ADC problem...

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vIEvod
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ADC problem...

Post by vIEvod »

Hi all, need some help with a circuit found at http://www.cae.wisc.edu/~mcardle/Gear_Indicator.htm . I don't know what values to use for R1, R3-R8 (not using R9) in order for the ADC to output the correct binary sequence to drive the BCD/7seg driver and get the LED to count 1 thru 6. My theory is really rusty, can anyone spare a minute to offer some help?<p>TIA :)
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Edd
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Re: ADC problem...

Post by Edd »

The value of R1 will be dependent upon the values of the other resistors in the rotary sensor/ selector unit.. So you need to ohm out each resistor and see what their values are. In order to get the most uniform and maximum spaced spread, the adjacent resistances are typically going to be in a digitally weighed inter relationship.
See data sheet : http://www.national.com/ds/AD/ADC0801.pdf
See Page 34..Locate op amp A4 section… that spread of resistors just below it is digitally weighed. Notice the initial 24k and then a doubling of the value with each adjacent resistors value……however I would suspect your values to be starting with lower values of resistances and not spreading on up to a 3 meg resistor on the high value end.
Your computed/selected R1 value in conjugation with the gear position selected resistor will make up a voltage divider and present a derived voltage to the AD’s input and it will then produce its coded output to go on to the 7447.
The selectors neutral position resistor would be the lowest value of resistor and with its bottom end grounded by the gear selector, it would pull the voltage passed thru R1 to the lowest value of voltage that would reach the DA input. The adjacent resistive values are going to pull the voltage progressively less to ground with a resultant evenly spaced higher output of voltage samples being introduced to the DA input as shifting up thru the gears. The encoded DA output steps on up accordingly to be sent to the 7447.
To initially calibrate, take a DC power supply thru a 1K isolation resistor and apply voltage to the bare AD input until the DB5 output shifts on, (as they are using the less touchy last three logic states of the AD to input into the 7447), log down that voltage and advance on up until DB6 switches and log in that voltage..Compute the midpoint voltage and ideally that is the voltage that you would like for your .voltage divider made up of gear 1’s selected resistor and the computed value of R1 that will produce that “in the middle”voltage level. The other values will fall into place as the progressively weighted resistors of the sequentially shifted gears resistor values switch in..
Now if perchance you are without any resistors in the unit to begin with, you would probably be best in selecting a 2400Ω resistor for gear position 1 and then computing /selecting R1 value to place the output voltage in the center of the range as mentioned above and then select the other sequential gears values to fall in the center of the range of their required voltages. Common EIA resistive values should
Be available values to accomplish that. <p>73's de Edd
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