My question concerns the filter response in Figure 14. on page 11 of the AD8283 datasheet. I have been testing this A/D converter and the programmable gain amplifiers, channel selection through SPI, digitization etc.. all seem to be working within specs, except for the programmable third order low-pass filter. According to Figure 14, for a cutoff frequency of 1 MHz, there is an attenuation of 28 dB at 2 MHz, since at 1 MHz the gain is ~16 dB and at 2 MHz the gain is ~ -12 dB. To test the filter response, the output of the AD8283 was connected to a D/A converter and the output signal was observed. The A/D was programmed to have a cutoff frequency of 1 MHz, however, as the input signal frequency (into A/D) was increased from 1 MHz to ~ 2 MHz, the output of the D/A didn't change in amplitude whatsoever.
Thanks in advance