use ADC without external opamps

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guys my analogue electronics is weak.. pls bear up

i want to use an ADC to measure voltages in the range of 0.55 volts down to 0.044 volts with respect to ground. im using atmega128 and was wondering could i use this microcontroller directly to digitse the values without using any amplifiers

I understand that i can apply a gain of 20db(2x) for differential inputs and no gain for single ended inputs

SO can i do an adc with a single ended input treating it as a differential(one end to the input and the othe end to ground)?

Thanks Rodney

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Yes you can,
But the resolution is gonna suck. Better to use an op-amp and raise the input level to get more trnsitions and then convert the number.

Jjim

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Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB user

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im not interested in accuracy. just want to monitor current passing trough a battery. will measure the drop across the resistor(2.2E 5 watt) which is between ground and the negative terminal of the battery. output will be in the form of a blinking led. blinking rate proportional to charging current..... high charging current - fast blinking and low trickle charge - slow blinking.

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Ok then

Use the direct method, keeping in mind the paramters of the input circuit.

Keep in mind that a simple op amp follower configured properly will/could save you a boatload of headaches

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB user

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If accuracy is not an issue, you CAN reduce the ADC reference voltage to get more "span" out of the voltage you have. The device spec sheet will give the minimum acceptable value for ADC Vref. A simple external reference could provide that with less hassle than an op-amp. It just might be enough to help.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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If you don't need the accuracy, which means that there is no need of an amplifier to amplify the voltage, you can use the ADC it self. An important note is not to use ADC external in series resistanse more than 10K. This is something that also the datasheet says.
Also if the Vmax you have to measure is near 550mV, use as a voltage referense the external AREF pin voltage, that you might set at a low voltage (check the datasheet), in order to be able to measure and the lower level which is 44mV. Also use 10 bits measurement.

Michael.

Michael.

User of:
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If you have the spare pin for a differential measurement, use it. If not, a low AREF voltage (e.g. 1V) and averaging of many AD Conversions can help. With a low Aref you should have enough noise to make oversampling effektive to increase resolution.

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If i'm stating the obvious then please ignore me...

Is the resistor still in the ground lead when you are discharging the battery? (i.e. When the battery is powering its usual load?)

The polarity of the voltage across the resistor will change based upon whether the battery is being charged, or discharged.

Make sure your uC's ADC input can tolerate a sustained negative voltage input, whose magnitude depends upon the maximun current drawn by the load attached to the battery.

JC