AD795JN View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD795JN Datasheet PDF : 16 Pages
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AD795
10mV
is & if
SIGNAL BANDWIDTH
1mV
in
WITH FILTER
NO FILTER
100nV
en
en
10nV
1
10
100
1k
10k
100k
FREQUENCY – Hz
Figure 45. Voltage Noise Spectral Density of the Circuit of
Figure 44 With and Without an Output Filter
10pF
RG
10kW
Rf
108 W
Ri
1.1kW
PHOTODIODE
AD795
VOUT
VOUT
=
I D Rf
(1+
RG
Ri
)
Figure 46. A Photodiode Preamp Employing a “T”
Network for Added Gain
Using a “T” Network
A “T” network, shown in Figure 46, 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 pH Probe Buffer Amplifier
A typical pH probe requires a buffer amplifier to isolate its 106
to 109 W source resistance from external circuitry. Just such an
amplifier is shown in Figure 47. 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 47 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 kW, 1%, +3500 ppm/∞C, available
from Tel Labs Inc.
PH
PROBE
GUARD
V OS ADJUST
100kW
–VS
1
4
3
5
AD795
2
7
8
+VS
+VS
+15V
0.1mF
COM
0.1mF
–VS
–15V
OUTPUT
6
1VOLT/pH UNIT
19.6kW
RT
1kW
+3500ppm/ ∞C
Figure 47. A pH Probe Amplifier
–14–
REV. B
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