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Output Noise of LDC500/501 Current Source
Rev B. Revised Oct 2012
Figure 1 Laser driver noise spectrum measurement setup
LDC500 and LDC501 are equipped with an ultra We measure the output noise through a 25
low noise current source for laser diodes. The noise in resistor which converts the output current noise into a
the output current is a combination result of thermal voltage noise which is then AC coupled to HP89410A
noise (Johnson Noise) in resistors, shot noise in signal analyzer. Figure 1 shows the measurement
semiconductors, flicker noise (1/f noise) , and setup. Figure 2 shows the measurement result. The
electromagnetic interference (EMI) inside or outside output current was set at 50mA for LDC500 and
of the instrument. The noise in the output current 100mA for LDC501.
could be regarded as an addition to the setting current.
Figure 2 Voltage noise and current noise measured across a 25 load driven by LDC500/LDC501
Stanford Research Systems www.thinksrs.com (408)744-9040 -1-
In order to see details in the noise spectrum, the measurement limit of HP89410A, we added a gain
measurement was divided into two bands: block of 100 before HP89410A and the measured
10Hz~10kHz and 10kHz~10MHz. Captured screens noise density is shown in the parenthesis.
from HP89410A are cascaded at 10kHz as shown in
Figure 2. During test, the LDC500 modulation input A comparative test was also done with a
was disabled to reduce internal cable pickup. The competitor's controller, and the result is plotted in
voltage noise is read close to 2kHz (Marker A, B, C and Figure 3. Both instruments are set at 500mA range and
D) and the output current noise is calculated using the 100mA output. Although the competitor's laser driver
formula: (Voltage noise)/25. has a bandwidth of DC~200kHz, its noise level goes up
after 7MHz. At 60Hz, the competitor's current noise is
Table 1 LDC500 and LDC501 Current Source Output Noise 5.1755uV/Hz, which is 30 times bigger than that of
measurement Result the LDC501.
Measurement result
(within 1kHz~1MHz)
Model and Range
Voltage noise Calculated output
on 25 resistor current noise
(nA/Hz)
(nV/Hz)
LDC501 500mA range 85.7 3.43
LDC501 250mA range 43.8 1.75
LDC500 100mA range 20.0 (18.0*) 0.80 (0.72*)
LDC500 50mA range 13.7 (10.0*) 0.55 (0.40*)
*A gain block of 100 is used before HP89410A.
Table 1 shows the noise level between 1kHz and
1MHz (the number is read at 2kHz). Because of the
Figure 3 Comparison between LDC501 and competitor's laser driver
Stanford Research Systems www.thinksrs.com (408)744-9040 -2-
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