
44-16 Optical
44.5.5.1 Equivalent Circuit
Like
th
e
GP
D
de
scription,
th
e
ex
pression
fo
r
th
e
eq
uivalent
ci
rcuit
of an SS
PM
co
ntains
a te
rm
fo
r
th
e
ga
in,
G, a
s
d
escribed
i
n
t
he
f
ollowing
e
quation:
I G I I G I
o db ds p
= ⋅ + + ⋅ (44.20)
where
I
db
is the bulk dark current
I
ds
is the surface dark current, which can include ohmic components
Notice
th
at
li
ke
th
e
AP
D,
th
e
ga
in
mi
nimizes
th
e
co
ntribution
of no
nbulk
le
akage
cu
rrent
. Un
like
th
e
GP
D,
ho
wever,
th
e
SS
PM
ca
n
ac
commodate
th
e
de
tection
of mu
ltiple
ph
oton
ev
ents
wh
en
th
e
ph
otons
ar
e
d
istributed
o
ver
t
he
a
rray
i
nto
d
istinct
G
PD
e
lements
.
44
.5.5.2
Qu
antum Efficiency
e
QE an
d
DE of an SS
PM
is sim
ilar ...