Biomedical Engineering Reference
In-Depth Information
V (
μ
V)
I
b
=120
μ
A
100
I
b
=110
μ
A
80
I
b
=100
μ
A
60
40
20
-1
0
1
Φ
ex
/
Φ
0
Fig. 3.15.
Calculated asmmetric
Φ
-
V
characteristics with the APF effect.
I
c0
=
0
.
1mA,
L
sq
=20pH,
R
s
=1
Ω
,
R
a
= 100
Ω
,and
M
a
= 300 pH
(
∂V/∂Φ
)
a
becomes larger with increase of
β
a
up to unity. But if
β
a
>
1, the
Φ
-
V
curve is no longer stable, which provides the same condition as indicated
in (3.52).
With a large APF gain, that is, with large M
a
/R
a
the output voltage is
reduced, if R
a
is too small. While with large L
a
, the current noise induced in
the resistor R
a
affects the SQUID through large M
a
[15]. APF circuit should
be designed to take into consideration the reduction of the output voltage
and the attribution of the noise induced in the APF circuit.
3.2.2
A System for Biomagnetic Measurement
Configuration.
In the previous section, SQUID sensors and their driving
electronics were described. The system configuration for high-sensitivity ma-
gnetic measurement can be accomplished by highly integrated digital and
analog signal processing technology. All the signals from the SQUID electro-
nics are acquired digitally for further numerical analysis. The analog signals
must be filtered and amplified before recording the signals as digital data.
The dynamic range of the amplifier and the cutoff frequency of the analog
filters are important for high resolution of the digitized data.
SQUID sensors must be kept in a refrigerant such as liquid helium. The-
refore, cryogenics are indispensable in SQUID magnetometry. The cryostat
specially designed for biomagnetic measurement is used in a biomagnetic field.
The cryostats for different measurement objects have different structures. For
example, the tail of the cryostat of the multi-channel system for magnetoen-
cephalography has a helmet-like shape in order to allocate the sensors around
the whole head at high density.
The magnitude of the biomagnetic field to be measured is extremely low.
The magnitude of the geomagnetic field is from one to ten billion times larger
