ADS1262: INPMUX register are not saved

Hi Hal,

Just to clarify, the register "groupings" just delay register updates in that group until after you've finished writing to all of the registers in that group when performing multi-register writes. The reason for this is if you were to increase the PGA gain and also switch MUX channels at the same time, you would might otherwise over-range the PGA momentarily if the MODE2 register were to update before the INPMUX register. Instead, these registers are grouped so that writing to both of them in sequence (during a multi-register write command) only takes effect after you finished clocking in the data to both registers. However, if you only write to one register at a time, registers will update immediately after completing the WREG command, as you'd likely expect.

Therefore, this issue should not have anything to do with the grouping.

There is a lot of overshoot on the DIN and SCLK signals, between 0.5V and 1V as best as I can tell. Are you able to clean this signals up at all? I am not sure you will get very good results with such noisy inputs.

You can try enabling the CRC and then reading back the value of the read with the CRC enabled. This can help you determine what is going on based on what you send to the ADC and then what data the ADC provides back.

Also, does this happen on any other registers? You said that this seems to work okay with the INTERFACE register, though that is just reading back the default value (which is the case when you read back the INPMUX register). Can you test out some other registers before and after writing to them to see what happens?

-Bryan

Hi Bryan, thank you for the support.

It actually an issue with the SPI on C2000 not the ADS1262. For some reason the first time I write/read on the SPI nothing happen.

So right now I just do a dummy write/read the first time. Then I can see all the messages that I write and read are OK.

Now I dig more with the ADS1262 and I experience some spikes in the reading.

I configured the registers pretty much the same as the example where I set the REF mux to RMUXP_INTP and RMUXP_INTN and then read MUXP_AVDD with MUXN_AVSS:

//setSTART(1);
    /*********DUMMY READ********/
    ADS126xSendResetCommand(); // dummy stuff to set the SPI

    //LoadInitRegSettings();

    //ADS126xReadRegister(POWER, 1, resultData);
    /**************************/

    //MODE0 (CHOP OFF)
    config[0] = (AdcRegData[MODE0] & DELAY_MASK);
    ADS126xWriteRegister(MODE0, 1, config);

    //MODE2 (BYPASS OFF, GAIN = 1 V/V, )
    config[0] = 0x04;//(AdcRegData[MODE2] & DR_MASK);
    ADS126xWriteRegister(MODE2, 1, config);

    //Reset OFCAL coefficients
    config[0] = 0x00;
    ADS126xWriteRegister(OFCAL2, 1, config);     //OFCAL2 = 0x00
    config[0] = 0x00;
    ADS126xWriteRegister(OFCAL1, 1, config);     //OFCAL1 = 0x00
    config[0] = 0x00;
    ADS126xWriteRegister(OFCAL0, 1, config);     //OFCAL0 = 0x00

    //Reset FSCAL coefficients
    config[0] = 0x40;
    ADS126xWriteRegister(FSCAL2, 1, config);     //FSCAL2 = 0x40
    config[0] = 0x00;
    ADS126xWriteRegister(FSCAL1, 1, config);     //FSCAL1 = 0x00
    config[0] = 0x00;
    ADS126xWriteRegister(FSCAL0, 1, config);     //FSCAL0 = 0x00

    //INPMUX (AINP = AIN0, AINN = AIN3)
    config[0] = MUXP_AVDD | MUXN_AVSS;
    ADS126xWriteRegister(INPMUX, 1, config);

    //REFMUX
    config[0] = RMUXP_INTP | RMUXN_INTN;
    ADS126xWriteRegister(REFMUX, 1, config);

    config[0] = STATUS|CRC_OFF; //0x04
    ADS126xWriteRegister(INTERFACE, 1, config);

    while(1){

        set_adc_START(1);
        DEVICE_DELAY_US(500);
//        result = ADS126xReadData(5, 1);
        result = ADS126xReadData(5, 1);
        VoltageReading = (double) result * IntRefVolt / ((double) Gain * exp2(29)); //2^29 to account for 1/4 supply & 1/2 FSR
        SupplyRefVolt = VoltageReading;
        ReadingmV = (int)(SupplyRefVolt*1000);

        if ((SupplyRefVolt>6.0))
        {
            GPIO_togglePin(LED_31);  //LED OFF
        }
    }

I read my AVDD with a DMM and measure 5.125V (+/- 1uV), pretty stable.

Now if in debug mode I look at the SupplyRefVolt, I have a good result 5.1258V but I also have spikes coming.

I can see those spikes on the SPI too.

First question, in my example the datarate is by default 20SPS, what happen when I read the register at 1.6KHz? I should expect to read the same value till a new value is written (every 50ms).

