MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 6

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

ETP-248: Managing Power, Ground And Noise
In Microcontroller/Analog Applications
By Bonnie C. Baker and Keith Curtis, Microchip Technology Inc.
Microcontroller applications often have low-level sensor
signals and moderate power-drive circuitry, in addition to the
microcontrollers. A peaceful coexistence with these three
extremes requires a careful power and ground distribution
design. This paper will discuss sources of noise and the paths
by which the noise travels. We will cover the theory behind good
layout practices and their impact on noise. We will also discuss
the proper selection and placement of noise isolating and limiting
components. Any designer who struggles to keep digital and
power noise out of sensitive input circuits will find this paper
useful.
Figure 1 shows a block diagram of the example system that
we will use to the discuss concepts in this paper. The overall
function of this system is to acquire weight and display the
results in an LED array, as well as on a laptop computer.
Additionally, a fan controller cools the board temperature as
needed.
In the example design, there is an analog and digital section.
One of the difficulties in this design is the segregation of these
two domains. Starting with the analog input to the circuit, the
design is capable of measuring weight. The analog interface
block in Figure 1 includes a load cell, gain block, anti-aliasing
filter and a 12-bit analog-to-digital converter (ADC). The load cell
is a weatstone bridge, as shown in Figure 2.
4
Figure 1: This block diagram models the circuit, along with noise
sources, of the system in this paper. The analog interface circuitry
measures weight with a load-cell sensor. The interface then
transmits those results to a microcontroller. The microcontroller
sends the sensor results to an LED display and laptop computer.
There is also circuitry for a fan-motor driver.
Analog and Interface Guide – Volume 2
Device
Noise
Techniques To Minimize Noise
RS-232 Transmitter
Microcontroller
LED Displays
Receiver
Conducted
Noise
Power Supply
Radiated
Noise
Interface
Analog
Motor
Driver
Radiated
Noise
Device
Noise
In the digital section, the microcontroller produces the digital
representation of the load-cell value. One of the activities of the
microcontroller is to display the measurement results in the LED
array. The microcontroller also uses an RS-232 interface port
to communicate the data to a desktop computer. The desktop
computer takes analog measurements from the microcontroller
and displays that data in a histogram chart. Finally, the digital
section also includes a PWM driver output for the fan.
This design includes sensitive analog circuitry, a high-power
LED display and a potentially noisy digital interface to a laptop
computer. The challenge is to design a circuit and layout that
allows the coexistence of all of these conflicting elements. We
will start by designing the analog section of this circuit and then
move on to layout concerns.
Analog Circuit Design
The analog portion of this circuit has a load-cell sensor,
dual operational amplifier (MCP6022, Microchip Technology
Incorporated) configured as an instrumentation amplifier,
a 12-bit 100 kHz SAR ADC (MCP3201, Microchip) and one
voltage reference. The ADC’s SPI™ port connects directly to a
microcontroller (see Figure 2).
The load-cell sensor has a full-scale output range of ±10 mV.
The gain of the instrumentation amplifier (A1 and A2) is 153 V/V.
This gain matches the full-scale output swing of the
instrumentation amplifier block to the full-scale input range of the
ADC. The SAR ADC has an internal input sampling mechanism.
With this function, each conversion uses a single sample.
The microcontroller acquires the data from the converter and
translates the data into a usable format for tasks such as the
LED display or the PC interface.
If the implementation of the circuit and layout design of this
system is poor (no ground plane, no bypass capacitors and
no anti-aliasing filter), it will be an excellent candidate for
noise problems. The symptom of a poor implementation is an
intolerable level of uncertainty about the digital output results
from the ADC. It is easy to assume that this type of symptom
indicates that the last device in the signal chain generates the
noise problem. But, in fact, the root cause of poor conversion
results originates with the PCB layout.
In the worst of circumstances, when noise reduction layout
precautions are not taken, the 12-bit system in Figure 2 will
output a large distribution of code for a DC input signal. Figure 3
shows data from the output of the converter.
The data in Figure 3 is far from optimum. Six bits of peak-to-
peak error changes the 12-bit converter system into a noise-
free 9.3-bit system. One can average this data in the digital
domain to recapture the full 12-bits, but this will ideally require
the averaging of 4
environment, it will be more.
(12 − 9.3)
or 36 samples. In a non-ideal