Oct 12, 2009
By M Ganesh Raaja
PSoC3 has 8 bit voltage and current DACs. Higher resolution DACs may be created by combining two 8 bit DACs, one for MSB and one for LSB and summing their outputs together. The DAC section of the TRM shows how to chain two 8 bit DACs to get a 12 bit DAC.
The MSB is implemented using an 8 bit DAC configured for 2.040mA full scale. Each bit of the MSB DAC now corresponds to 8uA. To create a 12 bit DAC, we need to extend the resolution by four bits and each bit should be equal to 1/16 of the MSB DAC, which is 0.5uA. This can be implemented by using Bits 2 to 5 of another 8 bit DAC configured for a 32uA output. The schematic diagram of a PSoC3 project to create a high resolution current DAC is shown below.
Below is the code that is used to split the 12 bit DAC value and update the MSB and LSB DACs.
DAC_MSB_SetRange(DAC_MSB_RANGE_2mA);
DAC_LSB_SetRange(DAC_LSB_RANGE_32uA);
DAC_MSB_Start();
DAC_LSB_Start);
DacValue = 4095;
DAC_MSB_SetValue((DacValue >> 4) & 0xFF);
DAC_LSB_SetValue((DacValue << 2) & 0x3C);
To implement a 10 bit DAC, the code would be:
DAC_MSB_SetRange(DAC_MSB_RANGE_2mA);
DAC_LSB_SetRange(DAC_LSB_RANGE_32uA);
DAC_MSB_Start();
DAC_LSB_Start);
DacValue = 1023;
DAC_MSB_SetValue((DacValue >> 2) & 0xFF);
DAC_LSB_SetValue((DacValue << 4) & 0x30);
But this method also has its disadvantages. Any mismatch in gain between the MSB and LSB DACs will result in non-linearity and unequal step sizes. This can be compensated by calibration, either by trimming the DAC using the DACx_TR register or by using the unused least significant bits of the LSB DAC. Stay tuned!!