Combining and Selecting Resistor and Capacitor Values on op-amps
Combining Op-Amp Stages
Combining op-amp stages to save money and board space is possible in some cases, but it
often leads to unavoidable interactions between filter response characteristics, offset voltages,
noise, and other circuit characteristics. The designer should always begin by prototyping
separate gain, offset, and filter stages, then combine them if possible after each individual circuit
function has been verified. Unless otherwise specified, filter circuits included in this document
are unity gain.
Selecting Resistor and Capacitor Values
The designer who is new to analog design often wonders how to select component values.
Should resistors be in the 1-ohm decade or the 1-Mohm decade? Resistor values in the 1-kohm to
100-kohm range are good for general-purpose applications. High-speed applications usually use
resistors in the 100-ohm to 1-kohm decade, and they consume more power. Portable applications
usually use resistors in the 1-Mohm or even 10-Mohm decade, and they are more prone to noise.
Basic formulas for selecting resistor and capacitor values for tuned circuits are given in the
various figures. For filter applications, resistors should be chosen from 1% E-96 values (see
Appendix A). Once the resistor decade range has been selected, choose standard E-12 value
capacitors. Some tuned circuits may require E-24 values, but they should be avoided where
possible. Capacitors with only 5% tolerance should be avoided in critical tuned circuits—use 1%
#From Application Report of Texas Instruments