Sensorless control approaches for low and zero speed determine the rotor position by injecting high frequency voltage signals. Current measurement noise and quantization necessitate a minimum amount of injection amplitude, which implies an acoustic noise emission that for many applications is unacceptable. This paper proposes to sample the current with the maximum A/D conversion frequency while processing the data with an FPGA. By means of a recursive least squares method and a algorithmic extension, high precision current slopes are calculated from low injection voltage. Experimental results show that the injection magnitude can be reduced by factor 7 to 10 compared to standard PWM centered current measurement.