A fully-differential 8th-order cascade bandpass AC-modulator is presented. The circuit is implemented using only two Opamps and operates at a sampling frequency of 80MHz.-The circuitcan be used in an IF-receiver to combine frequency downconversion with analog to digital conversion by directly sampling an input signal from an intermediate frequency of 6OMHz to a digital intermediate frequency of 20MHz. The measured peak SNDR is 75dB for a 1.25MHz bandwidth (IS-95). The circuit is implemented with a 0.35um CMOS technology and consumes 37mW from a 3.0V supply.
IntroductionA bandpass AX-modulator performs a high resolution N D conversion of a high frequency but narrow band signal. An obvious application for such a device is in radio receivers with a nonzero intermediate frequency (Fig.l)[ 13. The RF signal is downconverted to a fixed IF after which it is anti-alias filtered and undergoes a further down-conversion to second IF by a subsampling operation. The bandpass AX-modulator converts the sampled signal to digital domain where the final demodulation is performed. Performing an early A Dconversion eliminates some analog parts like the I/Q-demodulator and baseband filters and improves flexibility. Unfortunately, this happens at the expense of an increased ADC resolution requirement, which is not easy to meet. The scaling of the CMOS technology doesn't necessary help at all when designing high performance analog and mixed-mode circuits which calls for new circuit level solutions.The main parameters for designing a AX-modulator for a given signal-to-noise ratio (SNR) are the oversampling ratio (OSR), the order of the loop filter and the resolution of the internal quantization. The OSR can be increased up to a point, after which the power consumption starts to increase rapidly. For wideband applications (1-2MHz) OSR is typically limited to the order of tens which necessitates using a higher order loop filter to obtain adequate noise shaping with a low internal resolution.In this paper an 80MHz 8th-order BP AXM targeted at wideband IF-sampling receivers is presented.
