Have a question about ADCs or DACs? Ask our Expert, Nick Gray

.  I am trying to design a solar positioning  system  to control the  movement of a panel useing the cn7485 comparator  I need a  4 bit analog  to  digital  converter   to feed the 7485

THANK   YOU  .TIM   td1022@yahoo.com

.  I am trying to design a solar positioning  system  to control the  movement of a panel useing the cn7485 comparator  I need a  4 bit analog  to  digital  converter   to feed the 7485

THANK   YOU  .TIM   td1022@yahoo.com

Hi Tim,

In general, 4-bit ADCs (analog-to-digital converters are very high speed (hundreds of megasamples per second and higher) devices and expensive. I would look for an inexpensive 6-bit or 8-bit ADC and just use the 4 MSBs from it. This will be less costly than using an  ultra high speed 4-bit ADC. The reason that a really inexpensive ADC is adequate is because you will end up with more than enough accuracy for four bits and even the cheapest of today's 8-bit ADCs will perform well at the 4-bit level. The problem might be that today's loc speed, low cost ADCs generally come with serial output. This is to lower the packaging cost, which can be high with a parallel output. Here are some suggestions:

From Analog Devices: AD7170BCPZ-REEL7AD7170BCPZ-REEL7, AD7478, AD7171BCPZ-REEL7AD7171BCPZ-REEL7, AD7468.

From National Semiconductor: ADC081S021CIMF/NOPBADC081S021CIMF/NOPB

From Texas lnstruments; TLC5510 (parallel output, twice the cost of those above)

Having answered your question, I now offer what I think is a less costly solution. It certainly seems that you are considering a primaryly digital solution. I suggest an analog solution with two sensors placed within a fixture with two chambers side by side such that one light sensor is facing up in the bottom of each chamber. The chambers would be open only at the top and the side that is opposite from the other chamber. The output of the sensors would go to the "+" inputs of different analog voltage comparators (you can use a dual voltage comparator in a single package), Between the two sensors place a resistor, potentiometer and another resistor in series with each other. The two resistors connected directly to the sensor outputs could be 10 k-Ohms to 100 k-Ohms and the pot can be 50 k-Ohms to 1 Megohm. The arm of the pot and the "-" inputs of the two comparators are connected together. Between the output and the "+" input of each comparator put a 50K to 100K resistor for hysteresis to keep the comparators from oscillating.

With the sun shining fully on both sensors, adjust the pot all the way to one end of its travel and confirm that one comparator output is high and the other is low. Adjust the pot all the way to the other end and confirm that the output conditions of both comparators have changed. If this does not happen, increase the value of the pot. Once you settle upon a pot value, adjust the pot to one side, again with light shining equally on both sensors. Then move the pot toward the center until one of the comparator outputs just barely changes state. Measure and record the voltage at the arm of the pot. Move the pot all the way to the other end of its travel and then move it back until the other comparator output just barely changes state. Measure and record the voltage at the arm of the pot. Determine the average of these two readings, then set the arm of the pot for that average voltage. Connect the comparator outputs to a system that will turn the positioning motor one direction when one of the comparators is high and the other low, and turn the motor in the other direction when the opposite is true for the two outputs. When the comparator outputs are the same, then the motor should not be driven. I will leave the design of the drive system to you.

Good luck!

Nick Gray