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| Hobbyist Voltage Reference |
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| Gyro:
--- Quote from: 001 on October 14, 2019, 12:37:30 pm ---Is it a bad idea? :-// --- End quote --- It is if you're trying to calibrate a non True RMS voltmeter. Also a square wave has sharp edges, which means that the value will vary depending on the high frequency response of the meter. |
| rpetitho:
Got the PCBs yesterday. I ended up adding a bodge cap on the bottom on the noise reduction pin. Not sure if it will do much since it is battery powered. Since I have no standard to compare to, this is now the official 5v standard of my household. I have the parts to build up one more just to make sure I didn't get a bum chip. Battery should last about 12 hrs, but I made sure it was easy to change. I'm happy with the product! Pics below. |
| magic:
Nice, I have built a similar MAX6250 reference on a piece of perfboard but never bothered with enclosure. One thing you could add is reverse supply protection, otherwise you need to be careful not to touch the battery connector the wrong way even for a moment. Noise reduction capacitor is pointless for DMM calibration. The datasheet shows its effect on noise and input ripple rejection - it only starts to make a difference above 10Hz. That means the output resistance of the zener cell has to be some 16kΩ and it would take 100µF to push noise reduction down to 0.1Hz. Foil capacitors in this size are rather big and electrolytics have leakage approaching 1µA which would drag the output several mV down. Not sure about MLCCs but IIRC they aren't that great either. |
| iMo:
--- Quote from: magic on October 27, 2019, 06:52:26 am ---One thing you could add is reverse supply protection, otherwise you need to be careful not to touch the battery connector the wrong way even for a moment. --- End quote --- Simply wire an 1N4001 between Bat+ and Bat-. I smoked 2 my references recently that way.. Also I made bad experience with trimpots. Do not populate the trimpot, stay with the voltage as-is. |
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