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ADS1278-EP: Readings not stable.

pavan
Intellectual 850 points
Part Number: ADS1278-EP
Other Parts Discussed in Thread: THS4521, ADS1278

Hi,

I am using ADS1278-EP  in conjuction with SM470R1B1M in my data acquisition system. The readings were quite unstable while i have stable voltage at ADC inputs. I am using channels 1,2 and 3. I have a voltage around 3mV , 260 mV, 2.4V respectively. Readings are stable only at channel 3. when i have low input voltage the readings are quite unstable.

I always read +/- 3mV mean at 260mV and quite unstable at 3mV range.

The readings are from a transducer, we connected through THS4521 Diff op-amp and without op-Amp each case it is same.

ADS1278 is operating low speed mode. CLOCK is = 3.75 MHz and SPICLK = 1.875MHz.

Any help appreciated.

Regards,

Pavan.

  • 0
    TI__Genius 14540 points
    Hi Pavan,

    Have you confirmed that the VREFP and VREFN voltages are stable and clean? Also, a schematic might be helpful.

    thanks
    Christian
  • 0 in reply to Christian Yots
    Intellectual 850 points

    Hi Christian,

    Thanks for your quick reply,

    Yes, VREFP and VREFN are stable. Yellow is VREFP and Blue is VREFN and the red is Difference. This is just to check noise level with AC coupling.

    We are using three channels as i mentioned. Channel 1 readings are quite stable and accurate up-to micro Volts. This is differential voltage with respect to ground, on Channel 2 and 3 we have differential inputs. Please see the attached schematic below. 

    We have one I/O pin to power down both channel 2 and channel 3. These values are unstable and completely wrong more often. I have tested with range from 0 to 3.3V through the range it was +/- mV mean and some times completely wrong values even we have stable input at ADC. I have to read sequentially because we have one I/O pin to power up/down. I am reading 48 bits sequentially. 

    I have a function read two channels like this

    unsigned int ADC1278_48( unsigned int value) 
{ 
    unsigned int data=0, data1=0;    
    
    SW_Delay(DELAY_200US);   
    
    while (I2C2DIN & 0x0001);        //DRDY
    
    data = spi(0);     // read hi-byte            //Channel2
    data = (data << 8)| spi(0);
    data = (data << 8) | spi(0);    // and lo-byte.   
    Temp = data; 
    data1 = spi(0);     // read hi-byte in next data     //Channel3
    data1 = (data1 << 8)| spi(0);
    data1 = (data1 << 8) | spi(0);    // and lo-byte.
    Temp1 = data1;    
   
}

    I am not sure why some times i read complete wrong data (out of range).

    I have attached log file with data you can see how often we are getting wrong and unstable.

    Log1: This log is only reading first 24 bits, channel 2. Real value at ADC input is 4mV. 

    Log2: is reading 48 bits but printing only channel 3. Real value at ADC input is 260mV. 

    Regards,

    Pavan.

    Schematic1 is analog part, connected to channel 3. Channel 2 circuit also similar.

    ADC Schematic.

