MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 9

• Manufacturer Part Number: MCP6271R
• Description: 170 ?a, 2 Mhz Rail-to-rail Op Amp
• Manufacturer: Microchip Technology Inc.

The first analog/digital layout routed the digital 5V and ground
connections through the analog section. In this configuration,
the LED high currents, motor switching and digital controller
noise is overlaid onto the sensitive analog power and ground
paths (Figure 6A). The pathways for noise in the PCB traces are
the power and ground currents interacting with trace resistance
and inductance. This produces AC offsets in both the power and
ground of the analog portion of the circuit. A quick solution to
this problem is to reroute the power and ground traces so that
the analog and digital lines are independent, up to a central
location. At this central location, they are connected (Figure 6B).
This strategy takes advantage of trace resistance, inductance
and bypass capacitors to create RC and LC low-pass filters on
the power and ground traces. This further isolates noisy and
sensitive sections of the design.
Key candidates for radiated noise are the LED traces (that carry
high currents), the charge pump in the RS-232 interface (which
can pull moderate currents) and the I/Os from the microcontroller
(which have fast rise times). The LED and RS-232 driver traces
inductively couple noise to adjacent traces in close proximity on
the board. This action manifests itself as voltage noise. Fast
rise-time signals from the microcontroller capacitively couple into
high-impedance, sensitive traces. This coupling activity manifests
itself as current noise if traces are in close proximity.
If these factors are considered in the circuit layout, the noise
coupled from the noisy digital to the sensitive analog is reduced.
The new layout for analog circuitry remains unchanged, as does
most of the digital. The difference is that the LED traces now
go around the analog block rather than through it. The RS-232
interface’s power supply and ground are also kept separate from
the sensitive analog and digital functions on the board. And, the
power and ground strategy from Figure 6B is used to guide the
layout.
Figure 7: Testing the new layout shows that the changes are
effective. 1024 samples where collected. The board exhibits true
12-bit operation.
Techniques To Minimize Noise
Conclusion
The first step to analog noise reduction is to choose low-noise
analog parts. One can remove signal and power noise with filters.
Use an anti-aliasing filter as appropriate. In the power bus,
use bypass capacitors and inductive chokes as needed. Also,
implement the PCB solution with a ground plane.
When you add digital circuitry, develop a ground and power
strategy for the entire circuit. In the plan, consider the resistance
and inductance of the traces in conjunction with the current
density traveling through those paths. The objective in the
composite layout is to minimize noise pathways, such as
capacitive and inductive coupling between traces, and to use the
inductance and resistance of the trace, together with the bypass
capacitors, to reduce and isolate noise.
References
Howard Johnson, Martin Graham, Prentice Hall, 1993.
Ott, John Wiley, N.Y., 1998.
Editor-in-chief.
Bonnie C. Baker, ADN007, Microchip Technology Inc.
Bonnie C. Baker, AN699, Microchip Technology Inc.p
Peripherals”, Bonnie C. Baker, AN695, Microchip Technology Inc.
Baker, AN688, Microchip Technology Inc.
Controllers”, Keith Curtis, AN823, Microchip Technology Inc.
“High-Speed Digital Design: A Handbook of Black Magic”,
“Noise Reduction Techniques in Electronic Systems”, Henry
“The RF Capacitor Handbook”, American Technical Ceramics Inc.
“The Circuit Designer’s Companion”, Tim Williams.
“Reference Data for Engineers”, 7th edition, Edward C. Jordan,
“ABC’s of Transformers & Coils”, Edward J. Bukstein.
“Techniques that Reduce System Noise in ADC Circuits”,
Anti-Aliasing, Analog Filters for Data Acquisition Systems”,
“Interfacing Pressure Sensors to Microchip’s Analog
“Layout Tips for 12-Bit A/D Converter Applications”, Bonnie C.
“Analog Design in a Digital World using Mixed-Signal
FilterLab ® Active Filter Design Software, www.microchip.com
Analog and Interface Guide – Volume 2
7