AD795
Rev. C | Page 16 of 20
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:
V
OUT
= I
D
(Rf) = Rp (P) Rf
where:
I
D
is the photodiode signal current, in amps (A).
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
B
, contri-
butes an output voltage error, which is proportional to the value
of the feedback resistor. The offset voltage error, V
OS
, causes a
dark current error due to the photodiode’s finite shunt resistance,
Rd. The resulting output voltage error, V
E
, is equal to:
V
E
= (1 + Rf/Rd) V
OS
+ Rf I
B
A shunt resistance on the order of 10
9
Ω is typical for a small
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.
R
D
I
D
I
B
C
D
50pF
C
F
10pF
V
OS
R
F
1GΩ
PHOTODIODE
OUTPUT
00845-046
Figure 46. A Photodiode Model Showing DC Error Sources
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:
2
2
2
2
2
2
1
1
1
1
RfCfs
RdCds
Rd
Rf
en
RfCfs
Rf
isifinV
OUT
R
D
I
S
C
D
50pF
C
F
10pF
R
F
1GΩ
PHOTODIODE
OUTPUT
00845-047
en
I
N
I
F
I
S
Figure 47. Noise Contributions of Various Sources
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.
10µ
1µV
100nV
10nV
1 10 100 1k 10k 100k
FREQUENCY (Hz)
OUTPUT VOLTAGE NOISE (V/ Hz)
00845-048
SIGNAL BANDWIDTH
WITH FILTER
NO FILTER
en
I
N
I
Q
AND I
F
Figure 48. Voltage Noise Spectral Density of the Circuit of Figure 47 With and
Without an Output Filter
