THORLABS PDA015AM, Руководство пользователя

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Si Amplified Photodetectors Chapter 5: Operation
Rev B. May 22, 2017 Page 9
5.3. Transimpedance and Conversion Gain
The Transimpedance Gain (TG) of the PDA015A is the total gain of both amplifier
stages (in units of V/A).
The Conversion Gain (CG) (in units of V/W) is simply the product of the photodiode
Responsivity (

( λ
p
)) and the Transimpedance Gain.
As a result, the output voltage for a given optical power and wavelength is given
by the input optical power times the Conversion Gain.
However, the user should be aware that changing the final load resistance on the
PDA015A changes the conversion gain of the system. This is due to the internal
50
Ω series resistor shown in the block diagram.
The final load resistance creates a voltage divider with the 50 Ω series resistor
(R
s
). This changes the CG according to the following Scale Factor.
The actual CG is then given by the CG times the Scale Factor. Note that for a high
impedance external load, the CG and actual CG are the same. For a 50
Ω external
load, the actual CG is a factor of two smaller than the CG.
It is for this reason that the maximum output of the PDA015A is 10 volts for high
impedance loads and 5 volts for 50
Ω loads. The user should also be aware that
the linear, low distortion output range of the PDA015A is limited to about 6 volts
(3 volts with a 50
Ω load) at maximum bandwidth to avoid saturation. This is limited
by the maximum slew rate of the amplifier. Larger, linear swing can be achieved
when not running at maximum bandwidth.
For best signal integrity, we recommend using a 50 Ω coaxial cable with a 50 Ω
terminating resistor at the output end of the cable. This will minimize ringing by
matching the cable with its characteristic impedance. If more output voltage is
required, the load resistance can be increased. However, when the load resistance
does not match the cable impedance, the length of the coaxial cable can have a
negative impact on the signal integrity, unless the cable is kept very short.
The PDA015A is DC coupled, which means there is no low frequency cutoff and
the output voltage will have a DC component that is proportional to the average
optical power. Some instruments such as spectrum analyzers may require a DC
block to prevent damage to the instrument. If a DC block is used, ensure that it
has a flat frequency response up to 1 GHz to avoid adding distortion.
Some instruments may not support the high output voltage capability of the
PDA015A. In these cases an RF attenuator can be added between the PDA015A
and the instrument to prevent damage to the instrument. If an attenuator is added
ensure that it supports at least 1 GHz bandwidth to avoid adding distortion. Also,
note that RF attenuators are typically only meant to be used in 50
Ω systems and
the attenuator will not perform correctly with an instrument with high impedance
inputs.