ADS1248 s/w management for RTD interface (PT1000)

Hi Paulo,

The basic steps are as follows:

• Power up the device
• Set RESET high and wait the determined start up time as per datasheet
• Set START high (required to read/write the registers)
• Set the register configurations used
  • PGA
  • Data rate
  • Mux configuration
  • Turn on internal reference
  • Select reference to be used as REF1
  • Select IDAC current
  • Select IDAC routing
  • Issue a SELFOCAL
• Wait for DRDY to transition from high to low
• Read data and change mux channel and IDAC routing
• Wait for DRDY to transition from high to low
• Read data and change mux channel and IDAC routing
• keep repeating.....

The actual register selections will be determined by your design.

Best regards,
Bob B

Hi Paulo,

The internal logic for the ADS1248 is rather complicated.  The logic for the resetting of DRDY is seperate from the SPI interface itself.  If you look at page 35 of the datasheet this is discussed in the CS section. At the end of the first paragraph of that section there is a statement "DRDY pin operation is independent of CS."  What you are seeing is the behavior of the device.  If this is problematic for you, read my response in this post:

http://e2e.ti.com/support/data_converters/precision_data_converters/f/73/t/160153.aspx

The above thread is for the ADS1247.  As the ADS1247 and ADS1248 are the same family of devices, the control application would apply to both devices.

Best regards,

Bob B

Dear Bob,

sorry I did not focus on that post before, it seems it's 100% same as my situation.

And we also are solving the issue by software: we do all things on 1st ADS1248 "forgetting" the 2nd one, then we do all things on 2nd ADS1248 "forgetting" the 1st one.

Next PCB revision I will bring independent MOSI and SCLK to the converters.

Thanks a lot for your valuable support.

My best regards

Paolo

Dear Bob,

I am reopening this issue because we're about to start MP for the product where we use the ADS1248 and we are having speed problems.

In other words, we cannot raise up the speed over 5SPS! 

Here below is the sequence of operations we are using now. Just FYI see that for each reading of a conversion, we have to read and discard the first reading from the ADC, otherwise we get strange readings.

I have tried a lot of different configurations, I have even thought of some mistake I made in the h/w design (attached too). The filters have 2.2 milli-second time constant, so I don't think this is the reason why it's impossible to run faster.

Honestly speaking, the 5 SPS seems to work fine and it was pretty acceptable solution, until some inside customer's tech office wanted to have a faster reading. Now, in fact, due to all delays and retries, temperatures are updated about every 2~3 seconds; the new request is to have a refresh every 200 ms or faster.

Thanks a lot for the help which surely you will give.

I have attached the latest schematic of the ADS1248 interface for 4x RTD PT1000

My best regards

Paolo

paolo.bozzzola@cjb.it 

ACTUAL SEQUENCE OF OPERATIONS

Startup & Initialization (just done once at the beginning)

1         set signal RESET# active (low level)

2         set SLCK clock not active (low level)

3         set chip select CS# not active (high level)

4         set START active (high level)

5         wait 10 milliseconds

6         release RESET# (set high level)

7         Wait 2 milliseconds                              (t RHSC = 0.6 milliseconds,  our delay is affordable)

8         write 0x28 into register 0x02 (MUX1)

- Internal oscillator in use

- Internal reference always on

- REF1 input pair selected

- Normal Operation (default)

9         write 0x00 into register 0x03 (SYS0)

- PGA Gain = 1

         - Output data rate = 5 SPS

10     write 0x04 into register 0x0a (IDAC0)

- DOUT/DRDY pin functions only as Data Out (however we use DRDY# on pin 25)

          - IMAG : Magnitude of the excitation current =  500 µA

11     Send SYSOCAL command                                                                                            

12     Wait 2000 milliseconds

13     Write 0x07 into register 0x00  (MUX0)

- Select AIN0 as positive input channel

- Select AIN7 as negative input channel

14     Write 0xf8 into register 0x0b (IDAC1)

- Output pin of Excitation current source 1 - disabled

- Output pin of Excitation current source 2 = IEXT1                                         

15     Wait 100 milliseconds

16     Wait conversion has been completed

17     Read conversion result and discard it                     

18     Wait 2 milliseconds                                                         

Loop :

(1^st RTD)              

1         (Initial activity to get conversion on AIN0):  Assert START Signal (high level)

2         Write 0x07 into register 0x00  (MUX0)

- Select AIN0 as positive input channel

- Select AIN7 as negative input channel

3         Write 0xf8 into register 0x0b (IDAC1)

- Output pin of Excitation current source 1 – always disabled

- Output pin of Excitation current source 2 – IEXT1 (pin 20 of ADS1248)                                  

4         Wait 2 milliseconds

5         Wait the conversion has been completed, checking when DRDY# goes low (pin 25 of ADS1248)      

6         Get data from chip and discard them                                     

7         Wait 1 millisecond

8         Wait another conversion has been completed, checking when DRDY# goes low (pin 25 of ADS1248)

9         Release START Signal (low level)

10     Read Conversion (Relative to couple AIN0 – AIN7) and save the result

(2^nd RTD)

