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AD645 Datasheet PDF : 8 Pages
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AD645
10pF
GUARD
PHOTODIODE
10 9 Ω
2
AD645 6
38
OUTPUT
FILTERED
OUTPUT
OPTIONAL 26Hz
FILTER
Figure 30. The AD645 Used as a Sensitive Preamplifier
Preamplifier Applications
The low input current and offset voltage levels of the AD645 to-
gether with its low voltage noise make this amplifier an excellent
choice for preamplifiers used in sensitive photodiode applica-
tions. In a typical preamp circuit, shown in Figure 30, the out-
put of the amplifier is equal to:
where:
VOUT = ID (Rf) = Rp (P) Rf
ID = photodiode signal current (Amps)
Rp = photodiode sensitivity (Amp/Watt)
Rf = the value of the feedback resistor, in ohms.
P = light power incident to photodiode surface, in watts.
An equivalent model for a photodiode and its dc error sources is
shown in Figure 31. The amplifier’s input current, IB, will con-
tribute an output voltage error which will be proportional to the
value of the feedback resistor. The offset voltage error, VOS, will
cause a “dark” current error due to the photodiode’s finite
shunt resistance, Rd. The resulting output voltage error, VE, is
equal to:
VE = (1 + Rf/Rd) VOS + Rf IB
A shunt resistance on the order of 109 ohms is typical for a
small photodiode. Resistance Rd is a junction resistance which
will typically drop by a factor of two for every 10°C rise in tem-
perature. In the AD645, both the offset voltage and drift are
low, this helps minimize these errors.
Cf
10pF
PHOTODIODE
Rf
10 9 Ω
VOS
IB
Rd
ID
Cd
50pF
OUTPUT
Figure 31. A Photodiode Model Showing DC Error
Sources
Minimizing Noise Contributions
The noise level limits the resolution obtainable from any pream-
plifier. The total output voltage noise divided by the feedback
resistance of the op amp defines the minimum detectable signal
current. The minimum detectable current divided by the photo-
diode sensitivity is the minimum detectable light power.
Sources of noise in a typical preamp are shown in Figure 32.
The total noise contribution is defined as:
V OUT =
in2
+if
2
+
is
2
1+
s
Rf
( Cf
)
Rf
2
+
en2
1 +
Rf
Rd
1+ s
1+ s
( Cd
( Cf
)
)
Rd
Rf
2
Figure 33, a spectral density versus frequency plot of each
source’s noise contribution, shows that the bandwidth of the
amplifier’s input voltage noise contribution is much greater than
its signal bandwidth. In addition, capacitance at the summing
junction results in a “peaking” of noise gain in this configura-
tion. This effect can be substantial when large photodiodes with
large shunt capacitances are used. Capacitor Cf sets the signal
bandwidth and also limits the peak in the noise gain. Each
source’s rms or root-sum-square contribution to noise is ob-
tained by integrating the sum of the squares of all the noise
sources and then by obtaining the square root of this sum. Mini-
mizing the total area under these curves will optimize the
preamplifier’s overall noise performance.
Cf
10pF
Rf
10 9 Ω
PHOTODIODE
en
if
in
iS Rd
iS
Cd
50pF
OUTPUT
Figure 32. Noise Contributions of Various Sources
10µV
is & if
SIGNAL BANDWIDTH
1µV
in
WITH FILTER
NO FILTER
100nV
en
en
10nV
1
10
100
1k
10k
100k
FREQUENCY – Hz
Figure 33. Voltage Noise Spectral Density of the Circuit of
Figure 32 With and Without an Output Filter
An output filter with a passband close to that of the signal can
greatly improve the preamplifier’s signal to noise ratio. The pho-
todiode preamplifier shown in Figure 32—without a bandpass
filter—has a total output noise of 50 µV rms. Using a 26 Hz
single pole output filter, the total output noise drops to 23 µV
rms, a factor of 2 improvement with no loss in signal bandwidth.
Using a “T” Network
A “T” network, shown in Figure 34, can be used to boost the ef-
fective transimpedance of an I to V converter, for a given feed-
back resistor value. Unfortunately, amplifier noise and offset
voltage contributions are also amplified by the “T” network
gain. A low noise, low offset voltage amplifier, such as the
AD645, is needed for this type of application.
REV. B
–7–
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