MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 8

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

Analog Design Conclusion
Concluding this first section, it is important from the beginning
of the design to verify that the circuit devices are low noise. In
this example, the key elements are the resistors and amplifiers.
Following the device selection, make sure that the signal path
is properly filtered. This includes the signal path as well as the
power-supply train. An uninterrupted ground plane is a critical
element in all analog designs. You will eliminate noise that you
might otherwise have problems tracking down. Finally, bypass
capacitors are critical in analog designs. When you install them,
place the capacitors with short leads as close to the power pins
as possible.
At this point, the analog design is deemed “good”. However, you
will see that the addition of the LED array and fan controller will
cause the performance of the analog section to become worse.
Digital Design
We will start with the first-pass digital/analog design integration.
In the first pass, we will add the digital section arbitrarily using
normal rules of thumb for a layout strategy. This portion of the
design adds the LEDs, motor driver, RS-232 transmitter/receiver
and microcontroller.
The design uses bypass capacitors and a flyback diode on the
motor drive. Bypass capacitors are kept near the IC power, and
the ground traces are short. This is done without changing the
analog circuit layout. Figure 5 shows the first-pass histogram
results from digital/analog layout.
The results of the ADC output from this new board are worse
than the first try at the analog section. We are going to recover
the original analog behavior by reworking the power and ground
strategy. The first corrective action is to separate the digital
portions of the power train from the analog portion. Figure 6A
shows the first attempt with the analog/digital combination.
Figure 6B reflects the second, more successful version.
6
Figure 4: This figure shows the data results from the circuit design
in Figure 2. The improvement indicates that our low-noise layout
strategies are effective.
Analog and Interface Guide – Volume 2
Techniques To Minimize Noise
Figure 5: The digital circuit corrupts the analog section, causing a
code width of 35 codes from 1024 samples. This data (the output
of the ADC) indicates that there are noise sources on the board
because of the two features in the data. The results from this data
would be difficult to digitally filter to one final ADC code.
Figure 6: This figure illustrates the first- and second-pass
strategies for power and ground. Note that the second pass
separates noisy and sensitive circuitry as much as possible.
(total samples = 1024)
Code Width
of Noise = 35
A) 1st pass
B) Final ground/
combined ground/
power strategy
power strategy