Tracking Voltage and Current in Your Circuit

October 19, 2020 Cadence PCB Solutions

Key Takeaways

  • It can be difficult to track voltage and current throughout a complex system due to the large number of nodes in a circuit. 

  • When you need to visualize voltage and current in different circuits or sub-circuits, you should use a circuit design program with an integrated simulator. 

  • This set of analysis features provide a simple way to examine current and voltage distribution in a circuit before creating a PCB layout.

Tracking voltage and current in electronics schematics

Tracking voltage and current starts in your schematics.

Your schematics are where your design begins and the real engineering happens. Your schematics should also be the place where you feel free to experiment with different design choices before committing to a specific design and layout. If you use the right set of schematic design tools with an integrated simulator, you won’t have to be constrained to a specific design and you can examine the electrical behavior of your system before you start your PCB layout.

An important part of designing schematics is tracking voltage and current throughout circuits. When your schematic editor includes a set of integrated simulation and measurement tools, it’s easy to visually track where power is distributed in your circuit. Not all SPICE simulation packages give you the tools needed to track where the voltage and current are distributed throughout a circuit, so you’re forced to read through lines of text to determine voltage and current at each node in your circuit.

Instead of using older SPICE simulators, you can visually track current in the time domain and frequency domain with the tools in PSpice. Your schematic design and capture tools in OrCAD/Allegro will integrate with the powerful simulation engine in PSpice, giving you a simple way to track voltage and current in your circuit as well as quickly make design changes when necessary. Here’s how you can use Capture CIS and PSpice to visualize power distribution before you create your PCB layout in Allegro.

Tools for Tracking Voltage and Current

The voltage and current in your circuit is distributed according to some basic physical laws. Specifically, the voltage and current need to be tracked between nodes and at nodes, respectively, using Kirchoff’s laws and Ohm’s law. For simple circuits, even when nonlinear components are present, it’s quite easy to track voltage and current around a circuit. You can usually do this with pen and paper, as well as some simple online calculators.

Real circuits being designed for complex PCBs or ICs can be very complex and often do not reduce down to a simple set of simultaneous equations that need to be solved. For a linear circuit with dozens of nodes, the matrix equations you formulate for Kirchoff’s laws are solvable in principle, but the problem is intractable and prone to serious errors. Once you add nonlinear components into the circuit, the standard matrix equations often reduce down to a set of transcendental equations that must be solved numerically.

SPICE simulators that integrate into your circuit design tools are ideal for tracking voltage and current at different nodes in a real circuit. This is a matter of simply using the measurement tools built into your circuit simulator. You can then watch how voltage and current distribution change as different parameters in the circuit are adjusted. In this example, we’ll look at a schematic with multiple components and desired measurement points to see how current and voltage can be tracked throughout a circuit.

Selecting Measurement Points in a Complex Circuit

The schematic below shows a simple sub-circuit that converts an input signal into a positive pulsed output. There are a number of other components used to show the various possible measurement points in this circuit. In this circuit, there is a sinusoidal source, an input pi filter (low-pass), and eventually, a CMOS inverter. We want to see how current and voltage are distributed in this type of circuit, which contains DC and AC sections.

Tracking voltage and current in a schematic

Schematic used for SPICE simulations

This schematic could be part of a hierarchical structure, or it could be copied from a larger system. When you want to simulate a circuit that is part of a larger structure, it’s best to copy it into a new schematic and run simulations in isolation. Once you’ve isolated the circuit block to be simulated, you can place measurement probes at different components. You’ll be able to measure current, voltage, or power within the PSpice Simulator.

The measurement probes for tracking voltage and current are shown in grey in the schematic. Here, we’ve placed current probes and differential voltage probes at the input and output ports of the circuit. Although we’ll look at the input and output, these probes could be placed anywhere in the circuit. Once you create a simulation profile and run the PSpice Simulator, there are a number of options you’ll have to view the calculated voltage, current, and/or power throughout the circuit.

