Simple Gadgets Help You Design Better Circuits


Circuit design can be very daunting because the reality of it can be very different from what we read about in books. Many circuits are made from combinations of resistors, inductors, and capacitors (often abbreviated ‘RLC’), such as filters or voltage divider circuits, etc. It seems simple, and the functions of these circuits can often be expressed using general equations, but in truth there is a lot of knowledge involved in circuit design. Obviously, engineers need to constantly practice circuit design and make a lot of exercises and corrections in the process in order to complete their circuit design work more efficiently. DigiKey’s online conversion calculators are not only helpful for PCB design and code mark identification (please refer to the previous article To Help Make Initial PCB Design and Production Easier - DigiKey’s Online Conversion Calculator), but also for circuit design.

Fig.1 Online Conversion Calculators by DigiKey

Low Pass/High Pass Filter Calculator

Passive filters are used to attenuate signals above or below a determined frequency. This passive filter calculator lets you choose the composition (RC – resistor/capacitor or RL – resistor/inductor) of your circuit and enter values to calculate the -3dB cutoff frequency. Depending on the configuration of the components, you can build a low-pass or high-pass filter. The calculation methods and circuit diagrams corresponding to each combination type will be displayed in the calculator. The description of each filter type is listed in Table 1.

Filter Type Circuit Diagram Description
RC Low Pass Filter image RC filters are the most basic filter building block and are commonly used for low power signal filtering. Since the load is in parallel with the capacitor, it is generally assumed that the load impedance is high, so the circuit is unloaded.
RC High Pass Filter image A low-pass filter can be converted into a high-pass filter by switching the locations of the resistors and capacitors used for filtering. It can also be set as a high load impedance circuit.
RL Low Pass Filter image The RL filter is another first-order filter combination. Due to the inclusion of inductance, a phase shift occurs. Commonly used as a DC power supply for RF amplifiers, where the inductor is used to pass the DC bias current and prevent the RF from returning to the power supply.
RL High Pass Filter image A low-pass filter can be converted into a high-pass filter by switching the locations of the resistors and inductors used for filtering.


The AD7980 PulSAR® ADC series from ADI uses an antialiasing filter, which is composed of a 20Ω resistor and a 2.7nF capacitor placed in front of the ADC in the application circuit. Since an ideal anti-aliasing filter is characterized by unity gain in the passband, no gain variation, and simple circuitry, RC filters are a good choice. Figure 2 is an application circuit diagram of AD7980.

Fig. 2: AD7980 Application Circuit (Source: AD7980 Datasheet, ADI)

To find out the -3dB cutoff frequency for this filter, select “Low Pass Filter” and “RC - Resistor Capacitor” in the tool, then input R= 20Ohm and C = 2.7nF. The resulting -3dB cutoff frequency 2.9473 MHz is shown.


Fig. 3: Calculation result display of “Low Pass/High Pass Filter Calculator”

Voltage Divider Calculator

In the minds of many engineers, the voltage divider circuit is just a circuit studied in a book. They feel that it is rarely used in real applications. Compared with the filters described above, many engineers think that filters are much important in a circuit and these are two individual circuit designs, so the voltage divider circuit principle is ignored. However, upon closer analysis, the filter works like a voltage divider between the resistor resistance value (R) and the capacitor reactance 1/2πfC (taking an RC circuit as an example).

In fact, voltage dividers have many applications in real circuits, and their application fields are also very wide. For example, the logic level conversion of ADC, environmental measurement of sensing systems and measuring instruments are all technologies or concepts applied to voltage dividers. Therefore, canny engineers will find great utility in our voltage divider tool.

Fig. 4: The “Voltage Divider Calculator” Input Interface


One typical application for voltage dividing circuits is adding them on the input front end of the protection element. The voltage dividing ratio is used to adjust the allowable input range of the circuit’s isolation element. Figure 5 shows ACPL-C87 from Broadcom being used as a circuit isolation component. The input terminal needs to be set from a voltage divider circuit, being used as a voltage sensor, and using the voltage ratio to control light generation to form an optical isolation amplifier. Although the ACPL-C87 has a very high input impedance, this tolerance cannot be ignored when used in highly precise instrument applications. In Figure 5, in the R1/R2 voltage divider, R2 and Rin (the input resistance of the ACPL-C87x) are also considered to be a shunt, thus introducing additional measurement error. Using this tool, you can quickly determine the error value with your load, so you can make circuit changes or adjust the resistor value with consideration for this error value.


Fig. 5: ACPL-C87X Input Stage Simplified Circuit Diagram (Source, ACPL-C87B, ACPL-C87A, ACPL-C870 Datasheet, Broadcom)

The values of R1 = 598kOHM, R2 = 2KOHM, and the input impedance = 1000MOHM@2V were provided in the application circuit from this device’s datasheet. The calculation results are shown in Figure 6:

Fig. 6: Calculation result display of “Voltage Divider Calculator”

Parallel and Series Resistor Calculator

In circuit design, it is very common to use the same types of components in parallel and series. For example, the above-mentioned voltage dividing circuit is a series connection of resistors, or a parallel connection of resistors in a DAC. In fact, the applications of parallel and series connections also extend to other non-passive components, such as LEDs or batteries, and the design of parallel and series connections of these components requires special consideration of the internal resistance of the components for some circuits. The values of the components may need to be adjusted or the circuit modified after the calculation, so there is an advantage for this tool to reduce time-consuming redesign and enhance the design’s efficiency.

Fig 7: The “Parallel and Series Resistor Calculator” Input Interface


Here’s how to calculate the total resistance of series and parallel resistors. Suppose we start with three resistors, one 1000ohm, one 470ohm and one 680ohm. If they are connected in parallel, the resistance values entered in the tool gives a result of 217.4741Ohm. Figure 8 shows this result.

Fig. 8: Calculation result display for parallel resistor option

If they are in series, the total resistance value 2150Ohm is obtained. Figure 9 shows the result.

Fig. 9: Calculation result display for series resistor option


To design a good circuit, an engineer needs analysis, study and practice. No matter what the different circuits or component requirements are in various fields, or what the specialty of the product is, some basic and practical circuits are unavoidable. We have introduced three practical and popular online tools: low-pass/high-pass filter calculator, voltage divider calculator, and parallel and series resistor calculator. Making good use of these gadgets will be a good helper!

For more technical information on circuit design, please click on the links below. You are also welcome to leave a message at the end of the articles for discussion.

Frequency Filters Explained
Overview of Batteries in Series or Parallel
How to select resistor pairs for op amp and voltage divider applications
DAC’s output Buffered and Unbuffered