AD795 Analog Devices, AD795 Datasheet - Page 17

• Manufacturer Part Number: AD795
• Description: Low Power, Low Noise Precision FET Op Amp
• Manufacturer: Analog Devices

Specifications of AD795

• -3db Bandwidth: 1.6MHz
• Slew Rate: 1V/µs
• Vos: 100µV
• Ib: 1pA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 9nV/rtHz
• Vcc-vee: 8V to 36V
• Isy Per Amplifier: 1.5mA
• Packages: SOIC

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.
A H PROBE BUFFER AMPLIFIER
A typical pH probe requires a buffer amplifier, shown in Figure 50,
to isolate its 10
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
Figure 49. Photodiode Preamp Employing a T Network for Added Gain
PHOTODIODE
6
Ω to 10
9
Ω source resistance from external
100MΩ
AD795
10pF
R
F
V
OUT
= I
R
1.1kΩ
D
10kΩ
R
I
R
F
G
(1 +
R
R
G
I
V
)
OUT
Rev. C | Page 17 of 20
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.
PROBE
PH
GUARD
Figure 50. pH Probe Amplifier
3
2
V
AD795
OS
1
100kΩ
8
ADJUST
4
–V
+V
7
S
S
5
6
+V
–V
S
S
19.6kΩ
RT
1kΩ
3500ppm/°C
OUTPUT
1V/pH UNIT
0.1µF
0.1µF
AD795
+15V
COM
–15V