Was the us constitution a product of innovation or imitation?

The U.S Constitution was a product of Innovation.   It's 3 main innovations were:

1. The principles of the Rule of Law

2. Separation of Powers

3. Recognition of individual Rights.

Other innovations in the process of writing the Constitution were:

The document was made to be additive and flexible (it has been amended 27 times).  It was the first document in modern times to include the Freedom as a central theme.  

The US Constitution was definitely a product of innovation as its administrative status differed considerably to those of the European governments, particularly the British, which was ruled under a constitutional monarchy. It was signed by the delegates of the Constitutional Convention on September 17, 1787.

Prior to this, the US federal government was a relatively weak institution, as each of the states operated as independent countries as a product of the "Articles of the Confederation" of 1781. The central government had the responsibility to oversee foreign relations, military affairs, and regulate currency. However, these powers were heavily limited, as Congress had no authority to enforce requests to the states regarding money or soldiers.

At this point, it was clear that the country needed a stronger central government. The Constitutional Convention focused on this and established the 3 branches of the federal government (Executive, Legislative, and Judicial).

The Congress had delegates from all of the states and passed laws that regarded nationwide interest, in contrast to state legislatures oversaw domestic issues.

powers  are  denied  to the  national governmentin three distinct ways: some  powers, such as the  power  to levy duties on exports or prohibit the freedom of religion, speech, press, or assembly, are expressly  denied  to the  national government  in the  constitution.