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Si Biased Detector Chapter 4: Operation
Rev I, September 28, 2017 Page 7
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.