Is there ADC for current measurement?
Or need external resistor circuitry to somehow convert current to voltage?

 

Thanks to an expert, Nigel.

I think he just taught me that it can be done by first converting the current to voltage through an op-amp circuitry.

Please correct me if I'm wrong. Need advice, please?

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Lord Robert Stephenson Smith Baden Powell\'s last message : -BE PREPARED-

 

You simply measure the voltage drop across a resistor in series with the load - simple ohms law, a 1 ohm resistor will drop 1V for every amp of current.

As you probably can't afford to lose 5V off your supply rail, it's usual to use a smaller resistor and amplify the voltage drop with an opamp - this also allows you to reference the voltage to 0V (by using a differential opamp circuit).

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and be sure to check the wattage of the resistor. if there is going to be 1A through the 1 ohm resistor then you need a 1W resistor. and remember that the power dissipation varies directly with the square of the current.

P = I^2 * R

 

Thanks! Now i've got rough idea of how things are done.

Now I am having difficulties in selecting components.

Suppose I use 5V supply, which Op-Amp and ADC is suitable??

There are too many op-amp, and there are quite a lot that can fit into this application as well. Which type of op-amp is more appropriate?

 

what are you thinking of trying to measure??
am i correct that you are not trying to measure 5A of current ?
hopefully ?
i have used a 1 ohm resistor on many occasions to measure current -in the 0 to 200mA range.
if you set your reference on the adc correctly To 1/2 your estimated maximum input Voltage .you should be fine..

 

what i am really trying to say is you should study the adc that you intend to use.. becaust there is a minimum voltage that can be converted on all ADC's .
dont quote me on this but 19 mV comes to mind on the 0804 adc
this means that 0 to 18 mV ouputs a 00000000 on the 0804
and 19 mV to 37mV or so produces a 00000001 ..and so on
anyway check the LSB voltage this will give ya an idea of what ya can expect..

 

the current input range is from 0 - 20mA.

The thing bothering me now is, I have freedom to use any ADC and op-amp, but I have no idea exactly which one the be used.

I need an ADC with SPI interface , in PDIP package. Do you have any ADC in your mind now?

 

the MPC 3004
http://www.microchip.com/stellent/id...cName=en010529