Sources and remedies of analog circuit noise

Analog design can appear to be more black magic than science when it comes to noise problems. A common ailment for a designer is to have the analog hardware portion of the circuit built only to find that somewhere a gremlin is generating enough noise to force him/her back to the drawing board. This "wait and see" design technique can bring about successful designs - eventually. Another approach to avoiding noise problems is to use a basic list of guidelines in conjunction with a knowledge of noise related fundamentals to influence decisions at the beginning of the design cycle. This article will explore various sources of noise in a 12-bit system, including device noise, emitted noise and conducted noise.

You may think that designing a low noise 12 or 10-bit analog-to-digital converter (ADC) board is easy. This is true, unless you ignore some basic low-noise concepts. For instance, you would think that most devices like amplifiers and resistors work effectively in 12- or 10-bit environments. Usually, these devices are selected for specifications that are unrelated to noise. Another area that contributes to circuit noise is conducted noise. Conducted noise is already in the circuit-board traces by the time the signal arrives at the input of the ADC. The origin of conducted noise is either device noise or emitted noise. In some instances, the requirements of the circuit dictate that device and emitted noise sources must exist. As an example, conducted noise can come from devices in the analog signal path as well as the power-supply devices. A very common power device that is used in circuits is a switched mode power supply or, worse yet, a regulated "wall-wart". These types of devices create supply noise, which is injected into the sensitive analog devices. A third source of noise is radiated noise. Commonly, this noise can occur because of capacitive coupling of signals from two traces that are parallel and in close proximity. And finally, radiated noise also comes from external electromagnetic interference (EMI) signals.

Poor device selection
If you are concerned about device noise, poor device selection can be a major factor in the success or failure of the circuit. A common place for this type of problem would be embedded in an amplifier/resistor gain stage. As a solution, you can reduce system noise by changing your amplifiers to low-noise devices and using lower-value resistors. Conductive noise problems are solved using other techniques. If the noise is in the ADC signal path, a low-pass filter can be placed in front of the ADC to effectively reduce aliasing noise.

Another source of conducted noise is the power supply, as mentioned before. With this problem, the power-supply lines can be filtered using inductive chokes or a resistor-capacitor (R/C) filter. Additionally, all active devices should have a bypass capacitor installed between their power pins and ground. But by far, a majority of conducted noise can be eliminated with a ground plane. Finally, if radiated noise is a result of capacitive coupling from trace to trace, the two traces can be separated. External noises can be shielded or avoided with the placement of the circuit board. If the device, conducted and emitted noise issues are addressed, then it is true; designing a low noise 12-bit ADC board is easy.

An example of a 12-bit circuit is shown in Figure 1. It illustrates that the signal originates at the resistive-load cell, part number LCL816-G. The differential output ports of the LCL816-G are connected to a discrete, two operational amplifier instrumentation amplifier (A1, A2, R3, R4 and RG). The signal then travels through a second order low-pass filter (A3, R5, R6, C1 and C2). This low-pass filter eliminates unwanted errors in the ADC by eliminating aliasing of higher frequency noise.



Figure 1: When low-noise devices, a ground plane, bypass capacitors, and a low-pass filter are used, it is possible to produce an accurate 12-bit system.

Finally, the signal couples into a 12-bit ADC (A4, Microchip's MCP3201). The converter is configured to accept signals from 0 to 5V. The output of the converter is sent to the microcontroller (Microchip PIC16C623). The power is generated from the wall socket, through a rectifier/ac-dc converter (wall-wart). This power converter produces a dc, 9V output. The power supply is regulated down to 5V using the LM7805. The inductive choke, L1, further reduces power supply ripple and noise.




Figure 2: When low noise precautions are not taken during circuit design and board layout, a 12-bit ADC system underperforms with approximately 5.45-bit accuracy (or 5.45 effective number of bits). In this figure the X-axis represents the digital output code of a 12-bit converter and the Y-axis represents the number of times the digital code in the X-axis occurs, given 1024 samples.





If this circuit is built without using the above low-noise precautions, it is very easy to produce an output similar to Figure 2. In this figure (Figure 2), 1024 samples were taken at the output of the ADC (MCP3201) at a data rate of 30ksps. These samples have a 44 code "spread" centered around code, 2982. From this data, the system is approximately 5.45-bit accurate. Clearly this circuit is not good enough even for a 12-bit system. The specific configuration of this board is:




A modified circuit and board will produce an accurate 12-bit solution. As a first step, device noise problems are handled by using lower-noise amplifiers and resistors. For instance, when the resistors' values are 10-times lower, the gain remains the same; because of this change the noise is reduced by approximately 33. Additionally, the amplifiers are changed from the MCP604 to the MCP6024. The MCP604 voltage-noise density at 1kHz is 29nV/√Hz (typ). The MCP6024 voltage noise density at 10kHz is 8.7nV/√Hz (typ). This is over a 33 improvement. Conducted noise problems are solved by using a ground plane on the back side of the printed circuit board (PCB). This ground plane is implemented so that interruptions in the metal are parallel instead of horizontal to the signal path.

The performance of the board changes dramatically with these modifications. Tests show that the histogram output of the ADC changes from a code width of 44 codes down to nine codes. This dramatic change converts the circuit in Figure 1 into approximately a 9-bit system.

This sounds good, but there is a 12-bit system to be found in this application. To Address the conducted noise problem a 2nd order low-pass filter is added before the ADC. This low-pass filter reduces aliased signals through the A/D conversion process. This filter is designed using the FilterLab analog filtering software tool. Additionally, conducted noise is reduced by including the bypass capacitors. Finally, the effects of conductive noise are minimized by filtering the power supply with an inductive choke, L1. These three modifications (in addition to the three modifications above) change this system into a true 12-bit accurate system. This is illustrated in Figure 3 where 1024 samples are collected from the converter at a data rate of 30ksps and all 1024 samples are equivalent to one code: 2941.




Figure 3: If low noise active and passive devices are used, a ground plane is included, bypass capacitors are added and a low-pass (anti-aliasing) filter is placed in the signal path, the code width of 1024 samples is equal to one.

It is easy to design a true 12-bit ADC system by using a few key low-noise guidelines. First, examine your devices (resistors and amplifiers) to make sure they are low noise. Second, use a ground plane when ever possible. Third, include a low-pass filter in the signal path if you are changing the signal from analog to digital. Fourth, include bypass capacitors. These capacitors not only remove noise but foster circuit stability as well. Finally, be sure that your power supply is properly filtered.

Low noise design checklist
Good 12-bit layout techniques are not difficult to master as long as you follow a few guidelines:

Examine circuit devices in circuit and verify they are low-noise.

Always have an uninterrupted ground plane in one layer of the board.

Make sure your signals in a mixed-signal circuit are properly filtered with a low-pass anti-aliasing filter.

Bypass all devices properly. Place the capacitors as close to the power pins of the device as possible.

Filter your power supply properly.

References:
"Reading and Using Fast Fourier Transforms (FFTs)", Baker, Bonnie C., AN681, Microchip Technology, Inc.

"Anti-Aliasing, Analog Filters for Data Acquisition Systems", Bonnie C. Baker, AN699, Microchip Technology Inc.

Author Information
Bonnie Baker has been involved with analog systems for nearly 20 years, having started as a manufacturing product engineer, then an IC designer, and corporate applications engineering manager at Burr-Brown. In 1998, she joined Microchip Technology as the analog/mixed signal applications engineering manager. Baker holds a Masters of Science in Electrical Engineering (U of A, Tucson, AZ). She can be reached at bonnie.baker@microchip.com