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Converting current output to voltage

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Kath

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Hi,

I'm interfacing a current output from a current sensor to a microprocessor's ADC. From the following circuit, how do I calculate both the current and voltage that goes into ADC? Thanks!


Iout
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\
/
\ Rlim
/
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|___________|ADC|
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\
/
\Rout
/
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GND
 

Kath said:
I'm interfacing a current output from a current sensor to a microprocessor's ADC. From the following circuit, how do I calculate both the current and voltage that goes into ADC?
Click to expand...
There shouldn't be any significant current flowing into the adc. If there is, you may need to buffer the adc input signal to prevent measurement error.

The voltage into the adc is computed from ohm's law: V = Iout * Rout

You don't need Rin because you are using a current sensor. This sensor should produce a current that is independent of the applied voltage.
 

lambtron said:
You don't need Rin because you are using a current sensor. This sensor should produce a current that is independent of the applied voltage.
Click to expand...

Did you mean Rlim? I know that the Iout will be independent of the applied voltage. But I'd like to limit the current into the ADC, this is why I placed Rlim there, so that the current into the ADC will be Vadc/Rlim. Is that correct?
 

Hi,
For protection of ADC input, you must use one resistor between sample voltage (upper side of Rout) and ADC input.
for more protection you can use following circuit:
R2 ≈ 1K
R3 ≈ 1K
Diodes = 1N4148 or equi.

Regards,
Davood Amerion
 

You don't need to limt input current to an ADC .. Just make sure that the ADC's input voltage is within rated range, which in many cases is GND-0.5V to Vcc+0.5V ..

The circuit suggest by Davood will work for you (I think you don't need R3) but the type of silicone diodes (1V4148 or similar) may push the input voltage below or above the limit - check that with the ADC's data sheet - if it can't tolerate anyting below GND-0.5V and above Vcc+0.5V consider low power schotky diodes with Vf(forward voltage) <0.5V ..

Regards,
IanP
 

what is maximum current thru the conductor. You can choose the shunt resistor accordingly . use instrumetation amplifier or differential amplifier and u can connect the amplifier's output to ADC . Care must be taken potential difference between the condutor and your ground is withing the differential amplifier's input limit.
 

Hi, The best is to select Rout in your drawing based on your ADC resolution !. So if its 10 bits (1024 steps) you should take a 1024 Ohm or 10.14 Kohm resistor if your current is 5mA or 500uA. So that maximum current is giving your maximum ADC voltage range. This way you do not have to do any calculations since your ADC value is directly related to the current !.

Paul.
 

Kath said:
I know that the Iout will be independent of the applied voltage. But I'd like to limit the current into the ADC, this is why I placed Rlim there, so that the current into the ADC will be Vadc/Rlim. Is that correct?
Click to expand...
No! There should never be any current through the adc input.

The function of the adc is to produce a digital value whose magnitude is proportional to the voltage applied to the adc's analog input. So, unless you apply to the adc input a voltage that is outside the adc's supply rails (this should definitely be avoided!), you will not need to limit its input current because there won't be any input current. You can avoid this problem by choosing a value for Rout that guarantees Vadcin will not exceed the adc's maximum input voltage when Iout is at its maximum current:

Rout ≤ Vadcin,max / Iout,max
 

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