The Capacitor

 

For correct operation it is necessary to select the right capacitor value and the right capacitor material. Following the recommendation for the comparator circuit, the discharge time is about

t = 0.7*R_SG*C_Load.

In measurement range 2 the time interval measurement uses not only the high-speed unit of the TDC but also the reference clock (pre-divider). Doing this the measurement range of the TDC is extended to 100 µs. This allows update rates up to 10 kHz. For higher update rates the discharge time must be reduced. At update rates higher than 10 kHz measurement range 1 is recommended.

 

The following values are recommended for measurement range 2:

RSG	CLoad
1 kOhm	68 nF
350 Ohm	220 nF

Capacitor material

 

The capacitor is an important part of the circuit and has direct influence on the quality of the measurement and the stability with temperature.

 

We recommend the following materials (group 1):

• CFCAP (Multilayer ceramic from Taiyo-Yuden)
• C0G or NP0 ceramics
• Polystyrene
• Polyvenylsulfid

 

Also these materials may be used but with some small losses in temperature stability (group 2):

• X7R
• Polyester

 

The following materials must not be used (group 3):

• Z0G