Also, the noise associated with all MOS switches and MOS amplifiers makes the dynamic range too small for demanding high-fidelity audio applications. At switching rates greater than 1 MHz, MOS switched capacitor filter performance begins to degrade, which makes the use of switched-capacitor filters impractical above the audio range (20 kHz). Future development of this technique, or similar techniques that eliminate diffused resistors from filter circuits, raises the possibility that integrated filters containing resistors, capacitors, op amps, and switches may offer a simple and inexpensive means of digitally tuning state-variable analog filters. Researchers have introduced techniques that eliminate resistors by replacing them with field-effect transistors (MOSFETS) biased in the linear (ohmic) region of operation. Thus any filter topology with a transfer function dependent on capacitor ratios is ideal for unipolar (MOS) IC fabrication. In fact, capacitors in MOS technology can be matched within 0.1%. For this reason the thickness of thermally grown oxide layers on a wafer does not change very much across the wafer surface. The growth of silicon dioxide on silicon is subject to similar processing variations (temperature and time), but in this case impurities are not added. Together these two effects cause RC products to vary by a factor of two when using the best processing available. Furthermore, this variation is uncorrelated to the variation of capacitance per unit area. Variations in processing parameters (temperature, impurity dose, time, etc.) can cause the resistivity of diffused resistors to vary more than 1% on the same chip. The biggest problem with the existing monolithic integrated-circuit technology for frequency-selective filters is the wide variation of the relative resistor values in a single integrated circuit. Jenkins, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 VII Switched Analog Filters % loop through to find every possible value of R2 % loop through to find every possible value of R1 R2best = 0 % we haven’t yet found the best value of R2īestError = 1e12 % our initial error is huge R1best = 0 % we haven’t yet found the best value of R1 % duplicate vR by multiples of 10 to create all % create vR with standard value resistors from 1 to 9.1 For example, if you need to find the value of two standard value resistors from 1 Ω to 1 MΩ in series, whose sum is as close as possible to a given desired value, enter the following:įunction = findclosestseries(rDesired)

Standard resistor values table