iC212
HIGHSPEED PHOTORECEIVER
Rev A2, Page 11/15
Noise Equivalent Power (NEP)
NEP(λ)
= INV(f) * 1/S(λ)
NEP specifies the lowest light power (Pmin) that can
be detected by the sensor. In that case the signal to
noise ratio (S/N) would be 1, which means the signal
to be measured is of the same magnitude as the noise.
NEP(λ = 473 nm) = INV(93 MHz) / S(λ = 473 nm)
NEP(λ = 473 nm) = 215 nV/ Hz * 1 mW / 0.67 V
√
√
= 320 pW/ Hz
√
Noise(BW)
= NEP(λ = 473 nm) * BW
√
√
Noise(93 MHz) = 320 pW/ Hz * 93 MHz
= 3.09 µWRMS
√
Smax
S(λ)
Pmin(λ) =
∗ NEP ∗ BW
As to be expected this value is slightly higher than in
the first estimation.
Pmin(λ) - minimum detectable power, which can be
distinguished from noise (only white noise,
1/f-noise ignored)
Mesurement of minimum optical power Pmin(λ)
S(λ) - photo sensitivity at wavelength λ
Smax - maximum photo sensitivity
NEP - NEP at maximum photo sensitivity
1. Homogenisation of the blue LED light with mi-
crolens arrays (Figure 23)
2. LED modulation with 1 MHz
BW
- bandwidth
3. Change distance between iC212 and LED un-
til signal is barely distinguishable from noise
(method imprecise but rather simple to get a ba-
sic estimation)
4. Put Newport sensor at same distance as iC212
into the LED beam and read the power: PM =
126 µW (Figure 25)
Example
Blue LED with λ = 473 nm, square wave modulated f =
1 MHz (T = 1 µs), bandwidth of measuring circuit BW =
93 MHz.
Smax
NEP
= 1.625 V/mW (Figure 4)
= 115 pW/ Hz (Item No. 305)
√
S(λ = 473 nm) = 0.67 V/mW (Figure 4)
Because of the duty cycle (50%), the measured power
has to be multiplied by 2. The Newport sensor is com-
pletely illuminated (100 mm²). Hence the irradiance
can be calculated to
√
93 MHz
1.625
0.67
pW
√
Pmin(λ = 473 nm) =
∗ 115
∗
Hz
= 2.7 µWRMS
126 µW
100 mm2
µW
mm2
E(Newport) = 2 ∗
= 2.52
This calculation is only valid, if the input noise is fre-
quency independent. Figure 22 shows the input noise
(INV = Input Noise Voltage) of the photo amplifier.
With the effective area of the iC212 sensor (Item No.
302, Aeff = 0.75 mm²) this yield a total power of
µW
mm2
Pmin(λ = 473, measured) = 2.52
∗ 0.75 mm2
= 1.9 µW
This matches the calculated value reasonably well.
Output noise without signal:
√
Noise(BW) = INV(f) ∗ BW
√
nV
Figure 22: Input Noise Voltage as a function of the
frequency - with lower frequencies there
is higher noise
√
Noise(93 MHz) = 215
∗
93 MHz
Hz
= 2.07 mVRMS
For frequencies around 93 MHz an input noise of A slightly higher value of µ = 3 mVRMS has been mea-
√
215 nV/ Hz can be estimated.
sured though.
ICHAUS [ IC-HAUS GMBH ]