Second question, the status byte is 0x41 when the data is good and I measure 5.125V. When I see those spike the status byte is garbage:

Status byte I receive: 0x86, 0x46, 0xA3, 0x14, 0xB1

I feel like I receive stuff that I should not receive, but during the configuration I made sure I disable CRC and enable status.

Do you see anything wrong with my config?

Thank you

Hi Hal,

How do you know when data is ready to be retrieved from the ADC? I don't see any calls to a function that monitors DRDY. Are you retrieving the STATUS byte to know when new data is available instead?

If you are using read data direct mode, then sending additional SCLKs will continuously retrieve whatever data is in the ADC output shift register. This data should not change between conversion periods, it should only change after each conversion period completes. Therefore if you read data multiple times during the same conversion period, you should be reading the exact same result each time.

As noted in your scope captures previously, there appeared to be a large amount of noise on your digital signals. I am not sure if this is causing the issue you are seeing, but it is certainly something that should be addressed. I would also suggest looking at the data output using a logic analyzer to better understand what is actually being supplied by the ADC. You mentioned that your previous issue has something to do with the C2000 operation, so it is certainly possible that these new issues are also a result of the processor.

-Bryan

Hi Bryan,

I added

while(GPIO_readPin(ADS_DRDY_74)); //wait fopr DRDY to go low

before reading the data register and now it is good thank you very much

I am confused now, how I was able to read the voltage of AVDD (5V) using the 2.5V internal voltage ref. Normally with a GAIN=1 and ref mux =internal 2.5V, the full scale range is+/-2.5V, but I was able to read my 5.12V supply:

        set_adc_START(0);
        DEVICE_DELAY_US(100);
        config[0] = MUXP_AVDD | MUXN_AVSS;
        ADS126xWriteRegister(INPMUX, 1, config);
        config[0] = RMUXP_INTP | RMUXN_INTN;
        ADS126xWriteRegister(REFMUX, 1, config);
        config[0] = 0x09;//(AdcRegData[MODE2] & DR_MASK);
        ADS126xWriteRegister(MODE2, 1, config);
        set_adc_START(1);
        result4 = ADS126xReadData(5, 1);

Other than that, I connected my load cell as below:

Reading AIN1 and AIN2 using the internal ref 2.5V to get a bigger range:

        set_adc_START(0);
        DEVICE_DELAY_US(100);
        config[0] = MUXP_AIN1 | MUXN_AIN2;
        ADS126xWriteRegister(INPMUX, 1, config);
        //config[0] = RMUXP_AIN0 | RMUXN_AIN3;
        config[0] = RMUXP_INTP | RMUXN_INTN;
        ADS126xWriteRegister(REFMUX, 1, config);
        config[0] = 0x59;//(AdcRegData[MODE2] & DR_MASK);
        ADS126xWriteRegister(MODE2, 1, config);
        set_adc_START(1);
        while(GPIO_readPin(ADS_DRDY_74)); //wait fopr DRDY to go low
        result = ADS126xReadData(5, 1);

The signal is pretty noisy, around (4500000-3500000)=1000000 counts:

For the full range of the loadcell (compression+tension) I have around 160000000 counts. This gives roughly (4500000-3500000)*100/160000000  = 0.625% noise.

Changing the Vref from internal to AIN0 and AIN3 gives pretty much the same results:

around (2200000-1700000)=500000 counts of noise, full range of the loadcell (compression+tension) I have around 80000000 counts, same ration signal / noise.

So from my understanding, I should keep using the internal Vref , at least I have more resolution. Do you have any thoughts on achieving better reading? I did my best for the layout, followed the design guidelines.

On my board, the excitation of the load cell is coming from a switching DC/DC 5V that power the rest of the board AVDD=DVDD. The signal is noisy and coupled with the SPI (around 430mV Vp-p), do you think having a low noise LDO 5V will increase the performance I can get?

I am planning on using a dedicated ultralow noise LDO (LT3045):

After that I want to connected 3 load cells on the ADS1206. They will have the same excitation and  same reference, do you see any problem with that?
ADS1262.pdf

Note that I put a UCC27524DR driver if I want to implement the AC excitation in the future.

Last question, what is the impact on the choice of clock, Internal oscillator, External clock or External crystal. I could not find in the datasheet what is the rational behind adding an external crystal. What is the gain, better performance?

Thank you

Thank you so much for you help Bryan.

Will they be connected in parallel then, where you are measuring the differential output of each bridge with a separate differential ADC analog input? I do not see any issue with this.

Yes they are: 

I will take a look at the bridge measurement guide and the TPS7A94 for later implementation, but it looks like it is out of stock for now.