I am using the ACS758KCB Hall Effect sensor for a project. On the front page of the Datasheet it list a .1uF capacitor between Vcc and GND, in addition it lists a capacitor, Cf, between GND and VIOUT. It says that CF is for optimal noise management, with values that depend on the application. I am very new to electronics, so I do not know how to calculate the value for optimal noise reduction. Is there a specific formula? In addition it specifies Rf, what determines that value?

Thanks for any help, I really appreciate it, Joel


Cf and Rf form a first-order low-pass filter with a cutoff frequency \$f_C = \frac {1}{2\pi R C}\$. The response will be reduced to 0.707 (1/2 power) at that frequency. A first-order filter response only drops by -6dB/octave, so attenuation only increases gently.

The maximum value of Rf is determined by the requirements of whatever it is connected to, and perhaps the leakage of the capacitor. The minimum value is determined by the capability of the part, and 4.7K is specified. If you were to try to use, say, 10M ohm you'd need a very low leakage capacitor and a very low input bias current/input impedance input on whatever its connected to. Often you can only go to a few K ohms if you're going directly into a micro.

There is no maximum or minimum for the capacitance value, provided you keep the resistance more than 4.7K. You could use a 10uF capacitor and a 100K resistor (if whatever it's connected to is okay with 100K) and get a 0.16Hz cutoff if you want. The trade-off is that the response will be sluggish and it will take many seconds to settle to a stable value (a bit less than 5 time constants or 5 seconds in this case to get within 1% of the final value).

  • \$\begingroup\$ Thank you for your response, I still have a few questions. On the table on page 6 of the datasheet it says that the Output Load Capacitance max is 10nF what is that for? I am using this with a 10 bit ADC, I want to get as accurate and as quick results as possible, so do I need a high cutoff frequency? It says on page 2 of the datasheet 120 kHz typical bandwidth should I make the cutoff frequency a little higher, if not what is that referring to? Here is the data sheet for the ADC , I am using: adafruit.com/datasheets/MCP3008.pdf. Which parameter determines the max Fc? \$\endgroup\$ – Joel Dec 16 '14 at 2:33
  • \$\begingroup\$ TIt's clear from the datasheet - 10nF is the maximum load right on the output- exceed that and it could go unstable. It does not apply to what's on the other side of a 4.7K or higher resistor. You have a trade-off between filtering out noise and possibly affecting the signal to an unacceptable degree. Fast setting and minimum noise are opposing requirements and filter cutoff (and order) are parameters. The right answer (if there is one) is the one that fits your (unstated) requirements, and is not inherent in the specs of those particular parts you've chosen to look at. \$\endgroup\$ – Spehro Pefhany Dec 16 '14 at 2:42
  • \$\begingroup\$ As far as the maximum Fc, there is none. Simply omit the capacitor and short the resistor and you'll maximize Fc (and noise). Note that the particular ADC you've chosen does not play well with that part if you add a resistor- you'll have to keep the sample rate very low in conformity to table 4-1 (the impedance drops as sample rate increases) or add a buffer amplifier or leave out the filter. \$\endgroup\$ – Spehro Pefhany Dec 16 '14 at 2:51
  • \$\begingroup\$ I need to read the current motors. I am trying to use the current to get a general idea (I would like to be as accurate as possible, but as long as I can get a general idea I should be fine) of how hard the motors are working. Also to monitor for over current. I would like to be able to have sampling rates of 20+ hz. \$\endgroup\$ – Joel Dec 16 '14 at 3:10
  • \$\begingroup\$ If I use a 10kohm and a .33uf cap I should get a cutoff frequency of 48.2hz. Is that sufficient for my use? Would this ADC work better: adafruit.com/datasheets/ads1015.pdf \$\endgroup\$ – Joel Dec 16 '14 at 3:21

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