AD795JN Ver la hoja de datos (PDF) - Analog Devices
Número de pieza
componentes Descripción
Fabricante
AD795JN Datasheet PDF : 16 Pages
| |||
AD795
GUARD
PHOTODIODE
10pF
10 9 W
2
AD795 6
3
8
OUTPUT
FILTERED
OUTPUT
OPTIONAL 26Hz
FILTER
Figure 42. The AD795 Used as a Photodiode Preamplifier
Preamplifier Applications
The low input current and offset voltage levels of the AD795
together with its low voltage noise make this amplifier an
excellent choice for preamplifiers used in sensitive photodiode
applications. In a typical preamp circuit, shown in Figure 42,
the output of the amplifier is equal to:
VOUT = ID (Rf) = Rp (P) Rf
where:
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 43. The amplifier’s input current, IB, will
contribute 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
Cf
10pF
PHOTODIODE
Rf
109 W
VOS
IB
Rd
ID
Cd
50pF
OUTPUT
will typically drop by a factor of two for every 10∞C rise in
temperature. In the AD795, both the offset voltage and drift are
low, this helps minimize these errors.
Minimizing Noise Contributions
The noise level limits the resolution obtainable from any pre-
amplifier. 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 photodiode sensitivity is the minimum detectable
light power.
Sources of noise in a typical preamp are shown in Figure 44.
The total noise contribution is defined as:
VOUT =
(in2
+
if 2
+
is2 ) ÊËÁ 1+
s
Rf
( Cf
)
Rf
ˆ2
¯˜
Ê
+ (en2 )ËÁÁ1+
Rf
Rd
Ê 1+ s (Cd
ËÁ 1+ s (Cf
)
)
Rd
Rf
ˆ¯˜
ˆ
˜
¯˜
2
Cf
10pF
Rf
109 W
PHOTODIODE
iS
Rd
Cd
iS
50pF
en
if
in
OUTPUT
Figure 44. Noise Contributions of Various Sources
Figure 45, 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.
Minimizing the total area under these curves will optimize the
preamplifier’s overall noise performance.
An output filter with a passband close to that of the signal can
greatly improve the preamplifier’s signal to noise ratio. The
photodiode preamplifier shown in Figure 44—without a
bandpass filter—has a total output noise of 50 mV rms. Using a
26 Hz single pole output filter, the total output noise drops to
23 mV rms, a factor of 2 improvement with no loss in signal
bandwidth.
Figure 43. A Photodiode Model Showing DC Error
Sources
REV. B
–13–
Share Link: 