On-Chip Resistors and Capacitors

Abstract

Resistors and capacitors are both very important basic building blocks in any analog and mixed signal IC design. Numerous applications in the area of analog signal processing use these two elements, either separately, or combined. They are fundamental elements in oscillators, filters, delay elements, termination networks, amplifiers, and so on. Usually, if simple designs are used to implement any of the above applications, the main characteristics (oscillation frequency of an oscillator, cut-off frequency of a filter ...etc.) of any of them will highly depend on the absolute values of resistors and capacitors used. The accuracy of those elements is always a problem that floats to the surface, especially if the targeted application requires high accuracy for adequate performance. Off-chip discrete resistors and capacitors have high accuracy (typically 1% for resistors and 5% for capacitors), but they have to be used externally. This implies a higher cost and also raises other issues, like the number of pins that can be used to connect those external elements to the internal circuitry. For example, if too many filters are implemented and each one is using a different resistor, it becomes impractical to assign a pin for each one. Also, parasitic elements introduced to the signal path due to bond-wire capacitance and inductance could affect the performance of the circuit causing loss of accuracy intended originally by using external components. The problems introduced by those parasitic elements become more significant at higher speeds.