18 • Frequency Response and Noise
NEW FOCUS, Inc.
Performance Data for Noise
Figure 4 shows the typical noise spectrum expressed as
photocurrent noise for Model 205X photoreceivers on
the highest gain setting.
To derive the receiver’s Noise Equivalent Power (NEP),
divide the photocurrent noise by the photodiode
responsivity. To convert to output voltage noise (RMS),
multiply the photocurrent noise by the gain setting
from the 205X front label, then by 630 V/A (the scaling
factor between the gain setting labels and the actual
amplifier transimpedance gain).
For example, the output voltage noise (RMS) for Model
2053 in the 3x10
3
setting is approximately:
0.34 pA/
x 3 x 10
3
x 630 V/A = 0.65 µV
rms
/
.
For the 700 kHz of amplifier bandwidth in the 3x10
3
gain setting, the equivalent noise bandwidth is:
( 2 x
π
/4 ) x 700 x 10
3
Hz = 1.1 MHz
,
so the predicted output noise voltage is approximately
0.65 µV
rms
/
x
= 0.7 mV
rms
.
Because the NEP is listed at the highest gain setting,
some additional considerations add to the NEP at lower
gain settings. First, the noise spectrum (Figure 4) is not
flat, rising at frequencies above 100 kHz. This
contributes an extra 20% to the output noise voltage in
the 3 x 10
3
setting compared to 3 x 10
4
. Also, as the
output noise voltage approaches 1 mV
rms
, the Johnson
noise limit of your measurement instrument will
become important. Note that the Johnson noise for an
oscilloscope with 100-MHz bandwidth (assuming
perfect roll off) and 1-M
Ω
input impedance is
1.2 mV
rms
.
Hz
Hz
Hz
1.1
10
6
×
Hz
205x 10MHz Adj Rcvr revA.fm Page 18 Monday, January 10, 2005 9:29 AM
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