Will adding LEDs as follows affect my R2R ladder and give wrong outputs?

 

No.

 

It will give incorrect outputs and with the values shown the LEDs won't light either as there won't be enough current through them.

 

I want the leds to light corresponding to the binary number I input, How can I do this without affecting the output?

 

You would need to use double pole switches. One pole switches the resistors and the other switches the LEDs.

 

You are there, listen to OBW0549 this time. If you were switching from ICs , then drivers might be needed.

 

It will give incorrect outputs and with the values shown the LEDs won't light either as there won't be enough current through them.

I though the same at first , but look at the circuit again.
The LEDs are switched between 10V and ground.
They will light up properly and have no effect on the DAC output voltage.

 

I though the same at first , but look at the circuit again.
The LEDs are switched between 10V and ground.
They will light up properly and have no effect on the DAC output voltage.

Yes, you (and TS) are correct. Please ignore all previous posts.

 

Hello guys , thanks for the replies. I know it should not affect the output buy im having a drop in output voltage when I add the leds and don't know why.

 

Hello guys , thanks for the replies. I know it should not affect the output buy im having a drop in output voltage when I add the leds and don't know why.

Is your 10 volt supply stable, the same voltage whether the LEDs are on or off?

Are you using actual switches, like toggle switches, or are your "switches" actually semiconductor devices (transistors, MOSFETs, etc.)? If the latter, be aware they will drop some voltage across them, even when on.

Can't think of anything else...

 

Is your 10 volt supply stable, the same voltage whether the LEDs are on or off?

Are you using actual switches, like toggle switches, or are your "switches" actually semiconductor devices (transistors, MOSFETs, etc.)? If the latter, be aware they will drop some voltage across them, even when on.

Can't think of anything else...

Instead of switches, I'm using arduino( and 5 V supply) to input the binary signal .I also need to connect the a 7-seg decoder to display the decimal number on 7 segment.Just for testing I'm using arduino, I'll later replace with toggle switches.I also tried adding opamps as buffer to the inputs and add the leds to +ve terminal- again there is distortion in the output signal - it goes high, then low again then high.

 

A logic HIGH output from an Arduino port pin (or from any other microcontroller, for that matter) does not go all the way to +5V, and may not go much higher than 4-4.5V when driving an LED. And the more current you draw from the pin, the lower the HIGH output voltage will be. That's why your LEDs are interfering with your DAC accuracy.

 

Use a CMOS buffer, such as a CD4050, to drive the network and a separate CD4050 to drive the LEDs.
It should get very close to 0V and 10V when driving the over 200kΩ equivalent resistance of the network.

 

Hello guys , thanks for the replies. I know it should not affect the output buy im having a drop in output voltage when I add the leds and don't know why.

Hi,

How much of a drop?

If there is significant resistance in the 10v power supply line then THAT will cause a voltage drop with LED's because the LED's then draw significant current though the line when before it was just very light current.

Are you by any chance using a solderless breadboard? (also called a plug board).
If so, that could be the problem. The resistance of these boards varies with different makes and models and age.

 

Thanks for all the answers guys , Yes i'm using breadboard maybe this is the problem.... I've removed the leds and I'm using a 7 seg decoder to display the decimal number on hex display and all is fine - I think I'll have to remove the leds from my project.

 

Thanks for all the answers guys , Yes i'm using breadboard maybe this is the problem.... I've removed the leds and I'm using a 7 seg decoder to display the decimal number on hex display and all is fine - I think I'll have to remove the leds from my project.

Hi,

If you ditch the plug board it may work fine just the way you had it. Actually, it should work fine.

 

Thanks for all the answers guys , Yes i'm using breadboard maybe this is the problem.... I've removed the leds and I'm using a 7 seg decoder to display the decimal number on hex display and all is fine - I think I'll have to remove the leds from my project.

Check if the voltage on your +10V supply drops (even slightly) when an LED lights up. The “internal resistance” of the 10v supply may be fairly high and not intended to be stable power unless it is driving 100k ohm loads.