Hi,Circuit note(CN-0345) is an example of multichannel data acquisition system,here there are some questions I want to ask.
1.It says that the maximum sample rate of the system is 1 MSPS in page1, but by calculating the settling time of ADG1207,AD8251,AD8475,AD7982, we get the expected maximum sample rate of the system is then 770KSPS in page7. What's the difference between these two descriptions? Is the maximum sampe rate of the system for one channel or for all channels?
2.Similarly,It says that the channel switching logic is synchronous to the ADC conversions, and the maximum channel switching rate is 1 MHz. Channel switching rates up to 750 kHz also provide 18-bit performance in page1.What's the difference between them?
3.A single channel can be sampled at up to 1 MSPS with 18-bit resolution in page1. If all 8 channels are used, for each channel ,is the actual sample rate 1/8MSPS=125KSPS or 770/8=96.25KSPS？
Thanks for your question! I should be able to clarify these for you:
1. Apologies for the confusion here. The CN-0345 data acquisition system can sample at 1 MSPS, as the ADC in use (the AD7982) has a maximum sample rate of 1 MSPS. What was meant on page 7 was that the settling aspects of the full signal chain are such that we only expect to achieve settling a full-scale step to 16-bit resolution in ~1300 ns, which corresponds to a sample rate of 770 kSPS. This estimation is based on simulation and reported specifications of the devices in use. Actual results may vary, however. You can see some measured performance results using this hardware later in the document.
2. This just means that the channels can be switched every 1 us (1 million times per second), but performance may not be optimal when switching at these rates. 750 kHz is the switching frequency (and by extension, the ADC sample rate) which we've measured to not result in any degradation in performance due to the settling time characteristics of the signal chain.
3. The effective sample rate for each channel is the ADC sample rate divided by the number of channels enabled in the sequence. For 8 channels in use, that means the maximum effective sample rate per channel would be 1 MSPS/8 = 125 kSPS. But as mentioned before, depending on the analog voltage levels at the MUX inputs, there may be performance degradation due to insufficient settling time between samples.