The frontend (Helix2.1)

Helix128-2 uses the Helix2.1 frontend (fig. 2) which has been carefully optimized with respect to noise, pulse shape (peak time and undershoot), linearity, space and power consumption. More detailed descriptions can be found in [2] and [3].

  

Figure 2: Helix2.1 frontend

To set the frontend operation mode and to compensate for radiation damage the frontend bias voltages and currents (not to be confused with the power supplies +2V,0V,-2V) may be adjusted via programming of the corresponding DAC registers as explained in section 4. The nominal values are listed in table 1.

• Ipre sets the preamplifier bias current. With Ipre being above a threshold of $\approx$ 100$\mu$A no impact on the peak time is observed [7]. A higher Ipre current reduces noise, so that a noise degradation of the preamplifier due to decrease of the g_m of the input transistor can be cured by increasing this current (at the cost of power consumption).
• Isha sets the shaper bias current. Isha influences the shaper core cell "load resistor" leading to a smaller undershoot for higher currents. Also, the peaktime decreases (see fig. 3) with rising Isha. Isha should be kept constant under irradiation (pulse degradation should be adjusted basically by Vfs).

  
  

    

  Figure 3: Impact of Isha on pulse shape: from slowest to fastest pulse: Isha=60$\mu$A,100$\mu$A,200$\mu$A, and 300$\mu$A

  
  
  
• Ibuf sets the buffer bias current. It hould be kept constant. If the buffer becomes too slow by a decrease of g_m (this can be observed e. g. by changing pulse heights at different Sclk frequencies), Ibuf can be increased.
• Vfp controls the value of the preamplifier feedback resistance. It should be as low as possible for optimum noise performance. Vfp should be decreased continously during irradiation, so that the preamplifier operates slighly above the Vfp cutoff edge (for DC coupled detectors this value can be quite large).

  
  
  

• Vfs controls the value of the shaper feedback resistance and thus determines the discharge of the shaper feedback capacitor (see fig. 4). Vfs should be increased continously during irradiation, so that the peak time remains constant.
  
  

  
  

    

  Figure 4: Impact of Vfs on pulse shape: from outside to inside Vfs=0.8V,1V,1.5V, and 2V

  
  
  

 

Name 2|c|Nominal value = Irradiation

2|c|and dec. reg. cont. = compensation

$$200\mu\/ \Rightarrow\Uparrow$$ = 80

$$100\mu\/ \Rightarrow$$ = 40

$$100\mu\/ \Rightarrow\Uparrow$$ = 40

$$50\mu\/ \Rightarrow\Uparrow$$ = 20

$$\ldots 100\mu\/ \Rightarrow$$ = 20..40

$$\mu \Rightarrow$$ = 16

$$20\mu\/ \Rightarrow$$ = 8

$$100\mu\/ \Rightarrow\Uparrow$$ = 40

$$90\mu\/ \Rightarrow$$ = 36

$$\Downarrow$$ 0.2V = 71

$$\Downarrow$$ 1.5V = 113

$$\pm \Rightarrow$$ = 71

$$\ldots \Rightarrow$$ 1V = 97

$$\Rightarrow$$ -1.1V = 97

$$\Downarrow$$ V = 65

$$\Downarrow$$ -840mV = 65

$$\Rightarrow\Uparrow$$ -0.5V = 55