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This is a CIFF loop filter: delayed integrator (left dotted square) + passive summer (right dotted square), where out[k] goes to the quantizer input. But the problem is: I intended u[k] + w[k] as a quantizer input, but the passive summer makes the output (w[k] + u[k])/2. What should I do here? Should I really use an extra opamp just for an active summer?