Hello

Could someone advise how I could speed up an R/2R DAC made of discrete resistors on a breadboard (8-bit)?
So far I'm using 10k/20k 1% and the output starts deteriorating after shorter than 1uS rise/fall times are required.

I'm guessing the input capacitance of the scope messes with the high output impedance of the R/2R chain but I've tried buffering it with an op amp with only marginally better results.

Will reducing the chain to 1k/2k give me better results?
Will building the dac on a PCB give better results?

My target is at least 100nS or shorter rise/fall times.
I'm using a 50Mhz FPGA to drive the R/2R chain.

 

What is the make, model and bandwidth of your scope?
What is the make, model and bandwidth of your scope probe?

 

Brand: OWON DS6062V
BW : 60Mhz , 500MS/s
Probes : T5060 -> Rated at 60Mhz with about 20pf @ 10x (which is what I use) .

I doubt that the issue is the scope because I've already measured signals faster than 40 and 50 Mhz with no problems.

Also I tried several Op amps with decent slew rates ( > 12V/uS ) . My input signal is 3.3V pk-pk so that should be the issue either.

 

It would seem like you should get a faster rise-time than that, even for typical breadboard stray capacitances. I doubt that putting it on a PCB will significantly affect that. Did you measure the risetime of the signal out of the FPGA to the R-2R input?

 

My guess is you are doing something wrong. You should be getting lower than 50ns rise/fall times.

 

I scope the FPGA outputs and the rise/fall times are < 10nS as expected. Well if anyone has an idea , let me know.

 

Post your circuit diagram and a picture of the layout, if possible.

 

Are you sure the problem is with the ladder and not the speed of the op amp used for it?

 

I'm guessing the input capacitance of the scope messes with the high output impedance of the R/2R chain

I haven't calculated that impedance, but 20k x 20pF = 400nS time constant; rather too close to your 1uS pulse width?

 

what swwitch are you using to do the r/2r switching? how fast can it switch? how much input output capacitance for the switch?

 

I got myself a bunch of 1% 1.1k / 2.2k resistors and remade the ladder using them . The results are much better. I can get <50 ns rise/fall times
when scoping the output of the dac directly.

The next problem is that i'm going to use the output of this dac to drive a 75Ohm vga signal. The question is , what do I buffer the output of the dac
with?
A simple try using an op-amp buffer failed. I'm getting very bad results for some reason , even at lower speeds. What can the issue be (the issue is not a failure of driving the output close to the rails)?
I'll upload a schematic and scope output pictures when I get home from uni.

 

Use a single NPN transistor in a voltage follower configuration.

 

Do you think that this will suffice ? Or should I put a simple two-transistor current source instead of the emitter resistor?

 

It seems to be working . Thanks