LTspice®︎: The Fastest Way to Final ADC Designs

This blog covers a full evaluation of Precision ADCs with LTspice. As this blog series grows, supply lines get longer and harder to maintain. Previous blogs covered high-level tools that are great for product selection and general evaluation. The last blog discussed the evaluation of DACs with LTspice and is a useful preamble to this entry.

Figure 1: Choosing an ADC product for simulation. 

As with DACs, Figure 1 shows ADC products available for simulation, and the list is constantly growing as ADI releases new products. Again, this blog will talk about HOW the converters are modeled, and then how to use LTspice to evaluate full signal chains without needing to prototype a system design. It is helpful to remember that the fastest way to get started with LTspice simulations is to use one of the high-level tools such as the ADC Driver tool or Signal Chain Designer Tool to quickly build a signal chain, and then output the netlist directly to LTspice. Alternatively, a simple but complete circuit can be generated by selecting “Open this macromodel’s example circuit” in the selection box (Figure 2). As of the publication date of this blog entry, LTspice 17.1 is now released. LTspiceXVII is now frozen and out of date.  There are a variety of improvements, including many new models. It is important to be aware that LTspice XVII will not automatically update to this new version.  Therefore LTspice 17.1 must be downloaded and installed explicitly. This is highly recommended to get all the changes and improvements. 

Figure 2: Auto-generation of sample simulation netlist 

A review: LTspice is a SPICE-based modeler that has been optimized for the analog and switching systems that are common for ADI’s 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.  

Analog-to-Digital Converters (ADCs) are modeled as a reasonable approximation of the sample-and-hold circuit and analog filtering in the real product, with the output appropriately quantized. The input section models noise, headroom, settling, and includes Easy Drive functionality, where appropriate. Figure 2 shows one such model. The output is analog, and modeled as if the ADC’s digital output were then reconstructed with an ideal DAC. This is a convenient way to allow for virtual analysis of output and overall system performance in the analog domain, as no digital words or formatting are involved – what you see is what you get! Unlike DAC models, Vref is modeled.  

There are a handful of digital I/O functions that are modeled as they are required, or may be desired, for evaluation. These include convert start, for example, and can be controlled with a PULSE or PWL (piecewise linear) waveform to evaluate sampling delays and charge injection voltage kickback. 

Figure 3: ADC Test Circuits in LTspice 

ADC models are capable of simulation of many characteristics: 

Transient Simulations 

• Transient glitches of ADC Analog Inputs 
• Includes Easy Drive features for relevant products 
• Acquisition time of ADC 
• Channel sequencing on MUX ADCs (e.g., AD4696) 
• Estimate settling performance of AFE 

Noise Simulations 

• RTI Noise of ADC 
• RTO Noise of Full System 

There are ready-to-simulate circuits for AD4000 and AD4001 as examples. It is easy to use these as a starting point for system simulation. 

Figure 4: Evaluation Circuit for AD4000 

Figure 5: Evaluation result for AD4000 

The latest precision converter products from Analog Devices include AD4630 and AD4858. AD4630 includes a system simulation on the web page, as shown in Figure 6. This can be very helpful, as it showcases a carefully designed system that includes all required sources and components, plus settings associated with AD4630. 

Figure 6: Full LTspice system simulation for AD4630 

The next blog in this series will cover LTspice ADC simulation in a little more detail.