    Log1.txt 
    Raw value of Temp : 4096.000000

Temp  value [mV] : 2.441380

Raw value of Temp : 17.000000

Temp  value [mV] : 0.010133

Raw value of Temp : 7358.000000

Temp  value [mV] : 4.385663

Raw value of Temp : 6527.000000

Temp  value [mV] : 3.890353

Raw value of Temp : 6559.000000

Temp  value [mV] : 3.909426

Raw value of Temp : 2715.000000

Temp  value [mV] : 1.618249

Raw value of Temp : 6991.000000

Temp  value [mV] : 4.166915

Raw value of Temp : 3116.000000

Temp  value [mV] : 1.857261

Raw value of Temp : 5920.000000

Temp  value [mV] : 3.528557

Raw value of Temp : 5376.000000

Temp  value [mV] : 3.204311

Raw value of Temp : 7680.000000

Temp  value [mV] : 4.577587

Raw value of Temp : 6656.000000

Temp  value [mV] : 3.967242

Raw value of Temp : 10.000000

Temp  value [mV] : 0.005960

Raw value of Temp : 7519.000000

Temp  value [mV] : 4.481625

Raw value of Temp : 6489.000000

Temp  value [mV] : 3.867704

Raw value of Temp : 6804.000000

Temp  value [mV] : 4.055456

Raw value of Temp : 5286.000000

Temp  value [mV] : 3.150667

Raw value of Temp : 5212.000000

Temp  value [mV] : 3.106560

Raw value of Temp : 6178.000000

Temp  value [mV] : 3.682335

Raw value of Temp : 6438.000000

Temp  value [mV] : 3.837306

Raw value of Temp : 4800.000000

Temp  value [mV] : 2.860992

Raw value of Temp : 7936.000000

Temp  value [mV] : 4.730174

Raw value of Temp : 6144.000000

Temp  value [mV] : 3.662070

Raw value of Temp : 3.000000

Temp  value [mV] : 0.001788

Raw value of Temp : 21.000000

Temp  value [mV] : 0.012517

Raw value of Temp : 4861.000000

Temp  value [mV] : 2.897351

Raw value of Temp : 8126.000000

Temp  value [mV] : 4.843421

Raw value of Temp : 6220.000000

Temp  value [mV] : 3.707369

Raw value of Temp : 3334.000000

Temp  value [mV] : 1.987197

Raw value of Temp : 6272.000000

Temp  value [mV] : 3.738363

Raw value of Temp : 5760.000000

Temp  value [mV] : 3.433190

Raw value of Temp : 7168.000000

Temp  value [mV] : 4.272415

Raw value of Temp : 6144.000000

Temp  value [mV] : 3.662070

Raw value of Temp : 4096.000000

Temp  value [mV] : 2.441380

Raw value of Temp : 21.000000

Temp  value [mV] : 0.012517

Raw value of Temp : 5525.000000

Temp  value [mV] : 3.293121

Raw value of Temp : 7154.000000

Temp  value [mV] : 4.264070

Raw value of Temp : 6620.000000

Temp  value [mV] : 3.945785

Raw value of Temp : 4836.000000

Temp  value [mV] : 2.882449

Raw value of Temp : 5184.000000

Temp  value [mV] : 3.089871

Raw value of Temp : 4588.000000

Temp  value [mV] : 2.734632

Raw value of Temp : 5132.000000

Temp  value [mV] : 3.058877

Raw value of Temp : 5888.000000

Temp  value [mV] : 3.509484

Raw value of Temp : 6656.000000

Temp  value [mV] : 3.967242

Raw value of Temp : 6400.000000

Temp  value [mV] : 3.814656

Raw value of Temp : 1.000000

Temp  value [mV] : 0.000596

Raw value of Temp : 21.000000

Temp  value [mV] : 0.012517

Raw value of Temp : 7591.000000

Temp  value [mV] : 4.524539

Raw value of Temp : 6752.000000

Temp  value [mV] : 4.024462

Raw value of Temp : 5276.000000

Temp  value [mV] : 3.144707

Raw value of Temp : 5916.000000

Temp  value [mV] : 3.526173

Raw value of Temp : 6061.000000

Temp  value [mV] : 3.612598

