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 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.