(19-02-2017, 20:50)Zappie Wrote: (18-02-2017, 21:24)Werk_AG Wrote: (18-02-2017, 08:34)Zappie Wrote: Hi,
Something i'm really missing is the possebillity to insert a calibrated external voltage reference. Very easy to add i think!
Regards, Zappie
Where you think that having it, will significantly improve the readings?
Where minivollts are readout from analog ports, The internal reference is stable, but still has about a +/- 10% error. To improve accuracy of the analog readings i mostly use little calibrated voltage reference modules. They have almost the same size as the transmitter module.
Regards, Zappie
It's easy to theorize about the design options, but let's go...
Some of the analog reads done on the TX unit, use the 1.1V internal reference, others use the 5V reference, we change it dynamically by software control. Arduino only have one AREF pin, so how to use two external precision reference voltage sources without adding additional switching circuitry? Another consideration... if we chose to use just one reference voltage for the ADC, it must be 5V or at least 4.096V (because of the wind vanes), but if we do that we are seriously penalizing the resolution when we are reading those millivolt signals... (5 / 1024 versus 1.1 / 1024)
It is right, that the 1.1V Arduino internal reference voltage have a tolerance of around 10% from chip to chip, but it doesn't float, so in the worst case, on some chips it can be just 1.0V and on another it can be 1.2V. There are a lot of confusion about this matter, as most people think that the bandgap voltage floats on the same chip.
On both cases where we are using the 1.1V internal reference (reading the Solar Radiation and UV sensors), both will require additional hardware / software calibration after installation of the sensors, so having a reference voltage of 1.0V or 1.2V doesn't have a significant impact on the final result.
Quote:...i mostly use little calibrated voltage reference modules. They have almost the same size as the transmitter module.
The use of a little MCP1541 chip, with its output connected to the Arduino AREF pin can be an easy way...
Regards