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SpectRx NIR Technology
illustrated in Fig. 10.9. The net input voltage is then denoted by V , 503
in
and the additional current and voltage associated with the operation
of the amplifier itself are labeled I and V . The input voltage
amp amp
power spectrum takes the simple form
|
|
S V = S ℜ[( R C|)] 2 + S I amp [ ℜ ( R C|)] 2 (10.25)
,
I
in
where S is the current power spectrum due to the resistor as illustrated
I
in Eq. (10.24), S I, amp is the current power spectrum due to the amplifier,
and [( ||)]ℜ RC 2 is the squared real part of the parallel resistor–capacitor
combination.
The voltage power spectrum output generated through the ampli-
fier can be represented by:
S V out = S ( V in + S V amp ) G 2 (10.26)
,
where S is the voltage power spectrum of the amplifier itself, and
V, amp
G is its gain.
Eqs. (10.23) through (10.26) relate the measured quantity S .
V, out
Another important feature of Nyquist’s model is that Johnson
noise is independent of the material chosen for the resistor. This feature
is exploited in accurate thermometry, allowing precise temperatures
measurements without worrying about the particular type of sensor
material having a contaminating effect. Additionally, Johnson noise
found many applications in classical information cryptography.

10.7.9 Quantization Noise
Quantization noise (/A Hz ) arises from the finite size of the quanti-
zation steps of the analog-to-digital converter (ADC). It is given by:

I quantization = R
N (10.27)
G⋅ 2 ⋅ 12 ⋅ f
N
where R = range of the ADC (V)
G = gain from detector to quantization (Ω)
N = number of bits for quantization (no units)
f = Nyquist frequency, which is half the sampling frequency
N
(Hz)
Quantization noise is of concern in photometric systems utilized
for observing object views of high radiance, such as the sun.

10.7.10 Dark Noise
Dark noise is the electronic noise observed when no radiation reaches
the detector. This noise depends on the type of detector and the elec-
tronic circuitry used. The values used in the case of an InSb detector
are 1.5 × 10 –14 A/ Hz for each detector. As for the quantization noise,