# Help designing a filter for a DAC

Okay, I want to add a filter onto the end of this circuit with the transfer function:

$$\frac{1 - 3e^{-sT} + 3e^{-2sT} - e^{-3sT}}{s^{3}T^{3}}$$
(T is the sampling period)

I'm using PSPICE. The denominator seems simple enough to implement, just 3 op-amp integrators in series (right?), but I'm not sure about the top. Seems like you could implement it as 1 - 3z^-1 + 3z^-2 - z^-3 when the signal's digital, but I'm not sure how to do that without getting lost in a mess of wires and I wouldn't know what parts to use. Seems like there should be an easy way to do it. I'm new to PSPICE so if anyone could help I'd be really thankful.