AD795KN Analog Devices, AD795KN Datasheet - Page 13

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

REV. A
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 43,
the output of the amplifier is equal to:
where:
An equivalent model for a photodiode and its dc error sources is
shown in Figure 44. The amplifier’s input current, I
contribute an output voltage error which will be proportional to
the value of the feedback resistor. The offset voltage error, V
will cause a “dark” current error due to the photodiode’s finite
shunt resistance, Rd. The resulting output voltage error, V
equal to:
A shunt resistance on the order of 10
small photodiode. Resistance Rd is a junction resistance which
Figure 44. A Photodiode Model Showing DC Error
Sources
Figure 43. The AD795 Used as a Photodiode Preamplifier
PHOTODIODE
V
I
Rp = photodiode sensitivity (Amp/Watt)
Rf = the value of the feedback resistor, in ohms.
P
D
OUT
= light power incident to photodiode surface, in watts.
= photodiode signal current (Amps)
= I
Rd
D
PHOTODIODE
(Rf) = Rp (P) Rf
GUARD
I
V
D
E
= (1 + Rf/Rd) V
50pF
Cd
2
3
AD795
I
B
10
10pF
8
9
OS
9
V
ohms is typical for a
OS
+ Rf I
6
10pF
Cf
B
OPTIONAL 26Hz
10
OUTPUT
Rf
9
FILTER
B
FILTERED
, will
OUTPUT
OUTPUT
E
, is
OS
,
–13–
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 45.
The total noise contribution is defined as:
Figure 46, 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 45—without a
bandpass filter—has a total output noise of 50 V rms. Using a
26 Hz single pole output filter, the total output noise drops to
23 V rms, a factor of 2 improvement with no loss in signal
bandwidth.
V OUT
Figure 45. Noise Contributions of Various Sources
i
S
(i n
Rd
PHOTODIODE
2
i f
2
i s
i
2
S
)
1 s (Cf ) Rf
50pF
Cd
Rf
2
(en
en
i
2
n
)
1
10pF
Cf
Rd
Rf
10
1 s (Cd ) Rd
1 s (Cf ) Rf
AD795
Rf
i
9
f
OUTPUT
2