3
Operating Considerations
The TIA-527 is comprised of two
fi
ber coupled detectors and two ampli
fi
er stages. The
fi
rst ampli-
fi
er is a transimpedance stage which converts the net detector output current to a voltage by passing
it through a resistor of 1 400 or 14 000 ohms. Additional ampli
fi
cation is optionally provided by the
second stage which also serves to provide 50 ohm drive capability. Either AC or DC coupling between
the stages may be selected.
The overall bandwidth of the unit is determined by the
fi
rst stage transimpedance. It is in excess of
125 MHz when the T R switch is in the 1.4 K position and is 35 MHz in the 14 K position. The over-
all responsivity of the unit in terms of Volts/Watt is the current responsivity of the detector multiplied
by the transimpedance and further multiplied by the second stage gain. For example, the sensitivity
of the unit at a wavelength of 1300 nm would be 0.7 A/W x 1400 V/A x 10 = 10, 000V/W. It is evident
that the same responsivity may be obtained by using a transimpedance of 14 K and a second stage
gain of one. However, the
fi
rst setting will provide a bandwidth of 125 MHz while the second will pro-
vide a bandwidth of 35 Mhz. Since the overall peak-to-peak output noise increases with bandwidth,
it it desireable to use the higher transimpedance setting assuming that the signal of interest does not
exceed 35 MHz.
The selection of AC interstage coupling is useful when the user needs to examine a small signal in
the presence of a large DC optical component, (e.g. baseband
fi
ber optic video). This will preclude
the last stage from saturating on the DC component. Otherwise, DC coupling should be employed.
When using the TIA-527 mounted on an oscilloscope, the scope may have its input set to either 50 or
1 Meg ohm input impedance. If driving a coaxial cable, the cable should have a 50 ohm characteristic
impedance and be terminated with a 50 ohm load. Note that the signal amplitude will be reduced by a
factor of two.
The following table summarizes the operation of the unit under various operating conditions.
