III. Gm-C Filtering - Epublications - Université de Limoges
III. Gm-C Filtering - Epublications - Université de Limoges
III. Gm-C Filtering - Epublications - Université de Limoges
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Furthermore, it should also be taken care of the input impedance of the filter, called Zin<br />
in the following. A limit of 15-20Ω has been set, which is already very low and will need a<br />
high driving capability from the LNA.<br />
IV.2.a.iii Chosen Tra<strong>de</strong>-off<br />
Table 21 summarizes the dimensioning of the Rauch filter components.<br />
Table 21. Passive Components Values<br />
Component R1 R2 R3 K C<br />
Value 50 Ω 25 Ω 150 Ω 10 from 12 to 84pF<br />
IV.2.a.iv Capacitors Implementation<br />
To enable frequency tuning, capacitors C are implemented by means of capacitors<br />
banks. MOS transistors used for the switches are supplied with Vddswitch=2.8V<br />
Figure 145. Capacitors banks <strong>de</strong>sign<br />
For all capacitors banks, P1 is connected to R1 and R2. This no<strong>de</strong> is set to<br />
VDD/2=1.65V. P2 is either connected to the output, which DC voltage is close to 1.3V, or to<br />
the inverting input of the OA, stuck at GND.<br />
As for the <strong>Gm</strong>-C filtering, this capacitor bank is composed of a main capacitor, which<br />
value is 12pF. Then, switch-controlled capacitors may be ad<strong>de</strong>d in parallel to this main<br />
capacitor. There are 8 switchable capacitors, starting from 280fF for the smallest one, up to<br />
36pF for the largest one. From one bit to the next one, the capacitance is doubled as shown in<br />
Table 22. To keep a constant capacitance Q-factor, MOS switches have sizes which also<br />
double. The smallest one is 5µm/0.25µm.<br />
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