Keithley 2520, Руководство пользователя, Страница: 58 / 329

2-8 Connections Model 2520 User’s Manual
Figure 2-5
Laser diode test connections
Equivalent circuit
Figure 2-6 shows an equivalent circuit for the connection scheme shown in Figure 2-5 .
The circuit includes the current source and voltmeter to excite and measure the laser
diode, two ammeters to make photodiode current measurements, and two voltage sources
to bias the photodiodes.
Polarity considerations
All measurements are unipolar. If a particular measurement polarity is set up incorrectly,
the Model 2520 will display the “OVERFLOW” message. For detector (I
PD
) measure-
ments, current flowing into the ammeter (center conductor of the DETECTOR triax con-
nector) is negative, while current flowing out is positive. For laser diode voltage (V
L
)
measurements, the measurement is positive when the HI terminal is at a higher polarity
than the LO terminal, and negative when the HI terminal is at a lower polarity than the LO
terminal.
CURRENT
OUTPUT
VOLTAGE
SENSE
CAT I
DETECTOR 1 DETECTOR 2
BIAS
CURRENT
INPUT
ISOLATION FROM EARTH: 10V MAX.
HI
HI
LO
LO
!
!
Model 2520 Testhead
Triax Cable
Triax Cable
Center
Conductor
Center
Conductor
Inner Shield
Detector 1 Detector 2 Voltage
Sense
Current
Output
Inner Shield
Shield
Outer Shield
(Chassis Ground)
Coax
Cables
BNC
Shields
Connected
Optional Earth
Ground
Connection
Back
Photodiode
Detector
Forward
Photodiode
Detector
Laser
Diode