MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 25

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

Enhancing the circuit in Figure 1 with the parasitic capacitances
of the resistors and amplifier, things aren’t as simple. The 100
kΩ resistor in the feedback loop will have a parallel
0.5 pF (approximately) and the parasitic to ground could be
as high as 2 pF or 3 pF. In the dc domain these capacitors will
operate as open circuits. In the ac domain the capacitors will
affect the perfect square wave from input to output and there
will be quite a ring at the Vout node. If you use the amplifier’s
SPICE macromodel, with input impedances in the model and
board parasitics, you will see this problem immediately in your
simulation. If you bread board the circuit, you most certainly will
see this ringing.
Changing the values of the two resistors in this circuit solves this
problem. Hand calculations will help you find the correct values.
A SPICE simulation will facilitate the process. This is a little
easier than swapping out resistors on the breadboard until you
find the right values. In SPICE, you can also look at the response
of the amplifier using various resistors. This will help you find
the “corner” of this oscillation. If you go back and change both
values to 10 kΩ you will have great success in SPICE and on the
bench. Figure 3. shows the simulation results.
Figure 3: You can quickly verify that this simple circuit will ring
using a SPICE simulation. If you need to double-check this with
a breadboard circuit that is also a good idea, however, reducing
the 100 kΩ resistors down to 10 kΩ resistors solves the problem.
You do need to understand where the problem came from before
you continue with your circuit design. But this simulation caught a
significant stability problem. This ringing problem was an easy one
to miss by inspection of the schematic.
Operational Amplifiers
You do need to understand where the problem came from before
you continue with your circuit design. but the simulation caught
a signficant stability problem which was an easy one to miss by
just an inspection of the schematic.
The nay say-ers in industry will tell you your computer based
simulation tools will not work and using them will be a waste
of time. These people are a bit misguided and in my opinion,
they have a superficial view of what this tool can really do. Sure,
SPICE tools can lead you astray. But like any tool, it is only as
good as the user. Any insight that you gain from your simulations
emerges if you understand and use you SPICE tools properly.
Better yet, SPICE simulations will point out problems that you
had never anticipated. In most cases, they use double precision
calculations. This makes it easier to detect low level problems
that are impossible to find on the bench. SPICE simulations of
analog circuits is just one more way to avoid the design of a
“singing” circuit when you actually want stability.
This article is excerpted from Bonnie Baker’s book, “A BAKER’S
DOZEN: Real Analog Solutions for Digital Designers,” published in
Spring 2005 by Elsevier.
Analog and Interface Guide – Volume 2
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