This work presents a low-power receiver in IHP SGB25V 250 nm BiCMOS technology. The analog frontend is based on a switched passive input network (SPIN) with a surface acoustic wave (SAW) resonator and receives on-off-keying signals with up to 100kbps (measured). Due to bit-level duty cycling, it consumes only 640nA (measured) from a 1.5V supply at 10kbps, which corresponds to a dc energy consumption of 96pJ per bit. A digital backend performs detection of codes with a length of 63 bit with two-times oversampling and has a dynamic energy consumption of 11pJ per bit (measured). The measured input sensitivity of the Rx system is -92.4dBm (BER < 10^-3) and -95.4dBm (CER < 10^-3) with 3 dB processing gain from the code. Since the bit rate is adjustable from 10kbps up to 100kbps, this Rx can be flexibly used either as a wakeup Rx (WuRx) or as a low-throughput data Rx.