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I am doing an experiment to build and test a 4-bit Digital to Analog Converter based on an R/2R ladder network. I build the DAC that looks like based on this schematic:

Schematic

This circuit was hooked up to the counter chip and function generator which was set to generate square wave and 10 kHz frequency. Then I connected the DAC to an oscilloscope to test the result. The oscilloscope shows a staircase shape. As I increase the clock frequency, the edge of the stair steps becomes rounded and no longer reaches the 0 volt line on the oscilloscope (why does it no longer reach 0 volt line?) and the DAC becomes unusable, why?

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  • \$\begingroup\$ Some component is limiting the slew rate. \$\endgroup\$ Oct 28, 2013 at 20:06

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The output impedance of your DAC is very high. Not a desirable trait if you want to get any speed out of it. Decrease all your resistors by a factor of 10 and you will get more speed out of it.

Also (the edge of the stair steps become grounded) I assume you meant rounded.

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  • \$\begingroup\$ oh yes, it should be rounded \$\endgroup\$ Oct 28, 2013 at 20:23
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I presume you built this on a breadboard, no? Breadboards are very well known for having a lot of capacitance between the contacts. Any parasitic capacitance in your circuit will end up acting like a low pass filter, which will smooth the edges of the stair steps and limit the usable frequency range. The cutoff frequency of a first-order lowpass filter is 1/RC. To increase this, either decrease R or C. You can't really do much about C, but you can rebuild the circuit with smaller resistors to decrease R.

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