MCP4725 DAC non expected DC output with MEGA2560

Projects General Guidance
1

Hello All,

Many thanks for viewing any input appreciated.

I am attempting to generate PWM output from the Mega2560 and get my MCP4725 DAC to generate a sign wave, full sketch attached below.

Sketch uploads and runs fine. Output DC voltage from DAC oscillates only between 2.3 to 2.6V. i.e. I do not see the full range in the region of 0 - 5V I expect using this sketch. Could this be a faulty DAC typically?

Attempted to attach image below to show simplicity of setup utilising the I2C protocol (5V-VCC, GND-GND, SDA(PIN20)-SDA, SCL(PIN21)-SCL. DAC out connected to multimeter.

MEGA
MEGA1672×1254 255 KB 



/******************************************************************************
  MCP4725 Example Waveform Sketch
  Joel Bartlett
  SparkFun Electronics
  Sept. 11, 2014
  https://github.com/sparkfun/MCP4725_Breakout
  https://learn.sparkfun.com/tutorials/mcp4725-digital-to-analog-converter-hookup-guide/all

  This sketch takes data from a lookup table to provide
  waveforms to be generated by the MCP4725 DAC.

  Development environment specifics:
  Arduino 1.0+
  Hardware Version V14

  This code is beerware; if you see me (or any other SparkFun employee) at the local,
  and you've found our code helpful, please buy us a round!

  Distributed as-is; no warranty is given.

  This code builds off the sketch written by Mark VandeWettering, which can be found here:
  http://brainwagon.org/2011/02/24/arduino-mcp4725-breakout-board/
*/

#include <Wire.h>//Include the Wire library to talk I2C
#include <Adafruit_MCP4725.h>

//This is the I2C Address of the MCP4725, by default (A0 pulled to GND).
//Please note that this breakout is for the MCP4725A0.
#define MCP4725_ADDR 0x60
//For devices with A0 pulled HIGH, use 0x61

//Sinewave Tables were generated using this calculator:
//http://www.daycounter.com/Calculators/Sine-Generator-Calculator.phtml


int lookup = 0;//varaible for navigating through the tables

int sintab2[512] =
{
  2048, 2073, 2098, 2123, 2148, 2174, 2199, 2224,
  2249, 2274, 2299, 2324, 2349, 2373, 2398, 2423,
  2448, 2472, 2497, 2521, 2546, 2570, 2594, 2618,
  2643, 2667, 2690, 2714, 2738, 2762, 2785, 2808,
  2832, 2855, 2878, 2901, 2924, 2946, 2969, 2991,
  3013, 3036, 3057, 3079, 3101, 3122, 3144, 3165,
  3186, 3207, 3227, 3248, 3268, 3288, 3308, 3328,
  3347, 3367, 3386, 3405, 3423, 3442, 3460, 3478,
  3496, 3514, 3531, 3548, 3565, 3582, 3599, 3615,
  3631, 3647, 3663, 3678, 3693, 3708, 3722, 3737,
  3751, 3765, 3778, 3792, 3805, 3817, 3830, 3842,
  3854, 3866, 3877, 3888, 3899, 3910, 3920, 3930,
  3940, 3950, 3959, 3968, 3976, 3985, 3993, 4000,
  4008, 4015, 4022, 4028, 4035, 4041, 4046, 4052,
  4057, 4061, 4066, 4070, 4074, 4077, 4081, 4084,
  4086, 4088, 4090, 4092, 4094, 4095, 4095, 4095,
  4095, 4095, 4095, 4095, 4094, 4092, 4090, 4088,
  4086, 4084, 4081, 4077, 4074, 4070, 4066, 4061,
  4057, 4052, 4046, 4041, 4035, 4028, 4022, 4015,
  4008, 4000, 3993, 3985, 3976, 3968, 3959, 3950,
  3940, 3930, 3920, 3910, 3899, 3888, 3877, 3866,
  3854, 3842, 3830, 3817, 3805, 3792, 3778, 3765,
  3751, 3737, 3722, 3708, 3693, 3678, 3663, 3647,
  3631, 3615, 3599, 3582, 3565, 3548, 3531, 3514,
  3496, 3478, 3460, 3442, 3423, 3405, 3386, 3367,
  3347, 3328, 3308, 3288, 3268, 3248, 3227, 3207,
  3186, 3165, 3144, 3122, 3101, 3079, 3057, 3036,
  3013, 2991, 2969, 2946, 2924, 2901, 2878, 2855,
  2832, 2808, 2785, 2762, 2738, 2714, 2690, 2667,
  2643, 2618, 2594, 2570, 2546, 2521, 2497, 2472,
  2448, 2423, 2398, 2373, 2349, 2324, 2299, 2274,
  2249, 2224, 2199, 2174, 2148, 2123, 2098, 2073,
  2048, 2023, 1998, 1973, 1948, 1922, 1897, 1872,
  1847, 1822, 1797, 1772, 1747, 1723, 1698, 1673,
  1648, 1624, 1599, 1575, 1550, 1526, 1502, 1478,
  1453, 1429, 1406, 1382, 1358, 1334, 1311, 1288,
  1264, 1241, 1218, 1195, 1172, 1150, 1127, 1105,
  1083, 1060, 1039, 1017,  995,  974,  952,  931,
  910,  889,  869,  848,  828,  808,  788,  768,
  749,  729,  710,  691,  673,  654,  636,  618,
  600,  582,  565,  548,  531,  514,  497,  481,
  465,  449,  433,  418,  403,  388,  374,  359,
  345,  331,  318,  304,  291,  279,  266,  254,
  242,  230,  219,  208,  197,  186,  176,  166,
  156,  146,  137,  128,  120,  111,  103,   96,
  88,   81,   74,   68,   61,   55,   50,   44,
  39,   35,   30,   26,   22,   19,   15,   12,
  10,    8,    6,    4,    2,    1,    1,    0,
  0,    0,    1,    1,    2,    4,    6,    8,
  10,   12,   15,   19,   22,   26,   30,   35,
  39,   44,   50,   55,   61,   68,   74,   81,
  88,   96,  103,  111,  120,  128,  137,  146,
  156,  166,  176,  186,  197,  208,  219,  230,
  242,  254,  266,  279,  291,  304,  318,  331,
  345,  359,  374,  388,  403,  418,  433,  449,
  465,  481,  497,  514,  531,  548,  565,  582,
  600,  618,  636,  654,  673,  691,  710,  729,
  749,  768,  788,  808,  828,  848,  869,  889,
  910,  931,  952,  974,  995, 1017, 1039, 1060,
  1083, 1105, 1127, 1150, 1172, 1195, 1218, 1241,
  1264, 1288, 1311, 1334, 1358, 1382, 1406, 1429,
  1453, 1478, 1502, 1526, 1550, 1575, 1599, 1624,
  1648, 1673, 1698, 1723, 1747, 1772, 1797, 1822,
  1847, 1872, 1897, 1922, 1948, 1973, 1998, 2023
};


