Datasheet

AD795
Rev. C | Page 17 of 20
USING A T NETWORK
A T network, shown in Figure 49, can be used to boost the
effective transimpedance of an I-to-V converter, for a given
feedback resistor value. However, amplifier noise and offset
voltage contributions are also amplified by the T network gain.
A low noise, low offset voltage amplifier, such as the AD795,
is needed for this type of application.
00845-049
10pF
V
OUT
R
F
100MΩ
R
I
1.1kΩ
R
G
10kΩ
PHOTODIODE
AD795
V
OUT
= I
D
R
F
(1 + )
R
G
R
I
Figure 49. Photodiode Preamp Employing a T Network for Added Gain
A QH PROBE BUFFER AMPLIFIER
A typical pH probe requires a buffer amplifier, shown in Figure 50,
to isolate its 10
6
Ω to 10
9
Ω source resistance from external
circuitry. The low input current of the AD795 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
coefficient. The buffer of Figure 50 provides an output voltage
equal to 1 V/pH unit. Temperature compensation is provided
by resistor RT, which is a special temperature compensation
resistor, Part Number Q81, 1 kΩ, 1%, 3500 ppm/°C, available
from Tel Labs, Inc.
00845-050
2
8
7
1
4
5
6
3
AD795
OUTPUT
1V/pH UNIT
19.6kΩ
RT
1kΩ
3500ppm/°C
V
OS
ADJUST
100kΩ
GUARD
–V
S
+V
S
0.1µF
0.1µF
+V
S
–V
S
+15V
COM
–15V
PH
PROBE
Figure 50. pH Probe Amplifier