III. Gm-C Filtering - Epublications - Université de Limoges

Using this structure and considering the model illustrated in Figure 123 for the
switches, when the switch is ON, Vds is minimal so as to minimize Ron. Indeed, the MOS
transistor is then in the triode region.
When the switch is OFF, the drain is set to a high potential Vdd_switch to minimize the
non-linearity of the drain-bulk capacitance, also called Coff when dealing with switches.
ON
OFF
GND
GND
2.8V
GND
2.8V
GND
Figure 123. Model for the switches
Table 22 describes the values taken for the capacitances of the capacitor bank. Cfix is
3pF. The frequency step at high frequency is achieved by means of small capacitances C1 to
C4 in order to ensure to be centered on every channel.
- 105 -
Ron
Coff
Table 9. Description of the capacitor bank
Bit number Cfix C1 C2 C3 C4 C5 C6 C7 C8
Capacitance (pF) 3 0.2 0.4 0.8 1.6 3.2 6.4 12.5 25
III.3.a.ii Current Source
Currents feeding the Gm-cells are provided by a bandgap and a series of current
mirrors [III.1], as depicted in Figure 124.
Figure 124. Current source generation