AD795 Analog Devices, AD795 Datasheet - Page 16

• Manufacturer Part Number: AD795
• Description: Low Power, Low Noise Precision FET Op Amp
• Manufacturer: Analog Devices

Specifications of AD795

• -3db Bandwidth: 1.6MHz
• Slew Rate: 1V/µs
• Vos: 100µV
• Ib: 1pA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 9nV/rtHz
• Vcc-vee: 8V to 36V
• Isy Per Amplifier: 1.5mA
• Packages: SOIC

AD795
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 45,
the output of the amplifier is equal to:
where:
I
Rp is the photodiode sensitivity, in amps/watt (A/W).
Rf is the value of the feedback resistor, in ohms (Ω).
P is the light power incident to photodiode surface, in watts (W).
An equivalent model for a photodiode and its dc error sources
is shown in Figure 46. The amplifier’s input current, I
butes an output voltage error, which is proportional to the value
of the feedback resistor. The offset voltage error, V
dark current error due to the photodiode’s finite shunt resistance,
Rd. The resulting output voltage error, V
A shunt resistance on the order of 10
photodiode. Resistance Rd is a junction resistance, which
typically drops by a factor of two for every 10°C rise in
temperature. In the AD795, both the offset voltage and drift are
low, which helps minimize these errors.
MINIMIZING NOISE CONTRIBUTIONS
The noise level limits the resolution obtainable from any
preamplifier. 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 47.
The total noise contribution is defined as:
D
is the photodiode signal current, in amps (A).
V
V
V
OUT
OUT
E
= (1 + Rf/Rd) V
Figure 46. A Photodiode Model Showing DC Error Sources

= I
R
D
D
PHOTODIODE

in
(Rf) = Rp (P) Rf
2

if
I
D
2

is
2
C
50pF
OS





D
1
+ Rf I

s
 
Rf
I
Cf
B
B
Rf
V
OS




10pF
C
2
9
F

Ω is typical for a small
 
en
1GΩ
R
E
2
F
, is equal to:




1

Rd
Rf
OUTPUT





1
1
OS


, causes a
s
s
B
 
 
Cf
Cd
, contri-
Rf
Rd





Rev. C | Page 16 of 20




2
Figure 48, a spectral density vs. 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 configuration. This
effect can be substantial when large photodiodes with large shunt
capacitances are used. Capacitor Cf sets the signal bandwidth
and limits the peak in the noise gain. Each source’s rms or root-
sum-square contribution to noise is obtained 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 optimizes 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 47, 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.
Figure 48. Voltage Noise Spectral Density of the Circuit of Figure 47 With and
100nV
10µV
10nV
1µV
1
I
I
en
Figure 47. Noise Contributions of Various Sources
Q
N
AND I
I
S
PHOTODIODE
R
F
D
10
Without an Output Filter
I
S
C
50pF
FREQUENCY (Hz)
100
D
SIGNAL BANDWIDTH
I
en
N
10pF
C
1k
F
1GΩ
R
I
F
F
OUTPUT
WITH FILTER
NO FILTER
10k
100k