Air carrier Captain reported starting an engine at the gate per Maintenance instructions and not being aware that ramp personnel were near the engine.

Narrative

While operating a flight we had to return back to gate in ZZZ because of an ADS (Automatic Data System) fail. Upon reaching the gate maintenance personnel boarded the aircraft and wanted to perform an aircraft power down/reset. Once the aircraft powered back up; maintenance wanted to start an engine at the gate as a test run for the ADS fail. Prior to starting the any engine at the gate I requested the First Officer to contact operations to inform ground personnel that there would be an engine running at the gate. I witnessed this communication take place and operations seemed okay with our request. While this was taking place I noticed the maintenance technician had stepped out of the flight deck. I could audibly hear him talking to; what I assumed the Gate Agent and/or ramp personnel; that we were preparing to start an engine at the gate as well. I clearly heard the communication happening and I heard the 'Gate Agent' ask which engine would be running. After conferring with me I yelled back 'Engine 1' and the maintenance technician echoed my statement. I then heard him step back into the flight deck and ask if we were ready to start the engine. I replied with a yes. I waited for him to communicate this with whoever he was talking to and after he did he came to the flight deck again and we began to start engine 1. Around the time the start sequence had finished; the maintenance technician made a comment asking why the ground personnel looked frustrated and I could visibly tell some of them were frustrated. I didn't give it much thought at first because there was a lot going on because we were a gate return aircraft then it occurred to me that they were possibly unaware of the engine start. After the engine run test was completed and the logbook was in my possession; we were preparing to close when a Ramper stepped into the flight deck. He expressed that he and his team did not know about the engine start at the gate and that he had a crew member close to that engine when it started. Shocked; I sincerely apologized to him and his team; asked if anyone was injured; and I informed him that we communicated our intentions clearly to operations and that our maintenance technician communicated with either the Gate agent or supervisor in the jet bridge and he gave me the okay twice before I started the engine. He accepted my apology and I told him we would file a report accordingly and he should file his and we should both chase down where the communication breakdown occurred.I think it is noteworthy that the maintenance technician that boarded the aircraft had voiced that he wasn't too happy that he had to deal with our situation prior to clocking out in an hour and a half. Comments like that; though small; had an effect on energy in the flight deck between the people in the flight deck. Now reflecting on the whole situation; his demeanor has raised doubt to how well he communicated to operations personnel which may have led to safety being compromised.As Captain; I ultimately hold the responsibility for the upholding the safety of the operation and this lies on my shoulders. In my statement I just want to cast out my perceptions of things; hopefully learn where the communication breakdown occurred; and be a resource for others to learn from to avoid an unsafe situation.