Chapter 4: Operation
13124-S01
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 the photodiode, 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, so it is recommended
to keep the cable as short as possible.
4.8. Shunt Resistance
Shunt resistance represents the resistance of the zero-biased photodiode
junction. An ideal photodiode will have an infinite shunt resistance, but actual
values may range from the order of ten
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 impact 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, and this
low resistance can generally be ignored. The series resistance arises from the
contacts and the wire bonds of the photodiode and is used to mainly determine
the linearity of the photodiode under zero bias conditions.
4.10. Battery Check
The DET series includes a battery check feature that will allow the user to
monitor the bias voltage on the output BNC. Simply hold down the “V
Bias Out”
button located on the bottom edge of the unit. The bias voltage will be output to
the BNC. If a high impedance load is used (>10 k
Ω
), the output will be equal to
the bias voltage. This feature includes a 1.05 k
Ω
current limiting resistor (RCL) to
prevent excessive loading of the battery if using small terminating resistors. For
example, a 50
Ω
load resistor with a 10 V bias will produce a 200 mA current
without this resistor. This will significantly decrease lifetime of the battery.