Raw value of Temp : 6400.000000

Temp  value [mV] : 3.814656

Raw value of Temp : 8704.000000

Temp  value [mV] : 5.187932

Raw value of Temp : 5120.000000

Temp  value [mV] : 3.051725

Raw value of Temp : 2.000000

Temp  value [mV] : 0.001192

Raw value of Temp : 26.000000

Temp  value [mV] : 0.015497

Raw value of Temp : 6405.000000

Temp  value [mV] : 3.817636

Raw value of Temp : 6437.000000

Temp  value [mV] : 3.836709

Raw value of Temp : 4273.000000

Temp  value [mV] : 2.546879

Raw value of Temp : 6176.000000

Temp  value [mV] : 3.681143

Raw value of Temp : 6986.000000

Temp  value [mV] : 4.163936

Raw value of Temp : 6528.000000

Temp  value [mV] : 3.890949

Raw value of Temp : 6656.000000

Temp  value [mV] : 3.967242

Raw value of Temp : 7680.000000

Temp  value [mV] : 4.577587

Raw value of Temp : 8704.000000

Temp  value [mV] : 5.187932

Raw value of Temp : 27.000000

Temp  value [mV] : 0.016093

Raw value of Temp : 5040.000000

Temp  value [mV] : 3.004042

Raw value of Temp : 6493.000000

Temp  value [mV] : 3.870088

Raw value of Temp : 5506.000000

Temp  value [mV] : 3.281796

Raw value of Temp : 8683.000000

Temp  value [mV] : 5.175416

Raw value of Temp : 4982.000000

Temp  value [mV] : 2.969471

Raw value of Temp : 6068.000000

Temp  value [mV] : 3.616771

Raw value of Temp : 4604.000000

Temp  value [mV] : 2.744168

Raw value of Temp : 3584.000000

Temp  value [mV] : 2.136207

Raw value of Temp : 7936.000000

Temp  value [mV] : 4.730174

Raw value of Temp : 7680.000000

Temp  value [mV] : 4.577587

Raw value of Temp : 2.000000

Temp  value [mV] : 0.001192

Raw value of Temp : 27.000000

Temp  value [mV] : 0.016093

Raw value of Temp : 7099.000000

Temp  value [mV] : 4.231288

Raw value of Temp : 8062.000000

Temp  value [mV] : 4.805274

Raw value of Temp : 5354.000000

Temp  value [mV] : 3.191198

Raw value of Temp : 5711.000000

Temp  value [mV] : 3.403985

Raw value of Temp : 7839.000000

Temp  value [mV] : 4.672358

Raw value of Temp : 6272.000000

Temp  value [mV] : 3.738363

Raw value of Temp : 5120.000000

Temp  value [mV] : 3.051725

Raw value of Temp : 5888.000000

Temp  value [mV] : 3.509484

Raw value of Temp : 4096.000000

Temp  value [mV] : 2.441380

Raw value of Temp : 21.000000

Temp  value [mV] : 0.012517

Raw value of Temp : 5328.000000

Temp  value [mV] : 3.175701

Raw value of Temp : 5192.000000

Temp  value [mV] : 3.094640

Raw value of Temp : 6887.000000

Temp  value [mV] : 4.104928

Raw value of Temp : 6422.000000

Temp  value [mV] : 3.827769

Raw value of Temp : 5026.000000

Temp  value [mV] : 2.995697

Raw value of Temp : 9051.000000

Temp  value [mV] : 5.394758

Raw value of Temp : 6552.000000

Temp  value [mV] : 3.905254

Raw value of Temp : 7680.000000

Temp  value [mV] : 4.577587

Raw value of Temp : 6912.000000

Temp  value [mV] : 4.119829

Raw value of Temp : 7168.000000

Temp  value [mV] : 4.272415

Raw value of Temp : 22.000000

Temp  value [mV] : 0.013113

    Log1. Channel 2 readings

    Log2.txt 
    Raw value of Temp: 439365.000000

Temp sensor value [mV] : 261.879120

Raw value of Temp: 393217.000000

Temp1  value [mV] : 234.373062

Raw value of Temp1: 439369.000000

Temp1  value [mV] : 261.881500

Raw value of Temp1: 439337.000000

