Acid dissociation constant and equilibrium constant
An acid dissociation constant is a particular example of an equilibrium constant.
For the reaction:
$$\ce{CH3COOH +H2O &lt;=&gt;CH3COO- +H3O+}$$
The equilibrium constant would be:
$$\newcommand{\ac}[1]{\text{activity of }\ce{#1}}{\bf K_{eq}}=\frac{\ac{CH3COO-}\cdot\ac{H3O+}}{\ac{CH3COOH}\cdot\ac{H2O}}$$
And the acid dissociation constant would be:
$${\bf K_a}=\ce{\frac{[CH3COO-][H3O+]}{[CH3COOH]}}$$
Now the relation between both would be:
$$\newcommand{\acc}[1]{\gamma_{\ce{#1}}}{\bf K_{eq}}={\bf K_a}\underbrace{\frac{\acc{CH3COO-}\acc{H3O+}}{\acc{CH3COOH}\acc{H2O}}}_{\Gamma_0}\frac1{\ce{[H2O]}}$$
Now:
$${\bf K_a}={\bf K_{eq}}[\ce{H2O}]\Gamma_0^{-1}$$ 
So you're missing some factors.They're not exactly the same.