Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors
Flight to Washington Dulles. ATC gave us instructions to descend to 7000 ft. Airplane was on autopilot mode and we descended to 7000 ft as I announced ALTS CAP. Right after that ATC cleared us for 19C visual approach. I asked [the] Captain to sequence the approach and I started to verify approach course and frequency and when I looked up we were descending to 6700 ft and Captain put the airplane in VSI mode to climb back up to 7000 ft. While he was doing it ATC called and gave us a low altitude alert and Captain responded to that we were climbing to 7000 ft. After discussing it with Captain we concluded that I had pressed VSI mode instead of NAV mode when ATC cleared us for the approach.We should have turned off the autopilot and climbed back up to 7000 ft as this procedure would have been much quicker as the VSI mode took about 5-7 seconds.
CRJ-700 First Officer reported receiving a low altitude alert from ATC after overshooting cleared altitude on descent into IAD; citing lack of mode awareness as contributing.