Why should we need DAC ICs?

Question

Please look at the schematic below. This is a very simple resistive adder that works fine with any standard ! (TTL,CMOS,...) or any arbitrary voltage that is fed into it. On the other hand as there is no active component in it, it is extremely fast. It is just made of a few resistors , so it is very cheap. On the other hand there is no limitation for number of input bits ( it can be easily expanded to 32, 64 or hundreds of bits ).

So, Why should we need DAC ICs? I am looking for a 32-bit high frequency DAC. Such devices are not found easily and also if found, they are rather expensive. I mean What is the advantage that I should pay for to find such devices? I think there must be some advantage that they worth to be bought. The only thing that I can think about is their inherent amplification ( for example TTL -> 10V or so), but this goal is simply achievable with any kind of amplification.

Answer 1

What you have right there is what is known in the field as an R-2R DAC, one of the many different kinds of often employed digital-to-analog converter topologies. You have answered your own question: why do we need DACs when we have this DAC topology? Because it is a DAC!

R-2R DACs purely by themselves are not great as a general purpose digital to analog converter. The output impedance of an R-2R DAC is very high, which means that the bandwidth will quickly be very limited. Even a few tens of picofarads capacitance on the output will reduce effective bandwidth and increase settling time to the MHz region. And this is equally true if you buffer the output with an opamp follower - well-trimmed opamps don't come in sub-pF input capacitances, and reducing the R-2R ladder resistances quickly increases power consumption to the point where it is unacceptably high. Don't get me wrong, there are super high bandwidth R-2R DACs on the market, but these are the kinds of chips that you find in arbitrary waveform generators in some scopes, and they have a bit heatsink and fan on top of them.

There are other tradeoffs you can do with other DAC topologies. For instance, delta-sigma DACs do not to have a precision buffer output opamp and thus can be extended to very high bit depths (24-32 bit), where R-2R - because of the output buffering criterion - seldomly surpasses 12-bit. Successive approximation is another topology that is used, which inherently has a sample-and-hold on the output that can be driven with extremely low impedance (the same reason why conversely SAR ADCs can have a very high input impedance).

Answer 2

What you have there is called an R2R-resistor ladder. The ICs you can buy also have such a network internally, but because it is integrated, it is much easier to guarantee the accuracy. See the Wikipedia entry as to why it is so important to have exact resistor values. I'd say it's almost impossible to achieve the accuracy of integrated circuits with discrete hardware.

Also many DACs have serial interfaces, so you don't need as many pins from your MCU to use them.

Answer 3

• Size vs bits of resolution (depends on the architecture)
• Power comsumption (passive, also depends on the architecture)
• Load effects (input / output impedances)
• Quantization steps level dependant on devices connected (see load effects)
• Precision
• Accuracy
• Noise
• Bandwidth dependant on devices connected (see load effects)
• Too many critical components ...