void setup()
{
  Wire.begin();

  // Set A2 and A3 as Outputs to make them our GND and Vcc,
  //which will power the MCP4725
  pinMode(A2, OUTPUT);
  pinMode(A3, OUTPUT);

  digitalWrite(A2, LOW);//Set A2 as GND
  digitalWrite(A3, HIGH);//Set A3 as Vcc
}
//---------------------------------------------------
void loop()
{
  Wire.beginTransmission(MCP4725_ADDR);
  Wire.write(64);                     // cmd to update the DAC
  Wire.write(sintab2[lookup] >> 4);        // the 8 most significant bits...
  Wire.write((sintab2[lookup] & 15) << 4); // the 4 least significant bits...
  Wire.endTransmission();
  lookup = (lookup + 1) & 511;
}

The ultimate goal is to use PWM input to Arduino from a radio control receiver to generate DC output voltage to control a hydraulic pump remotely.

Thanks

2

You need to rightshift 8 times, not 4 times. Remember that 1 byte is 8 bits, not 4 bits. Adjust the mask to oxff (255) insteaf of 15, and don't do any shift. You can likely drop the mask as well if the low byte is sent in the write.

1 Like
3

First, I'd try the Adafruit library for this chip. IME, their code is pretty reliable.

Second, the Adafruit tutorial suggests that the address is either 0x62 or 0x63, so perhaps try those first. Or run this I2C bus scanner to determine what the address of the chip is.

1 Like
4

Why don't you use the example that comes with this library? You don't have to do the Wire commands directly

2 Likes
5

OK thanks scanner address gave me 0x60

6

Thanks appreciate the advice. I will take a look

7

Hey Railroader,

Many thanks for your assist. unfortunately I still have tiny oscillations in voltage i.e. 0.02V! So something I did wrong. Also with adafruit library example not working. A bit lost to be honest.

image

8

Did you change the example in the Adafruit library to the 0x60 address?

9

On a 10 bit ADC that is fully normal. Try and a cap some hundred nF between analog input and gnd.

10

I just added the Adafruit library and my Sparkfun 4725 breakout runs the sine example just fine. I changed the address in code to 0x60 but that's expected.

11

Hi, Yes I did but no success unfortunately.

image
image801×350 11.5 KB

Also I had seen in the .h header file an I2C definition as 0x62 so changed it to 0x60 again no success

image
image717×564 22.9 KB

beginning to wonder if I have faulty DAC. Also loaded the sketch onto a Nano and had the same results using I2C on pins A4/A5.

Thanks for your thoughts on this its a slow plod on our own!

12

Sorry I should have clarified. The full sine waveform i would expect 0-5V DC output from DAC or thereabouts. I get tiny oscillations of 0.2V around centre value of say 2.5V i.e. I seem not to be getting the Analog DAC conversion of the 0-4095 range in the sketch

Many thanks

13

Well, if the I2C scan reports the device is found at 0x60, it's unlikely that it's malfunctioning. Unless the output driver is shot.

1 Like
14

Read the datasheet, "Gain error" of +/- 2%. 2% times 4096 give 82 bits of variation.

1 Like
15

You said that you're measuring this with a DVM. Remember that this is outputting a sine wave. What are you expecting to see?

1 Like
16

The signwave tabulated in the sketch is a digital 12bit reconstruction of a signwave form.

That is looped to the DAC which outputs an Analog voltage within the input (Vcc) range (5V)

Just to be clear its a Digital to Analog converter.

So im hoping that as the sketch cycles through the PWM values i should see a rise and fall of the duty cycle output of the DAC in VDC. I would like to know that im wrong that would make sense!

17

Aha OK that is a good point.

I had an expectation that the output voltage would rise and fall as the sketch loops through the PWM values and the digital duty cycle input effectively goes up to high of 5V and down to low of 0V or thereabouts.

For some reason I am stuck on one value, with evidence of the error budget fluctuation to that V value

18

Good point i will have to figure that out somehow cheers

19

Yes. You are wrong: the frequency of the waveform, according to my scope, is around 25Hz. You're not going to see that properly with a DVM. What you describe, however, is exactly what I'd expect.

Try removing this line

lookup = (lookup + 1) & 511;

and telling us what you see.

[edit]
OK. Instead of removing that line, add

delay(1500);

right after it. Now you should have a very slow sine wave that the DVM can track.

1 Like
20

Sorry. I might not be fully correct and clear. That 2% should be considered as a permanent error, not a fluctuation from time to time. Your device looks like being really good U would say.