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ADS1282 EVM without PDK interfaced with Maple Native, unable to read register or sample

Gaute Hope
Prodigy 100 points
Other Parts Discussed in Thread: ADS1282, ADS1281, OPA1632

Hi,

I'm trying to get the ADS1282EVM board to work with a Maple Native (Arduino like), I have set up the I2C bus and as far as I know connected SPI and nDRDY correctly, as well as powering the board properly. Could you please provide a list of how jumpers and switches should be configured so that I can check with how I have set it up.

I want to use the on-board 4 MHz crystal for clock, but use SCLK from the Maple for SPI. I have contact with the I2C device (U7/PCA9535RGE) and have configured outputs and signals as specified in datasheet. One question to this though: On the schematic for the EVM the lines from U7 is marked i.e. PDWN connected to NOT PDWN (active low) on the ADS1282 - should the I2C output a low or a high on that line? Same goes for RESET..

I do get nDRDY after a while when setting PDWN to HIGH, SYNC HIGH and RESET HIGH.

My startup goes like this:
- Configure U7 to output SYNC: HIGH, PDWN: HIGH, RESET: HIGH and the following LOW: M0, M1, MCLK, SUPSOR and the following as inputs: PMODE, MFLAG, EXTCLK.
- Reset by pulling RESET LOW for 100 ms, then HIGH
- Wait 5 s
- Issue RESET command on SPI (causes change in DRDY signal)
- Wait 5 s
- Issue SDATAC command on SPI
- Wait 5 s
- Issue RREG 1 to 6
- Shift in 6 bytes

I always get empty (0) registers and if I try to read DOUT after RESET I also get 0. DRDY continues to stay LOW after my start up sequence is done. 

Do you have any tips for what I can try? Or as mentioned above; how are the jumpers/switches supposed to be; and is my assumption with the output from U7 correct?

Regards, Gaute

  • 0
    TI__Guru 55466 points

    Hello Gaute,

    Setting the SYNC, RESET and PWDN high should be the default condition.

    When using the ADS1282EVM  M0, M1, MCLK are the modulator signals when running in modulator mode;  and on normal operation, they should be tied.  The EVM has them set up by default as M0 low, M1 high, and MCLK low through the pull resistors.  This EVM board design supports both the ADS1282 and the ADS1281.  The PINMODE function is not available on the ADS1282, it becomes DGND when the board has the ADS1282 device installed. 

    You must ensure that the 4.096MHz external clock oscillator is enabled.  You can enable it by installing jumper J3 (pin 3-2); or setting PO5 high on the PCA9535RGE.  You also need to have Jumper J2 (pin1-2) installed to connect the output of the oscillator to the ADS1282 device.   Ensure jumper J10 (pin 1-2) DIN is installed.

    Please let me know if you still have issues.

    Best Regards,

    Luis

     

  • 0 in reply to Luis Chioye
    Prodigy 100 points

    Thanks, that verified it. I eventually got contact over SPI, I had to change the way I clocked out the signal, a working version:

    with uint8_t v:

    /* Write each bit, MSB first */
    for (int i = 7; i >= 0; i--) {
      digitalWrite (AD_DIN, !!(v & (1 << i)));
      digitalWrite (AD_SCLK, HIGH);
      digitalWrite (AD_SCLK, LOW);
    }

    delayMicroseconds (11); // delay, min: 24 / fclk
  • 0 in reply to Gaute Hope
    Prodigy 100 points

    Continuing my quest I am struggling with the bipolar inputs and the references.. I wish to sample a signal originally +-6V, divided down to +-1.25V using resistors. I have left SUPSOR (P03) HIGH in +-2.5V mode and J1 connectd to REF5050 with S1 selecting onboard reference. If I am correct this should create a reference of 5V from -2.5V to +2.5V ? And should allow a signal to measure between (-2.5 + 0.7) V to (2.5 - 1.25) V, is this correct ?

    So with AINN1 connected to 0V, AINP1 should have a range from -1.25 to 1.25V ?

  • 0 in reply to Gaute Hope
    TI__Guru 55466 points

    Hello Gaute,

    If the ADS1282 device is biased with +/-2.5V; and the REF5050 (5V reference) is referred to AVSS = -2.5V; therefore the device has an effective 5V reference accross its VREP and VREFN inputs.

    The full-scale differential voltage that can be converted by the ADS1282 is +/-VREF/(2 x PGA); so if we let for example, VREF=5V and set  PGA gain =1; the device can convert differential voltages of AINP-AINN = +/-2.5.  The absolute voltage input range at the inputs of the ADS1282 is AVSS+0.7V and AVDD -1.25V; therefore the inputs of the ADS1282 have an absolute voltage range of -1.8V to +1.25V (voltage referred to GND on the input pins)

    On the ADS1282EVM board, the ADS1282 device is preceded by the OPA1632, which is a fully-differential amplifier (OPA1632) configured with Gain=1. The gain of the differential amplifier is set by the ratio of its feedback and input resistors. The differential output stage  will provide a differential output voltage with a common-mode voltage centered around the VOCM pin.  In the ADS1282EVM board, the VOCM pin is biased at the middle of the supplies; or in your example AVDD/AVSS =+/-2.5V and VOCM=GND.  The differential VOUT(+) and VOUT(-) will swing around the VOCM pin, keeping the differential signal centered around GND. 

    For example,  if you let on the analog input header J6, A0(-)=GND, and A0(+) = 2.5V; the Vout(+) of the fully differential amplifier (OPA1632) will be Vout(+) =+1.25V and Vout(-) = -1.25V; where the ouput differential signal is centered around common-mode of 0V; and the differential output voltage is VOUT(+) - VOUT(-) = + 2.5V

    So if you are using the setting with PGA=1, VREF =5V and you are also using the OPA1632 set with a Gain=1; the user could have obtain the full-scale input voltage range of +/-2.5V.  You could also adjust the gain of the OPA1632 by changing the input/feedback resistor ratio.

    Attached below is an application that explains in detail Fullly differential amplifiers.

    http://www.ti.com/lit/an/sloa054d/sloa054d.pdf

     Best Regards,

    Luis

  • 0 in reply to Luis Chioye
    Prodigy 100 points

    [Edit]: I put in an audio opamp between signal and input terminals to scale my signal and it seems to have done the trick. Thanks for the help so far.

    Hi Luis, thanks for the comprehensive answer - you are very helpful. Since I am using the EVM board, and powering it with the +5V and -5V (-6V actually, but from the datasheet of TPS72325DBV that should be fine..?) - these should be regulated down to +2.5V and -2.5V by U10 and U13. As long as SUPSOR / GPIO3 is left pulled up/set HIGH. I use REF5050 for a Vref 5V (-2.5 to 2.5V). The opamps are supplied with -12V and +12V, which should also be within range.

    When I try to attach a DC signal (using the +5V or -5V source) with a potentiometer to vary the signal between 0 and +-2.5V, setting A0(-) to GND and A0(+) to the divided voltage from the potentiometer, the inputs A0 connected to the differential amplifiers drive the voltage up over the resistor I am measuring over.. I am having trouble locating my error, would a higher frequency signal through a capacitor work differently? Is it current from D1A or D2A flowing back over the feedback loop, and is an high impedance signal source expected/required?

    Best regards, Gaute

  • 0 in reply to Gaute Hope
    TI__Guru 55466 points

    Hi Gaute,

    Yes, adding the buffer infront of the potentiometer may have help since the impedance of the potentiomerter may interact with the input resistors of the OPA1632. 

    Thank you and Best Regards,

    Luis