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ADS8691: Reading ADC port through SPI Communication

Sarathi Thirugnanam
Prodigy 100 points
Part Number: ADS8691
Other Parts Discussed in Thread: ADS8681

Hi..,

I need to read ADC Port of ADS8691 through SPI Communication using an External device. Please suggest me some ideas.

Thanks in Advance

  • 0
    TI__Mastermind 19177 points
    Sarathi,

    We will look into this and get back with you as soon as possible.

    Thanks,
  • 0 in reply to Art Kay
    Prodigy 100 points
    Thank you Art Kay

    For ur response.

    I am reading some data from ADC. But it is not accurate.Though the ads8691 was new. I didn't get any support from any forum.
  • 0 in reply to Sarathi Thirugnanam
    TI__Mastermind 38215 points

    Sarathi,

    Can you give us more information about the issue you have? what's your input signal and your conversion code? so we can see how not accurate,also more information about your design and test:

    • Are you using internal or external voltage reference?
    • Can you upload your schematic?
    • Your timing for digital bus(SCLK, SDO and CONVST) captured with a scope 

    Thanks.

    Regards

    Dale Li

  • 0 in reply to Dale Li
    Prodigy 100 points

    Dale Li..,

    I am using Raspberry pi 3 board SPI communication for ADS8691 ADC read. I have verified with oscilloscope SPI Signals from master (Raspberry pi 3).

    Problem is I didn't  get any return data from Slave (ADS8691).

    I am using internal voltage  reference  (3 x v ref).

    Below is our Schematic

  • 0 in reply to Sarathi Thirugnanam
    TI__Mastermind 38215 points

    Hi Sarathi,

    Your schematic shows that you are using AD7193 from ADI which is a 24-bit 4 differential/8 pseudo differential input sigma-delta (Σ-Δ) ADC, it's not a schematic for our ADS8691, can you please double check?

    Thanks.

    Best regards

    Dale

  • 0 in reply to Dale Li
    Prodigy 100 points

    Hi Dale Li..,

    Sorry for wrong attachment. Here I have attached the correct Schematic. Pls chk it

  • 0 in reply to Sarathi Thirugnanam
    TI__Mastermind 38215 points

    Hi Sarathi,
    I apologize for missing your post. From your schematic, you should get the data if your timing is correct and even the data is not accurate:
    1. Please capture your digital timing for CONVST/SCLK/SDI/SDO and upload the screenshot for check.
    2. A balanced input RC is recommended in ADC input, please refer to the schematic in ADS8681EVM user guide. Even ADS8691 has integrated AFE, it's better to have a buffer before ADS8691 to isolate the input with your another ADC(MAX11200, our apps expert from Delta-Sigma team can give your best suggestion for the replacement).
    3. A reset pulse signal is recommended on /RST pin after power up ADS8691, do not connect it to 5V supply directly with a pull-up resistor.
    4. Please disconnect the input with MAX11200 and front-end circuit, use one DC voltage signal as input to ADS8691 for test purpose.
    Please let me know the result after your implement these suggestions, thanks.

    Best regards
    Dale

  • 0 in reply to Dale Li
    Prodigy 100 points

    Hi Dale Li.

    Thank you, for your reply.

    I am working with your suggestions. Now I had done changes in the schematic. Beginning I was working in ADS8691 EVM. During power-up, some configuration is done.

    Can you please tell me the configuration need to be done in software while booting up

    we are using Range selection as 1.25 * V ref

    Thanks in advance.

  • 0 in reply to Sarathi Thirugnanam
    TI__Mastermind 38215 points

    Hi Sarathi,

    I suggest you to test according to two steps as below:

    Firstly, don't change input range, test directly by applying a DC signal on ADC's input after power up (default configurations:  ±3 × VREF and internal Voltage reference), check if conversion code is correct with ±3 × VREF range. If the code is correct, this means that your timing also hardware connection are correct and ADC is working well.

    Secondly, change input range to 1.5 × VREF according to the command as below:

    Data frame:        <11010_10><000010100><00000000><00001011>

    Then, by applying a same DC voltage on ADC input (make sure the signal is in the range), check if conversion code is correct with 1.5 × VREF range. If yes, this means that your register programming is successful.

    Remark:   use Half-word write command:     11010_10_<9-bit address>_ <16-bit data>     With this command, only the LS byte of the 16-bit data word is written at the specified register address.

    Please let me know your result, thanks.

    By the way, we don't have ADS8691EVM which is 18-bit version in same family, I suppose you are trying to test on ADS8681EVM, this command can be used for ADS8681, but you can't test ADS8691 by replacing ADS8681 on this EVM board, just let you know.

    Thanks&Best regards

    Dale

  • 0 in reply to Dale Li
    Prodigy 100 points
    Hi Dale Li..,

    Sorry for delayed response.

    I tried as per your suggestion, I am unable to receive any data in the SD0 pin.

    I am using master as Raspberry pi 3 board - bcm2835 Library.

    From the master board, I need to read the ADC port of ads8691.

    I did all the configurations which are listed below:

    Reset power control register (Bitorders mentioned in 0 - 7 direction)
    Initialization - 0xD000, 0x05, 0x00, 0x69 (To enable write of 5-0 Bits of Reset power control register)
    Write - 0xD000, 0x04, 0x00, 0x0C ( Active mode(0), Disable NAP mode of Converter(0), PoR(0),Reserved(0), Input
    alarm disabled(1), VDD alarm disabled(0), Reserved (00))
    SDI Control register - 0xD000, 0x08, 0x00, 0x00 (CPOL = 0, CPHASE = 0, Reserved (000000)
    SDO Control register - 0xD000, 0x0C, 0x00, 0x02 (Same protocol followed for SDI (00), 0000 (Reserved), 1(Internal clock),
    0(Reserved))
    Data out control register - 0xD000, 0x10, 0x00, 0x08 (000(Value output is the conversion data),1(Two parity bit are appended to the
    LSB's of the output data) ,0000(Reserved))
    Range selection register - 0xD000, 0x14, 0x00, 0x00 (0000(+/- 3 x Vref),00(Reserved), 0(Internal reference is enabled), reserved(0))

    Before configuration Reset pin made low followed by 1000ms delay and again made High. (Done in program)
  • 0 in reply to Sarathi Thirugnanam
    TI__Mastermind 38215 points

    Hi Sarathi,

    I didn't expect you programmed internal registers directly. As suggested above, firstly we should make sure your ADC is working and you can get correct conversion data after power up ADC, nothing should be programmed after power up ADC and only one reset signal is desired on /RST pin after power up, by default, input range is ±3 × VREF, internal reference is used,1-bit SDO mode, Standard SPI with CPOL = 0 and CPHASE = 0. Please work on this and check your timing with a scope as below including RVS signal, also check if your conversion code is correct with these default configurations.

    Please note that 32 are expected within a data transfer frame. Please let me know your result with this test. Thanks.

    Best regards

    Dale