11     (Initial activity to get conversion on AIN1):  Assert START Signal (high level)

12     Write 0x0f to register 0x00  (MUX0)

- Select AIN1 as positive input channel

- Select AIN7 as negative input channel

13     Write 0xf9 to register 0x0b (IDAC1)

- Output pin of Excitation current source 1 – always disabled

- Output pin of Excitation current source 2 – IEXT2 (pin 19 of ADS1248)                                                  

14     Wait 2 milliseconds

15     Wait the conversion has been completed, checking when DRDY# goes low (pin 25 of ADS1248)

16     Get data from chip and discard them                                     

17     Wait 1 millisecond

18     Wait another conversion has been completed, checking when DRDY# goes low (pin 25 of ADS1248)

19     Release START Signal (low level)

20     Get Conversion (Relative to couple AIN1 – AIN7) and save the result

(3^rd  RTD)

21     (Initial activity to get conversion on AIN2):  Assert START Signal (high level)

22     Write 0x17 to register 0x00  (MUX0)

- Select AIN2 as positive input channel

- Select AIN7 as negative input channel

23     Write 0xf4 to register 0x0b (IDAC1)

- Output pin of Excitation current source 1 – always disabled

- Output pin of Excitation current source 2 – AIN4 (pin 13 of ADS1248)                                   

24     Wait 2 milliseconds

25     Wait the conversion has been completed, checking when DRDY# goes low (pin 25 of ADS1248)

26     Get data from chip and discard them                                     

27     Wait 1 millisecond

28     Wait another conversion has been completed, checking when DRDY# goes low (pin 25 of ADS1248)       

29     Release START Signal (low level)

30     Get Conversion (Relative to couple AIN2 – AIN7) an save the result

(4^th  RTD)

31     (Initial activity to get conversion on AIN3):  Assert START Signal (high level)

32     Write 0x1f to register 0x00  (MUX0)

- Select AIN3 as positive input channel

- Select AIN7 as negative input channel

33     Write 0xf5 to register 0x0b (IDAC1)

- Output pin of Excitation current source 1 – always disabled

- Output pin of Excitation current source 2 – AIN5 (pin 14 of ADS1248)                                   

34     Wait 2 milliseconds

35     Wait the conversion has been completed, checking when DRDY# goes low (pin 25 of ADS1248)

36     Get data from chip and discard them                                     

37     Wait 1 millisecond

38     Wait another conversion has been completed, checking when DRDY# goes low (pin 25 of ADS1248)

39     Release START Signal (low level)

40     Get Conversion (Relative to couple AIN3 – AIN7) an save the result

41     Go to loop

Paolo,


At this point, I would concentrate on the reference and it's response time. If your reference is turned on and off, I would look at reducing C90 from 10uF to 1uF. It's unlikely that this is that large of a contributor, but it's something.

Another thing I can see is the REFP1 and REFN1 inputs. The combination of C91 and R89 plus R91 makes a time constant that reaches into the milliseconds. With settling, you'd need maybe 10x that amount. By switching in different current paths, it might be briefly changing the reference value and requiring extra settling time. Start by reducing R89 and R91 to 1k and see if that helps.


Joseph Wu

Dear All,

for the benefit of people using ADS1248, we have found the hidden tricks to get perfect ADS1248 behavior.

Our speed problems have been due to 2 things:

1) the incorrect "position" of execution inside the program of the SYSOCAL command. 
It must be done BEFORE any register initialization!

2) excess RC filter value for the 4 inputs, where the 2.2uF has been now changed ionto 100nF. We have also changed C90 to 1uF. See attached schematic which reports new values.

Here below there is the ultimate code:

void thermoThread(void)

{

        static embtime_t tmCold;

        static embtime_t tm;

        static u8 state = THERMOTHREAD_STATE_SETUP_START;

        static u8 sonda;

        u8 tmp;

        u32 val;

        if (time_fired(tmCold)) {

               tmCold = time_ms(100);         // cold joint temp read sometimes

               temperatureBase = getTemperatureBase();

        }

        switch (state) {

        case THERMOTHREAD_STATE_SETUP_START:

               state = THERMOTHREAD_STATE_SETUP_HARDWARE_SETUP;

               break;

        case THERMOTHREAD_STATE_SETUP_HARDWARE_SETUP:

               clrSpi1Clock();

               assertReset(); //Put Chip in ResetMode (no operations)

               releaseReset(); //Release Reset Pin (start chip)

               releaseCs(0);

               releaseCs(1);

               assertStart(0); // start pin must be asserted from now!

               assertStart(1); // start pin must be asserted from now!

