PSOC 5 DAC – 1mv or smaller resolution
When dealing with the PSOC 5, you generally have a choice of 4mv (4/1000 of a volt, or 4 millivolt) resolution per bit. If you choose 4mv, you are restricted to 1.024 volts. A resolution of 16mv gives you 0 to 4 volts (approximately).
If you use the DVDAC component you can create up to a 12 bit resolution from 0 to 4 volts. Unfortunately, this involves DMA, Tables, the DAC and an Opamp, plus an external capacitor. This DAC output value is also very noisy.
As I was working with a power supply control loop, I suddenly realized I had been ignoring a simple solution to the problem of finer voltage resolution on my PSOC 5’s DAC output. Use the Current DAC! (iDAC)
If you place a 1k resistor on a pin, (use 0.1% for best results), you can get 1 mv per bit by using the iDAC in 1 microamp per bit mode. The math is simple algebra: 1 x 10e-6 X 1 x 10e3 gives 1 x 10e-3, or 1 millivolt. This gives you 0.255 volts maximum voltage.
It gets better once you also understand you can chose more resolution with the iDAC in 1/8 microamp per bit (up to about 32ua), or less resolution at 8 microamps per bit (up to 2 milliamps). Size your resistors appropriately for your resolution needs.
The problem you may encounter is the output impedance (for this example) is 1K ohms, which is the voltage resistor. Any device that reads the voltage will have current flowing into it, which will change the voltage equation. (You can calculate using the standard parallel resistor formula).
The rule of thumb solution is to use an opamp with high input impedance to buffer the voltage at the resistor without tremendously affecting the voltage produced due to extra currents. The currents are usually so small to ignore, but can usually be measured and calculated into the design.
(This does require an opamp with decently high input impedance.) You can do this with the opamp inside the PSOC, but there are issues with this approach.
The issue is that the PSOC’s internal aluminum cross connect wires have enough resistance to modify the voltage the opamp follower will read, depending upon how close the follower is to the iDAC output and how far away the resistor is from the iDAC output. If the follower is close to the iDAC and the resistor is several switches away from the opamp, the voltage output from the follower will change.
If you tie the resistor to the PSOC’s dedicated iDAC output pin, and force the iDAC output to that dedicated pin, this problem is greatly reduced. However, there is also another way to handle this, at the cost of an additional pin.
If you connect the resistor to an arbitrary pin, and connect another pin to the same resistor, you can send the iDAC output to that pin. You will then connect an Opamp’s input in follower mode to that pin. Regardless of how the router functions, your voltage measurement will be mostly free of the resistance factors introduced by the internal aluminum wiring and cross connect resistances.
In the image following, you will see the simplicity of this solution.
If you have an external opamp connected to R_1 in the preceding image, you can reduce the pin count by 2. You will also loose the need for the internal opamp, the ResistorIn pin, and the Vout_1 pin.
There are limitations, assuming a 5 volt VDDA. For the current source, you can only do 0 to 2.55 volts. For the current sink, you can do 5 volts down to 2.45 volts. Pick your values.
With various resistors, you can get various results. In my case, I needed 2 volts out, so chose a 10K resistor, giving me 10mv resolution, which is extremely easy to calculate with. It allowed me to control a power supply with small enough voltage steps.
One strong caveat: Maximum voltage out for current source is 1 volt below VDDA. Minimum voltage out for current sink is 1 volt above ground.
So, if you have a 3.3v VDDA, you can only provide 0 to 2.3 volts DAC out, or 3.3 volts down to 1.0 volts DAC out. Keep that in mind.
Enjoy!
Fini
About The Author
W Maxfield
Firmware/hardware Engineer for too many years.