How to deal with the interference of circuit board parasitic components on circuit performance

writer: G March 05, 2019

Non-essential capacitors cause trouble

Two adjacent parallel traces form a wiring capacitor.

Note: Two traces are placed adjacent to each other to form a capacitor on a single board. Because of this type of capacitance, a fast voltage change in one go online can cause a current signal on the other line.

This capacitance can cause problems in sensitive mixed-signal circuits when high-impedance analog traces are placed close to the digital traces.

Note: A 16-bit digital-to-analog converter with an output voltage of 60,536 steps consisting of three 8-bit digital potentiometers and three operational amplifiers. If VDD is 5V in this system, the digital-to-analog converter resolution or LSB size is 76.3μV.

The circuit operates using three 8-bit digital potentiometers and three CMOS operational amplifiers to form a 16-bit digital-to-analog converter. Two digital potentiometers (U3a and U3b) are connected between VDD and ground. The center tap output is connected to the non-inverting inputs of the two operational amplifiers (U4a and U4b). The digital potentiometers U2 and U3 are planned using the SPI interface of the microcontroller U1. In this architecture, each digital potentiometer is programmed as an 8-bit multi-level digital-to-analog converter. If VDD is equal to 5V, these digital-to-analog converter LSB sizes are equal to 19.61mV.

The center taps of the two digital potentiometers are connected to two non-inverting inputs of the buffer op amp. In this circuit configuration, the input of the op amp is high impedance, which isolates the digital potentiometer from the rest of the circuit. The amplitude of the change in the output of these two op amps is programmed to not exceed the allowable range of the second stage op amp.

To make this circuit form a 16-bit digital-to-analog converter (U2a), the third digital potentiometer will vary within the output range of the two operational amplifiers U4a and U4b. Planning U3a and U3b are used to set the output voltage of the digital potentiometer. Furthermore, if VDD is 5V, it is possible to individually plan U3a and U3b to be 19.61 mV variation per step. This voltage is across the third 8-bit digital potentiometer R3, so that the lowest effective bit of this circuit corresponds to a voltage value of 76.3uV.

This circuit can be used in two basic modes of operation; the first mode is used for a programmable DC reference voltage, in which only the digital portion of the circuit is used occasionally but not in normal operation; The mode is used for an arbitrary waveform generator. In this mode, the digital portion of the circuit is the operating core and capacitive coupling may occur.

Observing the color traces in the wiring, the potential problems are obvious. The analog trace (blue) indicated by the arrow taps from the center of U3a to the high impedance amplifier input of U4a. The digital trace (green) indicated by the other arrow is used to transfer digital data to plan the setting of a digital potentiometer. On the lab table, it was found that the green walking online digital signal was coupled into the sensitive blue trace.

Note: In the oscilloscope photo, the top is the JP1 waveform (plan digital potentiometer digital data), the middle is the JP5 waveform (on the adjacent analog go online noise), and the bottom is the TP10 waveform (16-bit digital-to-analog converter output noise) )

In the system, the digital signal of the planned digital potentiometer has been sensed from the trace to another analog trace with a DC voltage, and this noise is transmitted through the analog portion of the circuit to the third digital potentiometer (U5a). The third digital potentiometer varies between the output states of the two operational amplifiers. The way to solve this problem is to separate the traces.

Note: This distance essentially eliminates the interference digital noise caused by previous wiring.

Note: The 16-bit digital-to-analog converter in this new wiring is displaying a single* conversion with no digital noise from the communication to the digital potentiometer.

The wiring change results carefully separate the analog and digital traces, and the circuit becomes a very clean 16-bit digital-to-analog converter. A single digital potentiometer with a single* conversion of 76.29μV is displayed in the green waveform. The oscilloscope scale is 80mV/div and the displayed code change range is approximately 80mV. Limited by the study room, multiply the output of the 16-bit digital-to-analog converter by 1000 times.

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