Hello,

I am trying to design a Frac N PLL using an all digital ΣΔ modulator.  All literature I have found so far refer to the ΣΔ as an ADC or DAC, but it seems to me that the modulator should have a digital input and digital output.  Can any help clarify this, and maybe point me to some literature that talks about the design of ΣΔ's used in Frac N PLL's?  Thanks.

Tim