FO OF ACR WDB ACFT INADVERTENTLY DSNDED BELOW APCH INTERCEPTION ALT RESULTING IN ATC INTERVENING AND REISSUING APCH CLRNC. ALTDEV OVERSHOT.

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WE WERE ON BASE LEG AT 4000 FT APPROX 15 MI N OF DFW. THE CTLR TOLD US TO TURN TO 150 DEGS; DSND TO 3000 FT; CLRED ILS 18R; MAINTAIN 300 FT UNTIL ESTABLISHED. I DID THAT; HOWEVER; AFTER INTERCEPTING FINAL I CONTINUED BY DSCNT TOWARD THE FINAL APCH FIX (OM) ALT OF 2300 FT. THE PROBLEM IS THAT THE MIN ALT IS 3000 FT UNTIL 9 DME. DURING THE TURN FROM BASE TO FINAL WE RECEIVED 2 TAþS FROM THE TCASII DISTRACTING BOTH PLTS TO SOME DEGREE. AT AROUND 2500 FT; THE APCH CTLR ADVISED US THAT WE HAD DSNDED BELOW APCH ALT AND RECLRED US TO INTERCEPT THE GS AT 2500 FT IF WE DESIRED. I ADDED PWR AND BEGAN A CLB BACK TOWARD 3000 FT. APCH AND LNDG WERE NORMAL FROM THIS POINT. IN RETROSPECT; I DO NOT BELIEVE THAT WE WOULD HAVE DSNDED BELOW 3000 FT HAD I HAD THE FLT DIRECTOR ENGAGED. HOWEVER; THE FACT THAT I MISSED THE ALT POINTS TO THE NEED FOR SOME BASIC INST WORK. IN THIS CASE; WHILE HDG 150 DEGS; I BECAME FIXATED ON THE LOC CDI. THIS WAS DUE; IN PART; TO THE FACT THAT THE APCH CTLRS AT DFW COMMONLY SET US UP FOR LARGE TURNS TO INTERCEPT OR GIVE US THE DOGLEG AT OR NEAR CDI CASE BREAK. ALTHOUGH; THAT WAS NOT THE CASE IN THIS SITUATION.