Working with 12 bit DACs

[Smartie]

Hey guys,

Update: Bit of a mistake, its a MCP4922 not MCP1922. sorry

Last week I ordered some sample components from Microchip and got them in the mail two days ago.
I got 2 MCP1922 (dual 12 DAC) and 2 24FC512 (64KByte EEPROM)

I'm trying to get the DACs working at the moment but I'm having a few problems.
I've followed the data sheet and set up the circuit accordingly, I'm sure I'm sending the the SPI commands and I'm sure I have the correct voltage levels set up.

In my first test, I had VREFA connected straight to +5V, When sending a 0x001 to the DAC, the voltage on VOUTA would be +2.4V I've checked the signals with My new Logic analyser and the signals are in-fact correct.

when I change the config settings to have a 2x gain and set the VREFA to +2.5V using a voltage divider. I get nothing on VOUTA no mater what value i set it.

Think you guys could help?

here is the code that I'm using:

int main()
{
	init();
	spi_init();
	int test = 0x01;
	char temp,temp2, conf = 0x00;
	conf = 0b00010000;
	while(1)
	{
		temp = conf | ((test >>8)&0x0F); //first byte is made up of the config bits and the most significant 4 bits of test.
		
		temp2 = test&0xFF; //the rest of test
		RA2 = 0;//bring CS low
		spi_write(temp);//send first byte
		spi_write(temp2);//send second byte
		RA2 = 1;//bring CS high
		RC1 = 0;//send LDAC low to latch the new value onto the output
		RC1 = 1;//bring it high again.
		test++;//increment the value 
		DelayMS(10);
		if(test>4096)//if gone over the max, then resest to 0
			test = 0;
			
	}
}

sorry for the large images, not sure how to resize them in the post

 

[Smartie]

opps, forgot to add
MCP1922 data bits:

bit 15 A/B: DACA or DACB Select bit
	1 = Write to DACB
	0 = Write to DACA
bit 14 BUF: VREF Input Buffer Control bit
	1 = Buffered
	0 = Unbuffered
bit 13 GA: Output Gain Select bit
	1 = 1x (VOUT = VREF * D/4096)
	0 = 2x (VOUT = 2 * VREF * D/4096)
bit 12 SHDN: Output Power Down Control bit
	1 = Output Power Down Control bit
	0 = Output buffer disabled, Output is high impedance
bit 11-0 D11:D0: DAC Data bits
12 bit number “D” which sets the output value. Contains a value between 0 and 4095.

 

[Smartie]

Opps, its a MCP4922, not MCP1922

 

[Smartie]

OK it was a mistake on my part.

When I set up the SPI, I set it up to transmit on the falling edge rather than the rising edge of the clock so it was changing the value when the DAC chip was reading the data.
I'm now getting values from 0V to +5V on the output

 

[Sceadwian]

I love self answering posts =)

 

[ericgibbs]

I love self answering posts =)

It saves us a chore..