LED does not shut down completely when driving with Op amp

Thread Starter

a.rezaei.1984

Joined Nov 13, 2017
3
Hi
I have a circuit including 15 LED. The system that I used is similar to the analog device reference design. it works fine but there is a problem.
when I turn the led off (+ input pin of op amp connected to GND), a very dim glow remains and the LED does not shut down completely.
the only difference between the attached schematic and my circuit is that I use AD8618 and ADG633. the rest is almost the same. LEDs are connected to +5 V.
I checked most of the pins and voltages and unfortunately I could not find the problem.
would you please help me to find the problem.
thank you
 

Attachments

wayneh

Joined Sep 9, 2010
17,155
I can’t see all of the schematic but I’ll just guess that your opamp can’t output close enough to the lower voltage rail to turn off your LED.
 

be80be

Joined Jul 5, 2008
2,057
You can't see the whole circuit be nice if you posted a link to "analog device reference design"
There's lots of them. And the full circuit.
 

Thread Starter

a.rezaei.1984

Joined Nov 13, 2017
3
here is the complete schematic,
http://www.analog.com/en/design-cen...ence-design/circuits-from-the-lab/CN0363.html

but, as I said, I just use a part of complete schematic, since I only need to turn the led on and off. also I have to control the light intensity and this circuit works fine.
The LED is connected to the MMBT Tr.
ADG633 is a triple double throw switch IC. I have 5 ADG633 and 4 AD8618 (4 op amp in a package).
 
Schematic is blurry. I had this issue when I tried to replace a incandescent lamp used as the Stereo indicator with a LED. I had to add a Zener diode. This was back in the late 1980's.
 

Alec_t

Joined Sep 17, 2013
12,311
Try connecting a resistor (~1k-10k) between Q3 base and ground.

Edit: .... or a diode or two in series with Q3 base.
 
Last edited:

ian field

Joined Oct 27, 2012
6,539
Hi
I have a circuit including 15 LED. The system that I used is similar to the analog device reference design. it works fine but there is a problem.
when I turn the led off (+ input pin of op amp connected to GND), a very dim glow remains and the LED does not shut down completely.
the only difference between the attached schematic and my circuit is that I use AD8618 and ADG633. the rest is almost the same. LEDs are connected to +5 V.
I checked most of the pins and voltages and unfortunately I could not find the problem.
would you please help me to find the problem.
thank you
The transistor appears to be configured inside the op-amp feedback loop to be a constant current generator - probably not surprising you have problems making it work also as a switch. You could probably get somewhere by pre biasing the other input so the DAC forces it to cut off. A single rail op-amp like the 324 is probably better at going rail to rail.
 
Top