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I am using a MEANWELL 35-B power supply to power the circuit(Datasheet). It says in Datasheet that the ripple and noise are about 80mV p-p for the 5v o/p.

I have Powered a strain gauge type pressure sensor using the 5v output of power supply. I am using an HX711 ADC to get digital values but the ADC output shows fluctuations and are unstable. I am guessing that the fluctuations arise from powersupply as i am powering ADC unit as well as sensor using the supply . I dont have (and dont know how to use)the DSO to check the ripples and noise.

Questions : 1)Can i reduce noise ?

2)Can i add capacitors (i havent used any yet) to reduce noise ? How can i determine the value of capacitors ? is there a way to calculate cap .value using the noise specs of powersupply ?

3)Any other ways i can try to reduce analog noise like series resistors on sense rails?

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  • \$\begingroup\$ A few things from the MEAN WELL datasheet: The minimum current for channels 2 and 3 is 100mA; the ADC draws "<1.5mA". Below minimum current power supplies often exhibit poorer load regulation. Also, the voltage ripple was measured with a 47uF and 0.1uF cap. Capacitors located near the ADC are required to smooth voltage ripples in any case. \$\endgroup\$ – Alex Jan 24 '17 at 5:43
  • \$\begingroup\$ Add 10uF across the power supply pins of the ADC, locating the capacitor as close as possible to the device. You can see this in the reference design in the datasheet you linked. \$\endgroup\$ – replete Jan 24 '17 at 5:44
  • \$\begingroup\$ @Alex So I should use 0.47uF and 0.1uF? Will that help ?.yes I understood the minimum current part but does it add to noise ? this is the board that I've bought ebay.in/itm/302149910918?aff_source=Sok-Goog \$\endgroup\$ – Gaurang Deshpande Jan 24 '17 at 5:51
  • \$\begingroup\$ @replete I will do that, I will directly add caps on HX711, anything else I can do to reduce noise further? should I add any more capacitors ? \$\endgroup\$ – Gaurang Deshpande Jan 24 '17 at 5:59
  • \$\begingroup\$ @GaurangDeshpande Adding at least a 10uF cap as close as possible to the ADC as its datasheet recommends is a good start, and you can add a 0.1uF cap or similar to help deal with higher frequency noise. Also of importance is the fact that the power supply specifies a minimum current of 0.5A on the 5V rail. Below that the regulation may suffer, so if you still see unacceptable noise after adding capacitors, I recommend loading the 5V line so at least 0.5A is being drawn. There are further tweaks you can make if that also does not resolve the issue. \$\endgroup\$ – Alex Jan 24 '17 at 6:19
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The points you mentioned are just hacks. If you want a stable power supply for the ADC then you're better off using a voltage reference IC. They will provide an accurate and stable fixed voltage, with the drawback being that they supply current only up to about 100mA typically. The good ones have temperature compensation as well.

Eg: I use this one for 5v http://www.digikey.com.au/products/en?keywords=LT1461DHS8-5%23PBF ... and this one for 3.3v http://www.digikey.com.au/product-detail/en/texas-instruments/LM4132AMF-3.3-NOPB/LM4132AMF-3.3-NOPBCT-ND/1206403

You just need a couple of generic caps to go with them. Almost too easy.

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  • \$\begingroup\$ That seems too simple,Thank you! Can I just use 7805 then? \$\endgroup\$ – Gaurang Deshpande Jan 24 '17 at 8:21

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