               tm = time_ms(10);              // puls must last 4 ms min

               state = THERMOTHREAD_STATE_SETUP_RESET_ADS;

               break;

        case THERMOTHREAD_STATE_SETUP_RESET_ADS :

               if (time_fired(tm)) {

                       spiAdcSendByte(0, ADS1248_CMD_RESET);

                       spiAdcSendByte(1, ADS1248_CMD_RESET);

                       tm = time_ms(5);               // post reset time (at least 0,6 ms but better longer)

                       state = THERMOTHREAD_STATE_SETUP_STOP_CONTINUOUS_MODE;

               }

               break;

        case THERMOTHREAD_STATE_SETUP_STOP_CONTINUOUS_MODE :

               if ((getDRDYConversionComplete(0) == 0) || (getDRDYConversionComplete(1) == 0)) {

                       break;

               }

               AdcStopContinuousRead(0);

               AdcStopContinuousRead(1);

               tm = time_ms(100);

               state = THERMOTHREAD_STATE_SETUP_OFFSET_CALIBRATION;

               break;

        case THERMOTHREAD_STATE_SETUP_OFFSET_CALIBRATION:

               if (time_fired(tm)) {

                       AdcSendSysOcal(0);

                       AdcSendSysOcal(1);

                       tm = time_ms(100);

                       state = THERMOTHREAD_STATE_SETUP_CONFIG_REGISTERS;

               }

               break;

        case THERMOTHREAD_STATE_SETUP_CONFIG_REGISTERS:              // set registers which stay the same along the whole conversion

               if (time_fired(tm)) {

                       tmp = 0x28; // internal ref on, ref1 selected, normal op

                       AdcWriteReg(0, ADS1248_MUX1_REGISTER, &tmp, 1);

                       AdcWriteReg(1, ADS1248_MUX1_REGISTER, &tmp, 1);

                       tmp = 0x03;                    // gain1 - 40 sps

                       AdcWriteReg(0, ADS1248_SYS0_REGISTER, &tmp, 1);

                       AdcWriteReg(1, ADS1248_SYS0_REGISTER, &tmp, 1);

                       tmp = 0x04;            //500uA current source DRDY normal mode

                       AdcWriteReg(0, ADS1248_IDAC0_REGISTER, &tmp, 1);

                       AdcWriteReg(1, ADS1248_IDAC0_REGISTER, &tmp, 1);            

                       tm = time_ms(500);

                       state = THERMOTHREAD_STATE_SETUP_FINALIZE;

               }

               break;

        case THERMOTHREAD_STATE_SETUP_FINALIZE:

               if (time_fired(tm)) {

                       if ((getDRDYConversionComplete(0) == 0) || (getDRDYConversionComplete(1) == 0)) {

                               break;

                       }

                       tm = time_ms(100); // wait 100mS before start loop readings

                       state = THERMOTHREAD_STATE_LOOP_START;

               }

               break;

        // Continuous Reading Loop

        case THERMOTHREAD_STATE_LOOP_START:

               if (time_fired(tm)) {

                       ConfigureMux(0, sonda);

                       ConfigureMux(1, sonda);

                       tm = time_ms(2);

                       state = THERMOTHREAD_STATE_LOOP_WAIT_DATA;

               }

               break;

        case THERMOTHREAD_STATE_LOOP_WAIT_DATA :

               if ((getDRDYConversionComplete(0) == 0) || (getDRDYConversionComplete(1) == 0)) {

                       break;

               }

               state = THERMOTHREAD_STATE_LOOP_READ;

               break;

        case THERMOTHREAD_STATE_LOOP_READ:

               val = AdcReadConversion(0);

               tcRawValues[0+sonda] = val >> 8;

               val = AdcReadConversion(1);

               tcRawValues[4+sonda] = val >> 8;

               if (++sonda == 4) {

                       sonda = 0;

               }

               state = THERMOTHREAD_STATE_LOOP_FINALIZE;

               break;

        case THERMOTHREAD_STATE_LOOP_FINALIZE:

               state = THERMOTHREAD_STATE_LOOP_START;

               break;

        }

}

As you can see the thread is a state-machine.

The code is quite readable.

After... so many efforts, whoever needs some help can contact us!

Paolo Bozzola

paolo.bozzola@cjb.it 

Paolo,


Calibration should be done AFTER register initialization. The device should be set into the final configuration that you need before calibrating it to remove errors. If you imagine that the input PGA has an offset error, you want to set the PGA, and then calibrate out the offset error. If you run the calibration first and then set the PGA, your offset is probably no longer the same in a different gain.

Incidentally, most people will not run a SYSOCAL. In a system offset calibration, you need to be able to short the inputs together outside the device and run the SYSOCAL. Often, people will configure the device they way they want and then run a SELFOCAL. In this case, a self offset calibration will tie the inputs together internally to the device and run the offset calibration. If you need an offset calibration, you may want to do it any time you change the gain or perhaps the data rate.

Excess RC filter values can cause some delay in measurement. This not only applies for the analog inputs, but for the reference inputs as well. If you consider that the ADC compares the analog input to a reference value, both are similar in what they require for bandwidth and settling.


Joseph Wu