Two possibilities come to mind from your description of the problem:
(1) A wheel not tightened correctly: retorque your wheel nuts to specs to make sure you don't have a loose wheel on the vehicle.
(2) A sticking brake caliper. When you installed the new brake pads you had to compress the caliper pistons back into their bores, to allow enough room to install the new pads. The pistons have not been that deep in their bores for many thousands of miles; if there is any sedimant or rust in the bottom of the bore, it can cause the piston to stick and not release fully when you take your foot off of the brake pedal. If a brake pad is held in contact with a rotor for an extended period of time, it will cause the rotor to expand and warp from heat, this can result in a shaking of the front end from the resulting vibration as the distorted rotor spins and presses on the brakes. When you installed the new pads, the pistons should have compressed fairly easily into their bores; if you ahd a difficult time getting them depressed or if they would not bottom completely and you had any difficulty getting the assembly over the rotor during asembly, that would be a pretty good indication that you may need a pair of calipers.
If you are experiencing any vibration in the brake pedal or floor that only occurs during braking, then the source of that vibration is most likely brake rotors with excessive runout or thickness variation.
Also, check to make sure any retaining slips and shims are still in position, and have not dislodged and jammed up a caliper or pad from sliding correctly in it's mounting bracket.
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