A first resistor (R.sub.1) and a first capacitor (C.sub.1) constitute a filter arrangement having a time constant which is not very accurate due to the inaccuracy of the resistor and the capacitor values in integrated circuits. The time constant is adjusted using a circuit comprising a reference resistor...http://www.google.es/patents/US4691171?utm_source=gb-gplus-sharePatente US4691171 - Integrated RC filter with resistor trimming
