LTspice®︎: Bench-level System Evaluation Without the Hassle

This blog moves to full evaluation mode with LTspice. The LTspice tool itself has been covered in a variety of media, so the information here will address DAC models and evaluation specifically. Previous blogs covered high-level tools for product selection and general evaluation.

When using LTspice, it is helpful to note that new models are released frequently, so the Sync Release feature (Figure 1) is useful. The Show Change Log is also of interest, it will list new features. LTspice also checks that it is current every time it is launched. As of the publication of this blog entry, LTspice 17.1 is now released. There are a variety of improvements, including many new models. It is important to be aware that LTspice XVII will not automatically update to 17.1. LTspice 17.1 must be downloaded and installed explicitly. This is recommended.

Figure 1. Keeping up-to-date 

Once LTspice is up-to-date, products can be chosen from the drop-down list. Figure 2 shows DAC products available for simulation, and the list is constantly growing as Analog Devices releases new products. As a preamble, this blog will talk about HOW the converters are modeled and how to use LTspice to evaluate full signal chains without needing to prototype a system design. 

Figure 2. Choosing a DAC Product for Simulation 

LTspice is a SPICE-based modeler that has been optimized for the analog and switching systems that are common for customers. Models for ADI products are mid-level functional models. They are more efficient than transistor-level models, but not as detailed. The models are carefully designed to show how the products will behave in a variety of applications, and how they will tend to interact with each other and third-party components. LTspice is NOT a mixed-model simulator, so the digital engines and I/O are not modeled. In the case of DACs and ADCs, models are analog in and analog out. Digital-to-Analog Converters (DACs) are modeled essentially as an amplifier, with the input appropriately quantized. The function of a DAC is to drive predefined analog signals out to downstream circuitry. This function is modeled in a way that gives appropriate behavior in response to both the input signal and the external application circuit. The output section models noise, headroom, settling, and drive capability. Figure 3 shows one such model. This is a convenient way to allow for the generation of input stimuli and overall system performance, as no digital word generation is required – what you see is what you get! Vref is full scale, so the range is 0V to Vref. The load may be modeled as load resistance and capacitance, or with additional active components, such as a gain amplifier. 

Figure 3. DAC Model in LTspice 

There are quite a few multichannel products in the precision portfolio. These are all modeled as single channels. The user can add as many channels as required for a given simulation. Select DAC parameters that would affect analog performance are also modeled. In a DAC product with a programmable output range, use the .param syntax to choose a range, or right-click on the symbol and alter “Range = x”. 

Figure 4: Adding Multiple Channels and Parameterization 

Figure 5. Altering LTspice DAC Parameters 

DAC models are capable of simulation of many characteristics: 

• Headroom/Footroom 
• Frequency Response 
• Slew Rate 
• Settling Time 
• Short-Circuit Current 
• Offset Error 
• Full-Scale Error 
• Load Stability 

In addition, new models, such as AD3552R and AD5791, add the following: 

• Output Range 
• Step Response 
• AC Bandwidth 
• Noise Density 
• Device-Specific Parameters: DIGITAL SCALING, OFFSET 

New LTspice features for Fast DACs are covered in this article here.  

An excerpt from this article is shown in Figure 6. The symbol shows the available signals, while the model illustrates the various features that are parameterized: Range, Gain, Offset, and Polarity. 

Figure 6. Model for AD3552R 

With LTspice models becoming more lifelike and feature-rich, it is helpful to see examples of system simulations, as shown in Figure 7. Setting up a true application circuit is not difficult, but requires several components, power supplies, and signal sources. To instruct the user in the detail required, products such as AD5791 show examples. These are accessible from the product landing page. Using these tools and examples, it is simple to evaluate and optimize a system design without the expense and delay of prototyping. 

Figure 7. Sine Wave Generator 

The next blog in this series will cover the LTspice modeling and evaluation of ADCs.