How did the Founding Fathers of the Constitution limit democracy (excluding the establishment of the Electoral College), with a specific example?
The first example of the limits written into the Constitution appears in the second section of Article 1. One of the many compromises that were made in order to bring the Constitution into existence addressed the question of how to recognize the significant number of slaves and indentured servants already living in the southern states when determining the number of representatives those states should be granted in the House of Representatives. The text of the Constitution stated that
Representatives...shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons.
Indentured servants were counted as whole free persons in determining the population of a state. Slaves were counted as three/fifths of a person for census purposes. The democratic ideal of "one person one vote" was not a possibility at that time.