Connecting a buffer to an AD converter

[gior]

Actually i am new at this forum and i am an undergraduate student. I work on a project in which i want to convert an analog signal to a digital one using LEDs. But the LEDs didn't seem to light sufficiently so i decided to use a buffer. But i don't know how to connect it with the ADC..

 

[alexan_e]

So you have a digital output of say 8bit and you wany to drive a led with each bit?
You can use transistors
Transistor Circuits check Using a transistor as a switch
Using Transistor as a Switch | ermicroblog

Alex

 

[gior]

I could i suppose but in this case i will need 8 BJTs right(one for every bit of the ADC)? Wouldn't it be more practical with a buffer?

 

[alexan_e]

You want to use a buffer IC with 8 channels?
you can use a ULN2003 but it has 7 channels

Alex

 

[gior]

I think this will work! Thank you for your help!

 

[alexan_e]

Don't forget to add one resistor in each led to limit the current.
also note that the output of the ULN2003 drives the cathodes, the output can either provide ground or leave the output floating so you have to connect the positive supply to the anodes of the leds using resistors and the cathodes to each output of the chip.
Usually a resistor of about 220-270 ohm if fine to drive the led with 10mA using 5v power supply

Alex

 

[gior]

I used 330 resistance and it works fine! But now i have a problem with the start of the conversion. When i connect the circuit with the voltage source the 6 first LEDs(the least significant ones) stay "on" regardless of the Vinput of the ADC..I change the Vin theoretically with a potentiometer but the digital word won't change. I have connected the ADC0820 with the Self-Clocking in Free-Running Mode as described in the datasheets.. Any idea? What am i doing wrong?

 

[alexan_e]

I haven't used it but you should provide some king of schematic of what you have connected to control pins and the signals you provide, maybe something is wrong.
Do you use a specific schematic from the datasheet?

Alex

 

[gior]

Yes i used the Self-Clocking in Free-Running Mode schematic that i found in the datasheets( **broken link removed** ) I thought of another way using the timer 555 to trigger the adc but it wouldn't work neither..

 

[alexan_e]

This schematic doesn't show the Vref, how did you connect it?
Did you use any of the schematics in page 8 (2.5v or 5v)?

Alex

 

[gior]

I connected the Vref with the 5V.
No i did not use any of these but i can't see how it will help since my problem is how to start the conversion of the adc..

 

[alexan_e]

So Vref/2 is floating and Vref to 5v.
Have you applied gnd for a short period to the WR pin as shown in your schematic to start conversion?
"*After power-up,a momentary grounding of the WR input is needed to guarantee operation."

Alex

 

[gior]

The schematic shows the WR connected to the INT and the operation starts when you connect them to the GND right? I have checked the connection one hundred times but i still can't find what's wrong..

 

[Iuri]

I should recommend you to take a look on IC 74LS541, it have 8 channels and tri-state mode: http://www.datasheetcatalog.org/datasheets/185/334969_DS.pdf
I think that it can be better than ULN2003 in this application.

Iuri~