Temp1  value [mV] : 261.862427

Raw value of Temp1: 64006.000000

Temp1  value [mV] : 38.150135

Raw value of Temp1: 1443507.000000

Temp1  value [mV] : 860.387939

Raw value of Temp1: 54920.000000

Temp1  value [mV] : 32.734516

Raw value of Temp1: 439709.000000

Temp1  value [mV] : 262.084167

Raw value of Temp1: 439484.000000

Temp1  value [mV] : 261.950043

Raw value of Temp1: 135181.000000

Temp1  value [mV] : 80.573280

Raw value of Temp1: 393217.000000

Temp1  value [mV] : 234.373062

Raw value of Temp1: 439303.000000

Temp1  value [mV] : 261.842163

Raw value of Temp1: 439358.000000

Temp1  value [mV] : 261.874939

Raw value of Temp1: 63238.000000

Temp1  value [mV] : 37.692379

Raw value of Temp1: 2754228.000000

Temp1  value [mV] : 1641.630005

Raw value of Temp1: 393217.000000

Temp1  value [mV] : 234.373062

Raw value of Temp1: 439299.000000

Temp1  value [mV] : 261.839783

Raw value of Temp1: 439291.000000

Temp1  value [mV] : 261.835022

Raw value of Temp1: 99078.000000

Temp1  value [mV] : 59.054451

Raw value of Temp1: 16385716.000000

Temp1  value [mV] : 9766.541992

Raw value of Temp1: 439482.000000

Temp1  value [mV] : 261.948853

Raw value of Temp1: 214.000000

Temp1  value [mV] : 0.127553

Raw value of Temp1: 67334.000000

Temp1  value [mV] : 40.133759

Raw value of Temp1: 6162100.000000

Temp1  value [mV] : 3672.858154

Raw value of Temp1: 393217.000000

Temp1  value [mV] : 234.373062

Raw value of Temp1: 439336.000000

Temp1  value [mV] : 261.861816

Raw value of Temp1: 439357.000000

Temp1  value [mV] : 261.874359

Raw value of Temp1: 600117.000000

Temp1  value [mV] : 357.693726

Raw value of Temp1: 439264.000000

Temp1  value [mV] : 261.818909

Raw value of Temp1: 16.000000

Temp1  value [mV] : 0.009537

Raw value of Temp1: 439422.000000

Temp1  value [mV] : 261.913086

Raw value of Temp1: 439296.000000

Temp1  value [mV] : 261.837982

Raw value of Temp1: 657076.000000

Temp1  value [mV] : 391.643585

Raw value of Temp1: 439347.000000

Temp1  value [mV] : 261.868378

Raw value of Temp1: 240.000000

Temp1  value [mV] : 0.143050

Raw value of Temp1: 1507435.000000

Temp1  value [mV] : 898.491577

Raw value of Temp1: 1312436.000000

Temp1  value [mV] : 782.264343

Raw value of Temp1: 439411.000000

Temp1  value [mV] : 261.906525

Raw value of Temp1: 439470.000000

Temp1  value [mV] : 261.941711

Raw value of Temp1: 82950.000000

Temp1  value [mV] : 49.441517

Raw value of Temp1: 3212980.000000

Temp1  value [mV] : 1915.064575

Raw value of Temp1: 393217.000000

Temp1  value [mV] : 234.373062

Raw value of Temp1: 439270.000000

Temp1  value [mV] : 261.822479

Raw value of Temp1: 439363.000000

Temp1  value [mV] : 261.877930

Raw value of Temp1: 72454.000000

Temp1  value [mV] : 43.185482

Raw value of Temp1: 3016372.000000

Temp1  value [mV] : 1797.878418

Raw value of Temp1: 393217.000000

Temp1  value [mV] : 234.373062

Raw value of Temp1: 439306.000000

Temp1  value [mV] : 261.843933

Raw value of Temp1: 439381.000000

Temp1  value [mV] : 261.888641

Raw value of Temp1: 89862.000000

Temp1  value [mV] : 53.561348

Raw value of Temp1: 3278516.000000

Temp1  value [mV] : 1954.126709

Raw value of Temp1: 262145.000000

Temp1  value [mV] : 156.248901

