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CCS/ADS8681: ADS8681 with 8:1 MUX Interface Issue

sampreeth meti
Prodigy 190 points
Part Number: ADS8681
Other Parts Discussed in Thread: MUX36S08, OPA192

Tool/software: Code Composer Studio

Hi,

  I have interfaced MUX36s08 with ADS8681and facing a problem of undesired converted value. the Tiva TM4C1294NCPDTI3 microcontroller is used to control the address of mux and the SPI interface with ADS8681.

Each input of the mux channels are applied different voltages through each individual voltage divider. MUX switching from channel 1 to 8 is proper, OPA192 driver used between MUX and ADC.with this arrangement checked with Digital scope a neat switching of various channels is observed,ADS8681 is in default configuration, with meeting the timing requirements for ADS8681(CONVST+ACQ) following steps are followed.

1. Enable Mux

2. Applied Address Lines S2,S1,S0

3.  delay of 30uS

4.  ADC timings for data acquisition.(16 clocks are sent to acquire converted data)

All this works well and data acquired is accurate. But if the delay after switching the channel is reduced then the acquired data shows undesired values.My intention is to achieve at maximum sampling rate.

Cross checked the settling time of MUX and it is 2uS.

Kindly help

  • 0
    TI__Mastermind 25020 points

    Hello

    The maximum sampling rate for the ADS8681 is 1MSPS, which means that entire circuit would need to settle within 1us. The OPA192, from its datasheet settles to 0.001% which is needed to achieve 16 bit performance, in 2.1 us. This does not include any surrounding circuitry, ie RC filters, which would onyl add to the settling time.
    If you can provide a schematic, including the resisiter dividers for the MUX, front end circuitry, and ADC, we can evaluate the performance acheivable and what can be done to optimize the ciruit. Also, the ADS8681 has integrated front end, depending on your applcation it might be possible to eliminate the need of the OPA192, further evaluation of your schemtic will determine this.

    Regards,

    Cynthia

  • 0 in reply to Cynthia
    Prodigy 190 points
    Hi,
    Is it because due to internal 15KHz filter, The output of OPA192 observed is as per settling time mentioned in datasheet. Because i am switching the channels of mux at high speed at 3uS per channel and the settling time required for internal 2nd order LPF (15K) is more, but when i plotted the sampled values for a single channel it showed a clearly rise time of approx 60 us.