If we have an LLC and use a dba can we just put the dba on contracts or do we have to list the LLC name and dba?

I agree with Mr. Fink's answer, with the proviso that the the signatory's appropriate title (perhaps other than Managing Member) should be used.
Both prior answers referred to the possibility personal liability. You might find the information at the link below helpful (refers to corporations, but similar principles apply to LLCs).
Disclaimer: This post does not constitute legal advice and does not establish an attorney-client relationship.

You can use the LLC's dba on contracts. However, make sure you sign the contract as Managing Member of Tim's LLC dba ___________. Otherwise, your individual signature might give you potential liability.
The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change.