Simulation Results

Measurement probes can be used in any of the simulation tools accessible in PSpice. Some example analyses include:

  • DC sweep: A DC sweep cycles the DC voltage in the circuit between two values and displays the measurement data as a function of DC voltage value. A single source or multiple sources can be used for analysis. 

  • Frequency sweep: The data captured at the probe will be displayed in a graph as a function of frequency. 

  • Transient analysis: This is the standard time-domain analysis used with analog circuits. Although transient analysis normally refers to an examination of stability and relaxation behavior when a system switches states, it generally refers to any time-domain simulation and analysis.

  • Parametric sweep: A circuit parameter can be swept, and data at the measurement point can be captured and displayed for each value of the swept parameter.

Transient Simulation

Once any of the probes are placed in the schematic and a simulation is run, data from those probes will be displayed in a graph in the PSpice Advanced Analysis program. Capture CIS makes this easy, as it automatically builds a SPICE simulation directly from your schematic data. The example results below show the current (yellow) and voltage (green), measured at the output of the circuit shown above.

PSpice transient analysis results for current and voltage in a circuit

Transient analysis results in PSpice.

In the above graph, the source voltage and current are hidden for clarity. Here, we can see that the output voltage and current are quite low, although the circuit does provide pulses with a duty cycle of 50%. One option for determining how to increase the power output from the circuit would be to use parametric sweeps with different components. This would give a series of curves for different component values and would allow a designer to spot which component values give maximum power output.

If you click back over to Capture CIS, you’ll see the probes are now colored, which corresponds to the curve colors that were generated in PSpice. In addition, there are a number of tags showing voltage measurements throughout the circuit. This shows you where reference points in your circuit are taken, as well as the bias points with respect to these reference values.

PSpice bias voltage and current in a circuit

Bias and reference points determined from a PSpice simulation.

Adding Nets to a Graph

Although the automatically generated plot in PSpice will show data from the measurement probes placed in the schematic, you can still extract results from specific points in the schematic without re-running the simulation. To do this, simply create a new graph and add a new trace, or you can add a trace to an existing graph. Right-click on the graph area and select “Add Trace” to see which nets have data and can be displayed on a graph.

From here, you can select from a list of nets and the data they contain (current denoted with [I], voltage denoted with [v], and power denoted with [p] attached to the net name). You can also use your data in a custom formula, display values on a logarithmic scale (dB), and select a number of viewing options for your graph. These options are accessible for both frequency and time-domain data. An example with time-domain results is shown below.

PSpice tracking voltage and current on a graph

Current through L1 (purple) and C1 (teal) plotted in PSpice.

Any time-domain results can be transferred into the frequency domain with a Fourier transform. In this case, the Fourier transform would show harmonic generation due to the rectifying action of the CMOS inverter stage. Instead of using Fourier transforms, another option is to work in the frequency domain directly.

Frequency Sweep

Another possibility is to look at the frequency response with a frequency sweep simulation. Doing this doesn’t require swapping out any probes in the schematic; simply enable a frequency sweep in your simulation profile. This gives designers a method for tracking voltage and current in a complex circuit while comparing circuit responses at different locations and different frequencies. Designers can also perform other important frequency domain analyses, such as calculating a Bode plot.

If we look at frequency sweep results for the above system, we’ll find that the output is heavily attenuated at high frequencies. One contributor is the bandwidth of the CMOS inverter stage, while another is the pi filter at the input. This workflow illustrates one way to diagnose potential design changes while also tracking voltage and current in a circuit.

Working with Larger Systems

We have just examined a single circuit block for tracking voltage and current in a circuit. If this block is part of a larger system, you can use your simulation results on the input to another circuit block. Simply create a new simulation with the desired circuit block, and define your output current/voltage from the first block on the input to the second block. This will give you a simple way to examine how different circuit blocks in the series interact with each other.

This process is simple with the integrated simulation and circuit design features in PSpice Simulator. When you use this set of design and analysis tools, you’ll be able to simulate all aspects of circuit behavior before creating a layout. For IC designers, critical circuits can be qualified using customized circuit models with the PSpice modeling application. For PCB designers, models for COTS components can be imported into a schematic and used for circuit simulations.

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