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ADS8862: SAR Needed for undersampling application

Antenna_Head
Genius 11395 points
Part Number: ADS8862
Other Parts Discussed in Thread: ADS8881, ADS7056, ADS7853, ADS7042

We have a spectrum analysis problem where we need to analyze a signal in the 490 kHz to 520 kHz range. 

The signal will be bandpass filtered before sampling occurs, so an undersampling approach is feasible.

In order to minimize signal chain hardware and maximize dynamic range, I'd like to throw a ~16b SAR converter at this problem, and then perform an FFT on the data stream (this is the simplified description).

Please suggest a few SAR converters with an appropriate sample and hold time that would work here.  Clearly there are a variety of Nyquist zones available, so there are some trade-offs to consider.

I'm thinking a device such as the ADS8862 will work, but the usual spec I would look for related to the sampling aperture time isn't listed. 

There is an "Acquisition Time" listed of 540 ns, but I'm not sure this has the same meaning. Please clarify.

There is an "Aperture Delay" of 4 ns; if this is the sample/hold time, this would be great.

If you have other suggestions for devices, please let us know.

Thanks, Best, S. Dunbar, AFA Colorado

  • 0
    TI__Mastermind 49635 points

    Hello Steven,

    The acquisition time for the SAR converter is the period of time that the sample and hold capacitor is connected to the external circuitry, where the ADC internal sampling capacitor charges to the external voltage.  At the end of acquisition period, the sample and hold capacitor disconnects from the external circuitry and connects to the internal ADC circuitry.   On the ADS8862, the sampling of the signal occurs on the rising edge of  the CONVST signal, where the acquisition period ends and the conversion period starts. The device is by default in acquisition mode, until a conversion is triggered (CONVST rising edge) placing the device in conversion mode.  The conversion period is limited to a maximum of 930ns.   The device returns immediately to acquisition mode after the conversion time has lapsed.  Please see below:

     

    Aperture delay is the delay in time between the CONVST pulse (on ADS8862, CONVST rising edge) and the actual time at which the sample-and-hold disconnects from the external circuitry where the signal is sampled or captured.

    The ADS8862 device is based on the ADS8881 (18-B) high precision core. The architecture of these devices implements internal error correction during their conversion process to accomplish the high precision/high resolution results and they are intended to be used to sample signals with frequencies below Nyquist.

    Devices such as the ADS7042 (12-B), ADS7056 (14-B), ADS7853 (dual, simultaneous sampling 14-B), use a conventional SAR ADC architecture that may be used on an undersampling application.

    Please let me know if you have questions.

    Thanks and Regards,

    Luis Chioye

  • 0 in reply to Luis Chioye
    TI__Genius 11395 points

    Thanks, I now see how the samples are staggered in this timing diagram.  We'll look at some of these other parts. 

    Thanks again, Best, Steve