For wearable devices that monitor personal health, secure access to private medical data becomes a crucial feature. Nowadays, device authentication based on biometrics such as fingerprint or iris has become increasingly popular. In this work, we investigate using electrocardiogram (ECG) signals as the biometric modality for device authentication, and we present accurate and low-power ECG-based authentication hardware. Deep neural networks (DNNs) have been employed with a cost function that maximizes inter-individual distance and minimizes intra-individual distance over time. During DNN training, we also introduce joint optimization of low-precision and structured sparsity, so that the real-time authentication hardware can consume minimal energy and area. Experimental results of custom hardware designed in 65nm LP CMOS technology exhibit low power consumption of 59.4 µW for real-time ECG authentication with a low equal error rate of 1.002% for a large 741-subject in-house ECG database.