DAC Essentials: The resistor ladder

Other Parts Discussed in Post: TINA-TI

In my last post, I discussed the string DAC architecture and its tendencies― if you missed it you can view that post here. This post will focus on two very similar architectures, the R-2R DAC and the MDAC.

Recall that the big limitation of the string DAC is the challenge associated with realizing a high resolution design and maintaining linearity. The number of resistors required for the string DAC increases exponentially as resolution increases, unless clever design tricks are implemented like cascaded resistor strings or interpolating amplifiers. The R-2R architecture directly addresses this problem by utilizing a binary weighted resistor ladder structure, as illustrated in Figure 1: 

 Binary weighted resistor structure

Figure 1: Binary weighted resistor structure

Each bit of resolution of the DAC is made of one set of R resistor, 2R resistor, and 1 switch that moves between the reference voltage and ground creating a voltage divider at the output node. Typically there is an on-silicon output buffer included. If you’re having a hard time understanding how the binary weighted divider works, I recommend creating this circuit in TINA-TI where you can simulate each switch position.

The R-2R DAC sees a variable impedance at the reference node, so a reference buffer is a requirement for applications where the DAC output is going to be changed very frequently in order to keep reference settling time from impacting DAC output settling time. Usually this buffer is included on silicon, just like the output buffer, but make sure you consult your device’s datasheet before making any assumptions.

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The MDAC, or multiplying DAC, as seen in Figure 2, uses a very similar topology – the same R-2R ladder in fact – except that the position of the reference input node and the output node are swapped. This change makes the R-2R ladder a current divider rather than a voltage divider. Because of this a transimpedance output stage is required to convert from current output to voltage output. Typically this buffer is not included on silicon but the feed-back resistor required for the transimpedance stage is included. Featuring the feedback resistor on silicon helps to ensure resistor matching to the rest of the R-2R ladder in terms of both value and drift behavior.

Not including an output buffer may seem like a disadvantage to the MDAC but in many ways it is actually quite helpful. Moving the amplifier to outside the device allows for great flexibility in design, applications with relaxed requirements can choose cheaper amplifiers and save money versus the price of a fully integrated solution. Meanwhile, in high-performance systems it is beneficial to utilize discrete amplifiers that are made on strong pure-analog processes.

 Multiplying DAC

Figure 2: Multiplying DAC topology

As far as performance tendencies go, the R-2R and MDAC architectures have very similar properties and we’ll discuss them simultaneously.

Since there are fewer resistors in these designs we can implement a much more comprehensive trimming scheme to achieve very strong linearity. Trimming circuits also take up space, though, so the R-2R and MDAC packages tend to be larger than the string DAC.

The number of switches moving in the R-2R and MDAC designs is code-dependent. In some cases, every switch of the DAC may have to move to increment to the next code which will create much higher glitch energy than the string DACs.

Some things to remember about the R-2R DAC and MDAC:

  • Excellent INL/DNL
  • Medium to high glitch energy
  • Larger packages

The R-2R and MDACs find homes in virtually any application that is very high performance. That includes industrial programmable logic controllers, automated test and measurement equipment, precision instrumentation, and various other applications. Since the MDAC does not feature reference or output buffers it can deliver good results in medium speed applications as well such as waveform generation or AC attenuators. If these devices are of interest to you be sure to check out a few TI R-2R DACs and MDACs such as: DAC8811, DAC8822, DAC8734, DAC9881, DAC8881, or DAC7654.

This post finishes out the discussion of the DAC basics. Tony Calabria will continue the DAC Essentials series with a deep dive into glitch and glitch reduction in the next few posts so stay tuned.

As always, leave your comments in the section below if you’d like to hear more about anything mentioned in this post, or if there is something you would like to see included in a future post!

  • Hi Cameron,

    The exact implementation of trim circuits varies from one implementation to the next and is kind of the secret sauce by which semiconductor manufacturers achieve high-performance DAC products. Segmentation is very common in string DAC applications, which I think I touched on in another post, where an interpolating amplifier is implemented. Segmentation also shows up in R-2R and MDAC applications in the upper bits which helps intrinsically create some level of monotonicity, but given the necessity of matching the R, 2R, and Rfb values in order to achieve high linearity - not just monotonicity - sometimes you will find laser trimming on the resistors (for older products) or small current sources to correct for mismatch. In the end it boils down to what your process node is capable of, what you can achieve in layout, and what the segmentation looks like - more and more often hybrid structures are deployed which combine string and R-2R architectures, and potentially even interpolating output stages, to try to balance size, cost, and performance. 

  • "Trimming circuits also take up space, though, so the R-2R and MDAC packages tend to be larger than the string DAC"

    Could someone explain this please? I never knew additional circuitry would be needed for trimming... curious what this looks like!

  • @user4252298

    You are correct, of course. Copy/paste mistake from old graphics. Updated. Thanks.

  • The second figure is not correct!

    The MSB is the LSB and vica versa.

    At the correct LSB side must be conneced an additional 2R resistor to ground.

  • @tendo ren: That is a very good question!

    As I briefly mentioned in the blog series, the string DAC frequently implements other tricks like segmentation, cascaded resistor strings, or interpolating amplifiers to augment the resolution of the basic resistor string. A 16-bit string DAC, for example, quite frequently doesn't actually have 2^16 resistors. Instead, maybe only 8 bits or so of the DAC are in the resistor string and the rest come from other novel tricks that help bring the size down. Furthermore, a trim program either doesn't exist or is very minimal for a string DAC.

    The R-2R and MDAC structures, however, really do use the structures I've illustrated in the blog series. Since these designs use fewer resistors it is possible to implement a very comprehensive trim scheme for each resistor to deliver strong linearity. This trim scheme also takes up space, though, and is what usually contributes to the size of the R-2R and MDAC structures being larger than their string brethren.