DET08CFC(/M)
Chapter 4: Operation
TTN020103-D02 Page
8
4.7. Terminating
Resistance
A load resistance is used to convert the generated photocurrent into a voltage
(V
OUT
) for viewing on an oscilloscope:
=
×
Depending on the type of photodiode, the load resistance can affect the
response speed. For maximum bandwidth, we recommend using a 50
Ω
coaxial
cable with a 50
Ω
terminating resistor at the opposite end of the cable. This will
minimize ringing by matching the cable with its characteristic impedance. If
bandwidth is not important, you may increase the amount of voltage for a given
light level by increasing R
LOAD
. In an unmatched termination, the length of the
coaxial cable can have a profound impact on the response; thus, we recommend
the cable length to be as short as possible.
4.8. Shunt
Resistance
Shunt resistance represents the resistance of the zero-biased photodiode
junction. An ideal photodiode has an infinite shunt resistance, but actual values
may range from the order of 10
Ω
to thousands of M
Ω
, and is dependent on the
photodiode material. For example, and InGaAs detector has a shunt resistance
on the order of 10 M
Ω
while a Ge detector is in the k
Ω
range. This can
significantly affect the noise current on the photodiode. For most applications;
however, the high resistance produces little effect and can be ignored.
4.9. Series
Resistance
Series resistance models the resistance of the semiconductor material. This
resistance is typically very low and can be ignored. The series resistance arises
from the contacts and the wire bonds of the photodiode. It mainly determines the
linearity of the photodiode under zero bias conditions.
4.10. Damage Threshold
Exposure to an intense light source can easily damage a photodiode. One of the
main characteristics of a damaged photodiode is the presence of increased dark
current, along with burn spots on the detector active area. The damage threshold
may vary from photodiode to photodiode, as this is generally dependent on
material. Silicon devices tend to be more durable than InGaAs and can handle
higher energy levels.
The formula below calculates the energy of each pulse, using the average power
and the repetition rate. If the pulse width is given, the peak power can also be
determined.
=
∗
=
ℎ
