Chapter 4: Operation
23155-D02, Rev A
Page 6
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.
The maximum output of the PDA30B is 2 volts for high impedance loads (i.e.
R
Load
> 5 k
Ω
) and 1 volts for 50
Ω
loads. Adjust the gain so that the measured
signal level out of the PDA30B is below 2 volts (1 volts with a 50
Ω
load) to avoid
saturation.
For low terminating resistors, <5 k
Ω
or 1% error, an additional factor needs to be
included in the above formula. As described above the output includes a 50
Ω
series resistor (R
S
). The output load creates a voltage divider with the 50
Ω
series
resistor as follows:
=
+
=
ℜ
∗
∗
4.6. Gain Adjustment
The PDA30B includes a low noise, low offset, high gain transimpedance amplifier
that allows gain adjustment over a 70dB range. The gain is adjusted by rotating
the gain control knob, located on the top side of the unit. There are 8 gain
positions incremented in 10dB steps. It is important to note that the bandwidth
will decrease as the gain increases. See the specifications table in
Chapter 6
to
choose the best gain vs. bandwidth for a given input signal.
