and reflects the period of the switching transients at the drain nodes of the NMOS transistors in the ring oscillator. From eqn. 1 the required capacitance is then 3.3pF and the dynamic resistance of the diode is 6.06 k 9 . This results in a Z ( q ) of 0.59 as opposed to the impedance without the guard ring of 559. The actual resistance to ground is then also limited by the parasitic contact resistance R,. We have assumed a minimal value of R,, = 0.19 in the simulations. This represents the contact resistance and vertical resistance through the 1000pn2 diode formed by the n+ source/ drain implant with a horizontal sheet resistance of 100OCYO.22 3731 20 z 1 7 5 2 15 212 5

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