
0.022 (0.558)
0.014 (0.356)
0.045 (1.14)
0.010 (0.25)
0.185 (4.70)
0.165 (4.19)
(1.27)
0.050
8
MAX
1
Dimensions shown in inches and (mm).
Plastic Mini-DIP (N) Package
0.430 (10.92)
0.348 (8.84)
TO-99 Header (H) Package
OUTLINE DIMENSIONS
0.100
(2.54)
BSC
0.250 (6.35)
0.750 (19.05)
0.500 (12.70)
MIN
REFERENCE PLANE
0.019 (0.48)
0.016 (0.41)
0.070 (1.77)
0.045 (1.15)
BASE & SEATING PLANE
4
5
0.021 (0.53)
0.016 (0.41)
0.060 (1.52)
0.015 (0.38)
0.280 (7.11)
0.240 (6.10)
0.130
(3.30)
MIN
SEATING
PLANE
(5.08)
0.200
BSC
0.100
(2.54)
BSC
3
4
2
0.325 (8.25)
0.300 (7.62)
0.100
(2.54)
0.015 (0.381)
0.008 (0.204)
BSC
5
1
0.034 (0.86)
0.027 (0.69)
6
8
BSC
45
0.195 (4.95)
0.115 (2.93)
7
0.160 (4.06)
0.110 (2.79)
0.045 (1.14)
0.027 (0.69)
REV. B