Different types of analog-to-digital converters (ADCs) are available for converting an analog signal into a digital signal. For example, a continuous-time delta-sigma modulating (CTDSM) ADC uses a delta-sigma modulator for converting an analog signal to a digital signal. The CTDSM ADC may use a loop filter for moving low frequency quantization noise up to higher frequencies outside a band of interest. The out-of-band noise may then be filtered out, which may improve a dynamic range of the analog-to-digital conversion.
A quantizer in the delta-sigma modulator may receive the output of the loop filter and perform the analog-to-digital conversion for outputting a digital signal. A feedback loop of the CTDSM ADC includes digital-to-analog converters (DACs) that convert the digital output of the quantizer to corresponding analog values. The analog values are then subtracted from the input signal of the loop filter or internal nodes within the loop filter, and the resulting signal is input back into the loop filter and then the quantizer.
One drawback of continuous-time delta-sigma modulating (CTDSM) ADCs is that their power/frequency scaling is not as robust as discrete time (DT) DSM ADCs. For example, a CT DSM ADC in 28 nanometer radio frequency exhibits similar power consumption as its DT counterpart in a LTE 40 bandwidth mode, but it burns more than twice the power compared to the DT DSM ADC when operating in a global system for mobile communications (GSM) bandwidth (BW) mode. This is an important distinction for CT DSM ADCs used in baseband receivers because they support multiple standards with wide bandwidth variations. Therefore, there is a desire for a CT DSM ADC with improved power/frequency scaling.