8250A Optical Power Meter Operation Manual
5. TECHNICAL NOTES
5-1
5.
TECHNICAL NOTES
5.1
Principles of Measurement
Figure 5-1 shows a block diagram of this instrument.
When light falls on the photodiode in the sensor, the photo-electric current, which is generated, is converted
into voltage through the IV converter, amplified, and then digitized through the AD converter as measured
data.
When the 823* series sensor is connected, the sensitivity data (power calibration data and wavelength sensi-
tivity data), which is contained in each sensor, is read by the CPU and is used to perform the correction cal-
culation to the measurement data, and then the calculation result is displayed on the LCD.
When the 821* series sensor is connected, the power calibration is performed at the calibration wavelength
by using the VR for the power adjustment, which is contained in each power sensor, the wavelength sensi-
tivity data (typical value), which is contained in the power meter, is read by the CPU and is used to perform
the correction calculation to the measurement data, and then the calculation result is displayed on the LCD.
The analog data before the AD conversion is directly output through the buffer and from ANALOG OUT.
Therefore, as described above, the operation is different between when the 823* series sensor is used and the
821* series sensor is used.
When the 823* series sensor is connected, ZERO (offset cancel), power calibration, wavelength sensitivity
correction, and CF (correction coefficient) calculation are not applied, the input current is amplified, and a
voltage of 2 V is output when the internal current range is full scale.
When the 821* series sensor is connected, ZERO (offset cancel), wavelength sensitivity correction, and CF
(correction coefficient) calculation are not applied and a voltage of 2 V is output when the optical power
range at the calibration wavelength is full scale.
The data, which is the same as the data that is displayed after the correction calculation, can be output from
the USB and GPIB digital interfaces.
5.2
Analog Out
A voltage, which is proportional to the measured photo-electronic current, is output from ANALOG OUT of
this instrument. Because the sensor sensitivity calibration, wavelength sensitivity correction, zero cancel, CF
correction, and dBm/dBr calculation are performed on the CPU, these are not applied to a voltage output from
ANALOG OUT. Check the relationship between the display and the voltage output from ANALOG OUT
under the measurement conditions to be used. Table 5-1 shows the relationship between the voltage output
from ANALOG OUT and the full scale of the current range.
Table 5-1 Relationship between the Current Range and the Full-scale Voltage Output from ANALOG OUT
Current range
Full-scale voltage
20 nA to 20 mA
+2 V
50 mA
+500 mV
