As shown in Figure 2-5, the Proposal 2 adopts the ADS8881 and THS4551 for the CWD signal chain, the ADS8881 with single-channel, 1-MSPS, 18-bits SAR ADC from TI in 2015, is now widely used in ultrasound CWD systems and is particularly designed for general ultrasound equipment. Because of the ADS8881 and THS4551 are both with single channel, so double ADS8881 and THS4551 are required to sample the CWD signals and since the ADS8881 does not have an internal reference. An additional reference source REF70 is used in this proposal, TI's ultra-low noise and ultra-low drift voltage reference source introduced in 2023.

Figure 2-5 Proposal 2 ADS8881 and THS4551

Figure 2-6 and Figure 2-7 show the circuit for simulation and simulation results. Simulation of the noise and amplitude frequency characteristics for Proposal 2.

Figure 2-6 Proposal 2 Simulated Circuit

Proposal 2 noise and amplitude frequency characteristics.

Figure 2-7 Proposal 2 AC and Noise Performance

As shown from the simulation results, the ADS8881 has an integral noise of 8.28uV in 1MHZ bandwidth, slightly exceeding the ADS9224, but due to the higher full-range input range, As a result, the overall SNR remains higher than the ADS9224R.