Ltspice Simulation Guide for AD736 RMS Converters Released

Engineers working with LTspice simulations of the AD736 true RMS-to-DC converter often encounter frustrating delays, where progress bars remain stagnant for hours. This article examines the root causes of these simulation bottlenecks and provides systematic solutions to optimize circuit design and application of the AD736.

A common mistake when simulating the AD736 in LTspice is confusing schematic symbols (.asy files) with complete circuit models. Simply having a well-designed schematic symbol is insufficient—LTspice requires detailed device parameters and behavioral descriptions in .lib or .sub model files. The core issue arises when users attempt to import AD736.cir files directly without properly loading them as simulation models, causing the simulator to enter infinite calculation loops.

Solutions:

• Obtain Correct Model Files: Official AD736 models typically come in .lib or .sub formats. If only a .cir file is available from Analog Devices, verify whether it contains a complete SPICE model or requires extraction of subcircuit definitions. As an alternative, consider using LTspice's built-in AD8436, which offers similar functionality with robust model support.
• Proper Model Loading: Place the correct model file in LTspice's library path or add it via "Place" > "Component" > "X Devices" > "Library." For .cir files, ensure proper subcircuit references in the schematic.
• Understand .options Directives: Commands like .options cshunt=10f gshunt=1n abstol=1n vntol=1m should be added to SPICE simulation settings via "Simulate" > "Edit Simulation Cmd" or by placing a "SPICE Directive" component. These adjustments can improve numerical analysis convergence.

The AD736's performance depends heavily on proper reference points and virtual ground construction in single-supply applications.

Input Signal Reference Points:

• Connection to Power Negative Terminal: While AC coupling filters DC offsets, this approach may introduce interference and stability issues due to interactions between source impedance and power supply characteristics.
• Connection to Virtual Ground (VG): Linking the input signal to a resistor-divided virtual ground ensures alignment with the AD736's REF/COM reference. However, the higher impedance of virtual grounds may require buffering with operational amplifiers or careful resistor selection to minimize loading effects.

DC Output Reference: The AD736's DC output is always relative to its REF/COM terminal (VG), which critically determines ADC sampling dynamics.

AD736 vs. AD737 Output Characteristics:

• AD736: Produces positive-going outputs relative to VG (e.g., 0mV to +200mV).
• AD737: Generates negative-going outputs relative to VG (e.g., 0mV to -200mV). This polarity difference requires careful ADC interface design to prevent signal clipping.

Maximizing ADC performance requires appropriate conditioning of RMS converter outputs.

Differential-Input ADCs: The ideal interface uses VG as one ADC input and the converter output as the other, leveraging differential measurement to reject common-mode noise.

Single-Ended ADC Strategies:

• Differential Amplifiers: Low-power instrumentation amplifiers (e.g., AD8226) provide gain while eliminating DC offsets.
• Signal Inversion for AD737: Operational amplifier circuits can invert negative-going outputs and apply DC biases to match ADC ranges.
• Gain Calculation: Scale outputs based on ADC reference voltage and resolution (e.g., 25x gain for 5V ADC reference with 200mV full-scale output).

Alternative Virtual Ground Solutions:

• Charge Pump Regulators: Devices like TC7660 generate negative supplies from positive rails, simplifying output measurement relative to battery negative terminals.
• Low-Power Instrumentation Amplifiers: Combined with charge pumps, these optimize ADC interfacing.

Software-Based RMS Calculation: For less demanding applications, microcontroller-based RMS computation eliminates hardware components but increases processing load.

Cost Considerations: The AD736's premium performance comes at higher cost—designers may opt for AD737 implementations or software solutions where appropriate.

By addressing model loading issues in LTspice and carefully implementing virtual grounds and ADC interfaces, engineers can overcome simulation challenges and achieve accurate RMS measurements in their designs.