| Electronics > Beginners |
| Using external reference voltage with a PWM DAC |
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| Buriedcode:
PWM DACS have great linearity - because clocks are generally very consistent, but tend not to have the best absolute accuracy due to the problem you've encountered - precise high/low levels. As suggested, using buffers powered by your reference and a high impedance filter will help, but I would still add calibration to account for any remaining offset if you're after absolute accuracy. Offset is much easier to calibrate (simply adding or subtracting values, or use a trimmer if in hardware) than linearity. |
| mduchalski:
Thanks for your responses. I tried the suggested BJT trick (and also increased filter input impedance) , and while performance is better and I'll definitely keep this approach in mind for the future, it's still not quite what I would like. I'll try experimenting with different component values but since it seems that as you suggested using a CMOS buffer is the best option here, I think I'm going to go for a dedicated DAC IC in this case to avoid the need for calibration and get better performance in general. Thank you all for your help. |
| JS:
The thing with jelly beam CMOS is that they are cheaper and more available than the ADC you will look for... And in the end linearity for PWM is very good as already stated. Also, with one part you could buffer several channels. JS |
| Buriedcode:
It's all a question of application. PWM DACs are great for some things, and not for others. For apps where you have explicit feedback and you don't know how much of an adjustment you'll need, then PWM can be very good. If you're trimming a voltage - that doesn't vary that much, but must be stable, then a DAC with absolute accuracy is better. A dedicated DAC chip will *generally* be better, if your application is say, trimming voltages, and will requires less filtering. PWM DACs can be excellent for audio (again, because its so linear, and absolute accuracy isn't needed) just look at class D amps! |
| mduchalski:
My application is digital control of a power supply - setting voltage and current, so it is exactly "trimming the voltage". That's why I was leaning towards a dedicated chip in my last reply. But in the meantime I found some CMOS logic chips and tested them. I must say I'm very satisfied with the performance - I tested CD4011BE and 74HC00 NAND gates used as inverters. The latter one worked much better with errors within a few milivolts over the entire range, max. recorded -6mV/1.5LSB (approximately, my multimeter is only +/-1% accurate in that range), as opposed to max. -16mV/-4LSB with the 4000-series one. This makes me think I'm going to stick with PWM after all. Thanks! |
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