MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 7

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

Figure 2: This is the analog portion of the block diagram in Figure 1. The amplifiers and voltage reference tie into the ADC, and the ADC
ties to the microcontroller. Building a 2nd order, low-pass filter (A3) requires two resistors, two capacitors and the MCP6022 (Microchip)
operational amplifier. The other amplifiers form an instrumentation amplifier circuit. Bypass capacitors are included. For testing, this portion
of the circuit is powered from a 110 VAC to 9 VDC wall cube. A linear regulator converts the 9V output of the wall-cube to 5 VDC.
Figure 3: A poor implementation of the 12-bit data acquisition
system shown in Figure 2 could easily have an output range of six
different codes with a 1024 sample size.
Code Width
of Noise = 6
(total samples = 1024)
Techniques To Minimize Noise
Analog Layout Guidelines
Ground and Power-Supply Strategy: The implementation for the
layout of the ground plane is critical when designing low-noise
solutions. It is a dangerous practice to omit a ground plane
with analog and/or mixed signal devices. Ground planes solve
problems such as offset errors, gain errors and noisy circuits.
These errors are more prevalent when the layout lacks a ground
plane because analog signals typically refer to ground.
When determining the grounding strategy of a board, one should
determine if the circuit needs one or more ground planes. If the
circuit has a “minimum” amount of digital circuitry onboard, a
single ground plane and triple-wide power supply traces may be
appropriate. The danger of connect ing the digital and analog
ground planes together is that the analog circuitry picks-up the
noise from the digital return currents. In either case, one should
connect the analog grounds, digital grounds and power supplies
together at one or more points on the circuit board. The inclusion
of a ground plane in a 12-bit system is critical.
ADC Layout Strategies: ADC layout techniques vary with converter
technology. When SAR ADCs are used, the entire device should
reside on the analog power and ground planes. ADC vendors
often supply analog and digital ground pins. If high-resolution
SAR converters are used, a digital buffer should be used to
isolate the converter from bus activity on the digital side of the
circuit. This is the correct strategy to use for delta-sigma ADCs
as well.
Figure 4 shows the performance of a board laid out with these
considerations. This data shows that the analog section of this
circuit works extremely well.
Analog and Interface Guide – Volume 2
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