AD795KN Analog Devices, AD795KN Datasheet - Page 14

• Manufacturer Part Number: AD795KN
• Description: Low Power/ Low Noise Precision FET Op Amp
• Manufacturer: Analog Devices

AD795
Figure 46. Voltage Noise Spectral Density of the Circuit of
Figure 45 With and Without an Output Filter
Using a “T” Network
A “T” network, shown in Figure 47, 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
Figure 47. A Photodiode Preamp Employing a “T”
Network for Added Gain
PHOTODIODE
100nV
10 V
10nV
e n
1 V
1
i n
i
en
s
&
i
f
10
FREQUENCY – Hz
100
AD795
10
10pF
R f
SIGNAL BANDWIDTH
8
WITH FILTER
NO FILTER
1k
V
OUT
= I Rf (1+
10k
R
1.1k
D
i
10
R
k
G
R
R
G
i
100k
)
V
OUT
–14–
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 pH Probe Buffer Amplifier
A typical pH probe requires a buffer amplifier to isolate its 10
to 10
amplifier is shown in Figure 48. 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 48 provides an output voltage
equal to 1 volt/pH unit. Temperature compensation is provided
by resistor RT which is a special temperature compensation
resistor, part number Q81, 1 k , 1%, +3500 ppm/ C, available
from Tel Labs Inc.
PH
PROBE
9
source resistance from external circuitry. Just such an
Figure 48. A pH Probe Amplifier
GUARD
3
2
AD795
V
1
OS
8
100k
ADJUST
4
–V
+V
7
S
S
5
6
+V
–V
S
S
OUTPUT
19.6k
RT
1k
+3500ppm/ C
1VOLT/pH UNIT
0.1 F
0.1 F
REV. A
+15V
COM
–15V
6