@component function FilteredInput(; name, x0=0, T=0.1)
    params = @parameters begin
      k(t) = x0
      T = T
    end
    vars = @variables begin
      x(t) = k
      dx(t) = 0
      ddx(t)
    end
    systems = []
    eqs = [
      D(x) ~ dx
      D(dx) ~ ddx
      dx ~ (k - x)/T
    ]
    return ODESystem(eqs, t, vars, params; systems, name)
end

@mtkbuild sys = FilteredInput()
ModelingToolkit.observed(sys)

julia> ModelingToolkit.observed(sys)
3-element Vector{Equation}:
 dx(t) ~ (k(t) - x(t)) / T
 dxˍt(t) ~ (-dx(t) + Differential(t)(k(t))) / T
 ddx(t) ~ dxˍt(t)