Noise Analysis of CMOS Readouot Ciruits
 “A CMOS 0.35 μm Analog Front-End Electronics for Gamma Ray Medical Imaging” suggest a noise model and analysis for circuit of analog
signal processing circuit as following figure and equations.
 They focus on three separate contributions determine the ENC:
1) ENCd : The input transistor channel thermal
2) ENCf : Flicker noise
3) ENCo : Detector leakage current and feedback resistor thermal noise
 These factors are depend on the circuit design.
ENC
 They figure out a optimum transistor width(W) which makes noise minimized with
fixed drain current of CSA.
 Following their analysis, optimum width of input transistor is about 1 mm.
 Although the optimal value from the analysis minimize the noise, 1 mm is too large to
be integrated.
 And also, we have to consider about gain and linearity of CSA.
 If W is increased with fixed ID (100 μA), VOV and gm would be decreased by W.
Input Transistor Width [Meter]
 Large transistor with respectively low drain current causes low gain and linearity.
Noise Analysis of CMOS readouot ciruits
gm
 As we mentioned above, 1mm width transistor has only 0.02 V overdrive voltage depicted on a left figure.
 0.02 V of overdrive voltage is very close to threshold voltage of transistor.
 In the right graph, we can see the gm which is small and nonlinear.
New Value => good gain & linearity
Old Value => Poor gain & linearity
Gate Voltage [V]
ENC
 We suggest constant overdrive voltage analysis to find out the optimum width of the transistor has small size and high linearity.
 Fixing the overdrive voltage to 1 V, we recalculate ENC and show on the left figure.
 With same noise performance, new optimum value of transistor width is much smaller size (15 μm), much higher linearity.
Input Transistor Width [Meter]
Input Transistor Width [Meter]