Anti-Alias Filter for 24-bit ADC

For many ADC applications a simple RC filter at the buffer input will provide adequate anti-alias filtering. For applications that require a higher order filter an active filter is often used. The active component in that filter must have sufficient bandwidth, fast settling, low noise and low offset so that it doesn’t corrupt the signal before it gets to the ADC. The LTC6363 is a differential op amp optimized to drive low power SAR ADCs. The LTC6363 has 500MHz GBW, 780ns settling to 4ppm, 2.9nV/√Hz and a maximum offset voltage of 100µV.

Figure 1 shows a 30kHz 3rd order filter using the LTC6363 that has been optimized for use with the LTC2380-24 1.5Msps/2Msps low power SAR ADC with an integrated digital filter. The LTC2380-24 can average from 1 to 65536 conversion results in real time providing an increase in SNR. Both inputs of this circuit can be driven differentially with a ±2.5Vpp signal range or one input can be grounded with the other input driven with up to a ± 5Vpp signal.

Figure 1. 30 kHz 3rd order filter drives LTC2380-24 24-bit ADC

Figure 1. 30 kHz 3rd order filter drives LTC2380-24 24-bit ADC

Figure 2 shows the combined frequency response of the filter and ADC with a sample rate of 1.5Msps and number of averages (N) set to 1 and 8. Figure 3 is a PScope screen capture that shows an FFT, SNR and THD for the circuit of Figure 1 with N = 1. Figures 4 and 5 show THD and SNR vs input frequency for the circuit of Figure 1 with N equal to 1 and 8. At input frequencies below a few kilohertz performance is close to the typical data sheet numbers for SNR and THD. THD gracefully degrades as the input frequency increases beyond that.

Figure 2. Combined Frequency Response of Filter and ADC

Figure 2. Combined Frequency Response of Filter and ADC

Figure 3. PScope Screen Capture shows FFT, SNR and THD of Circuit of Figure 1 with N=1

Figure 3. PScope Screen Capture shows FFT, SNR and THD of Circuit of Figure 1 with N = 1

Figure 4. THD vs fin for the Circuit of Figure 1

Figure 4. THD vs fin for the Circuit of Figure 1

Figure 5. SNR vs fin for the Circuit of Figure 1

Figure 5. SNR vs fin for the Circuit of Figure 1

作者

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Guy Hoover

Guy Hoover是一名拥有超过30年凌力尔特公司工作经验的工程师,曾担任技术员、IC设计工程师和应用工程师。

T他的职业生涯始于LTC,当时的职位是技术员,并向参与多个产品工作的Bob Dobkin、Bob Widlar、Carl Nelson和Tom Redfern学习,涉及产品包括运算放大器、比较器、开关稳压器和ADC。在此期间,他还花了大量时间来编写测试程序以对这些器件的特性进行表征。

他在LTC的下一段职业生涯是学习PSpice和设计SAR ADC。由Guy设计的产品包括LTC1197系列10位ADC和LTC1864系列12位和16位ADC。

Guy目前是混合信号部的应用工程师,专门从事SAR ADC应用支持工作。这包括为SAR ADC演示板设计、编写Verilog代码和测试程序,帮助客户优化包含LTC SAR ADC的产品,并希望编写有用的应用文章,将有关使用这些器件的知识传递给客户。

Guy毕业于德锐技术学院(现为德锐大学),拥有电子工程技术硕士学位。