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Ask for questions about circuit note(CN-0345)

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!

  • jerrytang,

    Please see my answers below aligned to your questions

    1)  The maximum conversion rate for a fixed, single channel measurement is 1MSPS which is dictated by the data converter itself.  In multiplexed applications we now must take into account the settling times of the front-end due to the change in the input voltage and thus if any more than one channel is used then to achieve full performance the throughput is limited to 770 KSPS.

    2) The statement is meant to indicate that the mux can be switched at a rate of 1 MHZ.  However, because of the settling time requirements of the front-end conditioning circuits to achieve the full 18-bit accuracy the overall system conversion rate must be reduced to 770 KSPS.  Remember that the mux and the converter will be run at the same rate to maintain synchronization.

    3) As switching limits the data converter throughput to 770 KSPS then the effective conversion rate per channel will be 770/N where N is the number of active channels for N > 1.

    Sean