Non-linear Blind Source Non-linear Blind Source Separation Applied to Ion- Separation Applied to Ion- sensitive Field Effect Transistor sensitive Field Effect Transistor Sensor Arrays Sensor Arrays Guillermo Bedoya Advanced Hardware Architectures (UPC) Laboratoire des Images et des Signaux (INPG) UNIVERSITAT POLITÉCNICA DE CATALUNYA UPC INPGrenoble
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Non-linear Blind Source Separation Applied to Ion-sensitive Field Effect Transistor Sensor Arrays Guillermo Bedoya Advanced Hardware Architectures (UPC)
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Non-linear Blind Source Separation Non-linear Blind Source Separation Applied to Ion-sensitive Field Applied to Ion-sensitive Field
-How does BSS apply to semiconductor-based chemical sensing
OverviewPotential advantages of integrated circuit technology applied to the field of physiological data acquisition and water monitoring systems :
• small size
• reliability
• low cost & rapid time response
• multi sensor chip
• on chip signal processing
Our objective is to design a low cost/high performance smart sensor system for Biomedical and environmental monitoring applications, using ISFET/CHEMFET sensor arrays.
IntroductionBlind Source Separation deals with the « separation » of a mixture of sources, with a little prior information about the mixing process and the sources signals
..
....
SensorsSensors
S1
S2
SN
Sources
Environment
Source Separation Algorithm
W...
..
.x2
xN
x1 Ŝ 1
Ŝ2
ŜN
Observations
x = As Ŝ = Wx
What is Blind Source Separation
x1[n]=a11s1[n]+…+a1psp[n]..
xm[n]=am1s1[n]+…+ampsp[n]
x[n]=(x1[n]…xm[n])=As[n]
y[n]=(y1[n]…ym[n])=Wx[n]=s[n]
s x yA W
Identifiability & Algorithmic solutionsPrinciples of information theory can be applied to the BSS problem.
We suppose that source signals are independents (realistic assumption).
Then, we minimize a measure of the independence (e.g., the mutual
information (MI) I(.)) of the outputs I(y), where y=Wx.
s x= As y= Wx
How do we do it?
We consider a processing function g, which operates on a scalar X using a function Y=g(X;w) in order to maximize the MI between X and Y. Parameter w is chosen to maximize I(X;Y).
I(X;Y)=H(Y) – H(Y|X)
The MI is maximized when H(Y) is maximized (for g deterministic).
H(y)=sum[H(yi)] - I(y1,…, yN)
In order to maximize H(y) (we can maximize each H(yi) or minimize I(y1,…, yN)). g must be the CDF of x.
The mutual information is minimized when all the outputs are independent !
W g(x,W)x y
How do we do it?
An adaptive scheme is to take: Δw α
Δw = ;∂ (ln |∂y/∂x|) ∂w
1w
then,Δw α + x(1-2y)
Assuming a particular functional form:
y = g(x) = 1/(1+e-(wx))
∂y/∂x = wy (1-y)
∂H(y)
∂w
And we have a weight update rule :
w[k+1]=w[k]+ stepsize Δw
Applications to semiconductor-based sensor arrays
Ion-Sensitive Field Effect Transistors (ISFETS and CHEMFETs) are basically metal oxide semiconductor field-effect devices. The construction of an ISFET differs from the conventional MOSFET devices, in that the gate metal is omitted and replaced by a
membrane sensitive to the ions of interest.
Potentiometric sensors!
ISFET/CHEMFET sensors
S D
(1) Reference Electrode (Vref)
(2) Solution (Electrolyte)
(3) Membrane (MOSFET gate
metal)
(4) Gate Insulator
VG VD
ID
Silicon Substrate
ISFET/CHEMFET sensors (Short view)
ID for the conventional MOSFET is:
ID = α[(VG –VT)-0.5VD]VD (1)
where α = μCoW VD/L
We have to establish new expressions for VG and VT to adapt equ. (1) to the ISFET.
Preliminary Results• The algorithm recovers the wave forms of the main ion activity (ai) and the interferent ion activity (aj). aj is considered as noise in other approaches. Selectivity is improved.
• The algorithm works well for sensor arrays with poor selectivity coefficients. Sensor arrays with poor performance bring to the algorithm spatial diversity and more statistical information. We can use low cost sensors.
• The adaptive scheme allows the separation when environment characteristics varies (e.g., temperature). We hope to study the algorithm behaviour as a function of the device drift.
• The scheme can be adjusted to build a multi-parametric system, adding more sensors (sensitive to other ions) and adjusting the algorithm parameters.
• Hardware implementation using a DSP card is currently being developed. Previous implementations had shown good performance.
Non-linear Blind Source Separation Non-linear Blind Source Separation Applied to Ion-sensitive Field Applied to Ion-sensitive Field