Raw value of Temp1: 439367.000000

Temp1  value [mV] : 261.880310

Raw value of Temp1: 439384.000000

Temp1  value [mV] : 261.890442

Raw value of Temp1: 2375894.000000

Temp1  value [mV] : 1416.127808

Raw value of Temp1: 425984.000000

Temp1  value [mV] : 253.903503

Raw value of Temp1: 439458.000000

Temp1  value [mV] : 261.934540

Raw value of Temp1: 439370.000000

Temp1  value [mV] : 261.882080

Raw value of Temp1: 77830.000000

Temp1  value [mV] : 46.389793

Raw value of Temp1: 15074996.000000

Temp1  value [mV] : 8985.300781

Raw value of Temp1: 439221.000000

Temp1  value [mV] : 261.793274

Raw value of Temp1: 4.000000

Temp1  value [mV] : 0.002384

Raw value of Temp1: 439520.000000

Temp1  value [mV] : 261.971497

Raw value of Temp1: 1312436.000000

Temp1  value [mV] : 782.264343

Raw value of Temp1: 1836724.000000

Temp1  value [mV] : 1094.760986

Raw value of Temp1: 4.000000

Temp1  value [mV] : 0.002384

Raw value of Temp1: 439392.000000

Temp1  value [mV] : 261.895203

Raw value of Temp1: 6456154.000000

Temp1  value [mV] : 3848.125977

Raw value of Temp1: 393216.000000

Temp1  value [mV] : 234.372467

Raw value of Temp1: 439370.000000

Temp1  value [mV] : 261.882080

Raw value of Temp1: 439391.000000

Temp1  value [mV] : 261.894623

Raw value of Temp1: 80390.000000

Temp1  value [mV] : 47.915657

Raw value of Temp1: 2426548.000000

Temp1  value [mV] : 1446.319702

Raw value of Temp1: 393216.000000

Temp1  value [mV] : 234.372467

Raw value of Temp1: 439587.000000

Temp1  value [mV] : 262.011444

Raw value of Temp1: 439487.000000

Temp1  value [mV] : 261.951843

Raw value of Temp1: 136198.000000

Temp1  value [mV] : 81.179459

Raw value of Temp1: 438272.000000

Temp1  value [mV] : 261.227631

Raw value of Temp1: 219702.000000

Temp1  value [mV] : 130.951187

Raw value of Temp1: 439400.000000

Temp1  value [mV] : 261.899963

Raw value of Temp1: 87302.000000

Temp1  value [mV] : 52.035484

Raw value of Temp1: 5341613.000000

Temp1  value [mV] : 3183.814941

Raw value of Temp1: 393217.000000

Temp1  value [mV] : 234.373062

Raw value of Temp1: 439435.000000

Temp1  value [mV] : 261.920837

Raw value of Temp1: 439404.000000

Temp1  value [mV] : 261.902374

Raw value of Temp1: 34054.000000

Temp1  value [mV] : 20.297546

Raw value of Temp1: 1312436.000000

Temp1  value [mV] : 782.264343

Raw value of Temp1: 13658950.000000

Temp1  value [mV] : 8141.280762

Raw value of Temp1: 439418.000000

Temp1  value [mV] : 261.910706

Raw value of Temp1: 336.000000

Temp1  value [mV] : 0.200269

Raw value of Temp1: 331802.000000

Temp1  value [mV] : 197.767258

Raw value of Temp1: 439364.000000

Temp1  value [mV] : 261.878510

Raw value of Temp1: 23.000000

Temp1  value [mV] : 0.013709

Raw value of Temp1: 439359.000000

Temp1  value [mV] : 261.875549

Raw value of Temp1: 439296.000000

Temp1  value [mV] : 261.837982

Raw value of Temp1: 9832116.000000

Temp1  value [mV] : 5860.334473

Raw value of Temp1: 5.000000

Temp1  value [mV] : 0.002980

Raw value of Temp1: 439360.000000

Temp1  value [mV] : 261.876129

Raw value of Temp1: 2580694.000000

Temp1  value [mV] : 1538.196899

Raw value of Temp1: 393217.000000

Temp1  value [mV] : 234.373062

    Log2. Channel 3 readings