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Bipolar adc...

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gatas

New Member
Hi there..I am not sure about the exact english word (I think it is bipolar) but i am searching for a simple-cheap adc that accepts input in the range of -5V to +5V...
I had used adc0804 before but i am not sure if it can convert negative input. In the datasheet there is an interface for -5V to +5V signals but they are using a Beckman Instruments #694-3-R10K resistor array, but i dont understand how it works. I 'd appreciate it if somenone could enlighten me on adc0804's ability of accepting negative input, or maybe suggest me another adc. A complementary dac (bipolar again) i could use in conjuction with the adc, would be fine...

Thanx in advance.....
 

Scubasteve

New Member
gatas,,

The input stage on that converter is just a 10k-10k resistive voltage divider. I think that any two 10k resistors will work, but I am not sure about why they chose that beckman part.. I believe that company has either changed names or went out of business, so I couldn't check the spec. sheet. Just try to use the most accurate resistors you can get..

Steve
 

acrid_22

New Member
gatas

I think that you should put pin 7(-Vin) to +5V, if you want to read a input from -5V to +5V, but you should need to change the Vref/2 value to +5V.
Im not sure about this, but it would work.

gil
 

Nigel Goodwin

Super Moderator
Most Helpful Member
It would be unusual for an ADC to cover positive and negative voltages, nor is it required. You simply bias the input of the opamp to +2.5V, a negative voltage will move it down towards zero, and a positive value will move it towards +5V.

It's a simple circuit, you need the output of an opamp that can move between -5V and +5V (10V range), when it's input is zero, then the output should be zero as well. Connect the output of the opamp via a resistor to the input of the A2D, and a second resistor from the input of the A2D to 5V, the two resistor should be identical. Notice that the two resistors do two tasks, they shift a negative to positive swing to a zero to positive one, and also divide by two - converting the 10V swing to a 5V one.

Your software needs to take care of the rest, it's quite straight forward to do.
 
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