DAC doesn't give quite the right output

Daanii · April 7, 2011, 4:28am

I'm trying out a TI chip that has 8 8-bit DACs, with the chip number being TLV5628CN. Looking at the datasheet, I thought I could use the SPI commands.

But the following code gives the results that follow in the second code section. The numbers should match, but don't.

// include the SPI library:
#include <SPI.h>
#define dacInputPin 4

// set pin 9 as the slave select for the DAC:
const int dacSelectPin = 9;
int dacValue;

void setup() {

  // start serial port at 9600 bits per second
  Serial.begin(9600);

  // set the slaveSelectPin as an output:
  pinMode (dacSelectPin, OUTPUT);

  // initialize SPI:
  SPI.begin(); 

}

void loop() {
    // change the resistance from min to max:
    for (int level = 0; level < 256; level = level + 1) {

      dacWrite(level);
      Serial.print("  ");
      Serial.print(level);
      Serial.print("\t");      

      dacValue = analogRead(dacInputPin)/4;
      Serial.print("  ");
      Serial.print(dacValue);
      Serial.print("\n");      

      delay(10);

    }
 }

int dacWrite(int value) {

  unsigned char commandMask, byteOne, byteTwo;  

  commandMask = 0x0;
  byteOne = commandMask;
  byteTwo = value;
  
  // take the SS pin high to write to the chip:
  digitalWrite(dacSelectPin,HIGH);

  //  send in the value via SPI:
  SPI.transfer(byteOne);
  SPI.transfer(byteTwo);

  // take the SS pin low to write into DAC:
  digitalWrite(dacSelectPin,LOW);
 
  // take the SS pin high to finish the write:
  digitalWrite(dacSelectPin,HIGH);
  
}

It seems like the problem should be obvious, but I'm not finding it. Anyone else see what is happening?

system · April 7, 2011, 4:36am

Looks like this DAC takes data in on falling clock edges, so the default SPI mode 0,0 is not right. Try switching to a 0,1 or 1,0 mode.

--
The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons

Daanii · April 7, 2011, 5:55am

Thanks, that does seem to be the problem. I tried changing the clock phase to falling edge, by the following:

SPCR = (1 << CPHA); // set SPI mode to send data on falling edge of clock

That got me better results, as follows:

But it is still not quite right. I also tried changing the CPOL bit, and that led to the same results as changing the CPHA bit. Changing both bits to ones? Same results. (That's a little strange.)

I'll keep puzzling over this. But it does have me puzzled.

system · April 7, 2011, 4:02pm

Daanii:
...puzzled.

What VDD are you applying to the DAC? What voltage(s) are you applying to REF1 and REF2 on the DAC?

What makes you think that the ADC reading divided by 4 (with integer arithmetic, which throws away the lower two bits of the quotient) should be numerically equal to the DAC code byte? (Your table values would be pretty close if the DAC reference voltage is something like 3.7 Volts. But I still wouldn't throw away the lower two bits of the ADC readings.)

Regards,

Dave

Footnote:
It may be possible to isolate the puzzling aspects like this by something like this:

Instead of trying to figure out what's wrong with a list of 256 different words, write a particular DAC code byte, say 128. Measure the voltage with a meter. It should be 1/2 the DAC reference voltage for that channel, right? If not, then you can debug the DAC end of your system.

Repeat the experiment with a code byte of 64. Try 192. See if the results are consistent with your expectations.

Then for a particular DAC code byte:
Read the value with analogRead() on the Arduino. Print that integer value. Convert to (floating point) volts in the Arduino. Print that out. It should be (close to) the voltage you read on the meter. If not, then you can debug the Arduino end of the system.

Anyhow, once you get the value for a few particular code bytes to meet your expectations, then run the program that gives results for all of the DAC code bytes. (I would probably print DAC code word and expected voltage and then calculate Arduino voltage and relative error (or percent error) and print those out for each reading.)

Daanii · April 7, 2011, 6:30pm

Thanks for your comments.

davekw7x:
What makes you think that the ADC reading divided by 4 (with integer arithmetic, which throws away the lower two bits of the quotient) should be numerically equal to the DAC code byte? (Your table values would be pretty close if the DAC reference voltage is something like 3.7 Volts. But I still wouldn't throw away the lower two bits of the ADC readings.)

