SBAA558 October 2022 ADS9218 , ADS9219
For the traditional SAR architecture, the filter at the output of a driver amplifier is used to provide a charge reservoir to help respond to the input transients. Normally the bandwidth of this filter needs to be set to a high frequency to be effective as a charge reservoir. Tuning the filter to lower frequency introduces significant distortion due to the input signal not fully settling during the acquisition period. Thus, it is generally not possible to use the charge reservoir filter as an antialiasing filter as this impacts distortion performance. Notice that the input filter on the traditional SAR (Figure 1-1) is set to 7.2 MHz which is higher than the 1-MHz Nyquist frequency. Furthermore, the process of optimizing this filter often requires parameter stepping in simulation to find the optimal RC value. This process is covered in the Driving SAR Converters video series.
The ADS9218 does not require the charge reservoir filter. Thus, the same filter structure on this device can act as an antialiasing filter. In Figure 2-1, the output filter is tuned to 500 kHz which provides some antialiasing for the 10-MHz sampling rate. Furthermore, the low cutoff frequency of this filter acts to minimize the overall intrinsic noise from the amplifier. Table 2-2 illustrates the total simulated RMS noise from the amplifier stage. Note that the highest noise is 20.4 μV for the wide bandwidth option. The ADC noise is approximately 40.2 μV, so the ADC dominates the noise and all amplifier configurations have minimal impact on SNR. The last two columns of Table 2-2 show the SNR of the amplifier and the SNR of the system. The SNR of the system ranges from 95 dB to 94 dB, so the SNR is dominated by the noise of the ADS9218. The calculations for the table are shown in Equation 2, Equation 3, and Equation 4. A detailed explanation of the noise equations is provided in the THS4551 data sheet.
| Device | Suggested Max. Freq. for low Distortion |
Output Filter Cutoff | En (nV/√Hz) | In (pA/√Hz) | Total RMS Noise of Amplifier | SNR of Amplifier for FSR of 6.4 VPP | SNR of ADC+Amp FSR of 6.4 VPP |
|---|---|---|---|---|---|---|---|
| THS4561 | 20 kHz | 100 kHz | 4 | 0.35 | 4.52 μV | 113 dB | 95.0 dB |
| THS4551 | 100 kHz | 500 kHz | 3.3 | 0.5 | 9.29 μV | 107.7 dB | 94.8 dB |
| THS4541 | 1 MHz | 5 MHz | 2.2 | 1.9 | 20.40 μV | 100.9 dB | 94.0 dB |
In general, the input filter of the ADS9218 can be adjusted to any frequency to provide antialiasing and optimize noise performance whereas the traditional SAR architecture output filter is tuned to a specific frequency to facilitate settling and maintain good THD. Besides providing flexibility in the output filter design, the new topology simplifies the design process. For traditional SAR architectures, the selection of the output filter can be challenging. The Driving SAR Converters video series covers the selection of the amplifier and output filter for a traditional SAR ADC. In these types of circuits, a common approach is to choose an amplifier with a bandwidth 10 times faster than the acquisition period. Once the amplifier is selected, parameter sweep simulations are used to optimize the output filer. The result of the design process is that the amplifier bandwidth needs to be higher than is needed for the new topology. The filter is restricted to a very specific frequency which generally does not provide antialiasing and perhaps is not optimal from a noise perspective. The new topology provides the freedom to choose any amplifier and output filter that is designed for the frequency range of interest.