Prior to testing this DAC, I tested two other DACs and three digital potentiometers, using the same approach of reading the analog input to the Arduino and dividing by 4 (to scale it from 0 to 1023 down to 0 to 255). As you note, that takes off the lower two bits, but I found that it was still very close to the digital output I sent to the device. Only in this case is there a discrepancy of more than plus or minus one.

The differences between this DAC and the other DACs and digital potentiometers are that this one does not use pure SPI, and this one is 3.3 Volts instead of 5 Volts. Following RuggedCircuit's comment, I had thought the SPI problem has been taken of.

Maybe the problem now is with the supply voltage and the reference voltage. The datasheet is a little unclear on what those should be, and I've tried various combinations. None worked. Some close, but the closest was the one I posted.

I'll keep trying.

system · April 7, 2011, 7:44pm

Daanii:
...
Prior to testing this DAC, I tested two other DACs ....

I hate to repeat myself but

davekw7x:
What voltage(s) are you applying to REF1 and REF2 on the DAC?

Now, sometimes I ask questions out of idle curiosity, but in this case I actually had a reason.

The reason that I asked is that for proper operation of this particular device, the maximum value of reference voltage is VDD-1.5 Volts. So: If you are applying 5 Volts to VDD and 3.3 Volts to REF1,REF2, it will be operating within spec.

However...

But the full-scale voltage of the DAC will be Vref (minus 1 lsb) not 5 Volts. If you apply 3.3 Volts to VDD, then you should not put 5 Volt logic signals (SPI Clock and SPI MOSI) to the device. Furthermore, if you use 3.3 Volts VDD, then the reference voltage(s) should be no more than 1.8 Volts.

Here's my take on it: For most convenient use with a 5-Volt Arduino, apply 5 Volts to VDD and 3.3 Volts to Vref on the DAC.

Daanii:
...I had thought the SPI problem has been taken of.

I think that SPI can work and, in fact your values are monotonically increasing as you increment the DAC code byte, so it is (probably) OK. I, personally wouldn't even have thought of using SPI (I would just have waded in and done it the "hard way" with bit banging), but I can't see why SPI can't work for this.

Have you tried the experiment that I suggested: Put a single DAC code byte and read the voltage with a meter? Output voltage should be equal to

(Reference Voltage) * (DAC code word) / 256

Apply 5 Volts to VDD and 3.3 Volts to REF1 and REF2. Write 128 to the DAC. Read the voltage with a meter. Should be 1.65 Volts, right?
Now, read the voltage with Arduino analogRead. The ADC reading should be something like 338.

Anyhow convert the ADC reading to a voltage with something like this:

    const float aRef = 5.0; // Make this value equal to the Arduino VCC (nominally 5 Volts but could be off by a percent or two).
.
.   // write 128 to DAC
.
    int adcValue = analogRead(0); // Or whatever channel you use
    float adcVoltage = adcValue * aRef / 1024.0;
    Serial.print("Analog reading = ");Serial.print(adcValue);
    Serial.print(" => ");Serial.print(adcVoltage);
    Serial.println(" Volts");

Bottom line: If the full-scale 8-bit DAC voltage is equal to the full-scale 10-bit ADC voltage, then you can divide the ADC reading by 4 and it should just about match the DAC code word. That's not the case for this particular DAC and your Arduino. There is also a factor of (DAC Reference)/(ADC Reference) But---See Footnote.

Regards,

Dave

Footnote:
You could, in fact, use 3.3 Volts for the Arduino ADC reference by applying 3.3 Volts to the Arduino Aref input and changing the software to use the external reference. In fact, for best precision in any ADC operation, it's usually a Good Idea to use a reference voltage that is somewhere in the neighborhood of the maximum expected input voltage. At any rate, the formula for converting analogRead value to voltage is what I show above. Just change the aRef value to whatever you decide to use.

Daanii · April 8, 2011, 1:55am

Thanks for the detailed comments, including the repeat of your earlier comment. You were exactly right to focus on the supply and reference voltages. Once I used 5 Volts as supply and 3.34 Volts as reference, I got the results I expected. (Which were exactly the numbers you predicted.) Those voltages were the problem -- not the SPI clocking as I thought.

So this TI chip tests out fine. But I need a voltage that varies between 0 and 5 Volts. Since I can't use 5 Volts as the reference voltage on this TI chip, and I can on the others I tested, I'll use one of them.

It's good to have solved the puzzle, though. Thank you for doing that for me.

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DAC doesn't give quite the right output

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