MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 24

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

Amplifiers And The SPICE Of Life
By Bonnie C. Baker, Microchip Technology Inc.
What is operational amplifier circuit stability and how do you
know when you are on the hairy edge? Typically, op amps are
used with a feedback network in order to reduce the variability
of the open loop gain response from part to part. With this
technique, circuit stability is provided. But it is possible to design
an amplifier circuit that does quite the opposite. You can design
an amplifier circuit that is extremely unstable to the point of
oscillation. In a closed loop amplifier system, stability can be
determined if the phase margin of the system is known. In this
analysis, the Bode stability analysis technique is commonly used.
With this technique, the magnitude (in dB) and phase response
of both the open loop response of the amplifier and circuit
feedback factor are included in a Bode plot.
They say a computer-based simulation of your analog circuit is
important. This is because the use of your preferred computer
SPICE (Simulation Program with Integrated Circuit Emphasis)
program can reduce initial errors and development time. If you
use your SPICE simulator correctly, you can drum out circuit
errors and nuances before you go to your breadboard. In this
manner, you will verify your design before you spend the time to
solder your circuit. SPICE helps troubleshoot on the bench; it is a
great place to try out different hypotheses. It is great at “what if”
scenarios (i.e., exploratory design).
You can view the results from these software tools on a PC with
user-friendly GUI suites. This tool will fundamentally provide DC
operating (quiescent) points, small signal (AC) gain, time domain
behavior and DC sweeps. At a more sophisticated level, it will
help you analyze harmonic distortion, noise power, gain sensitivity
and perform pole-zero searches. This list is not complete, but
generally, SPICE software manufacturers have many of these
fundamental features available for the user. By finessing the
Monte Carlo and worst-case-analysis tools in SPICE, you can
predict the yields of your final product. If you use your breadboard
for this type of investigation, it could be very expensive and time
consuming. All of these things will speed up your application
circuit time-to-market.
But, beware. You can effectively evaluate analog products if your
SPICE models or macromodels are accurate enough for your
application. The key words here are “accurate enough”. Such
models, or macromodels, should reflect the actual performance
of the component, without carrying the burden of too many circuit
details. Too many details can lead to convergence problems and
extremely long simulation times. Not enough details can hide
some of the intricacies of your circuit’s performance. Worse
yet, your simulation, whether you use complete models or just
macromodels, may give you a misrepresentation of what your
circuit will really do. Remember that a SPICE simulation is simply
a pile of mathematical equations that, hopefully, represent what
your circuit will do. It is in essence a computer product that
produces imaginary results.
So you might ask, “why bother?” Are SPICE simulations worth
the time and effort? A pop quiz will help you clarify this question.
The circuit in Figure 1 shows a fundamental, basic circuit. Is
this circuit stable or does it oscillate? Would the output of the
amplifier have an unacceptable ring? I would think that you would
quickly look at this and say, “That is a silly question. Of course it
is stable!” But then again, if you are always looking for the trick
question you may be suspicious. So what is the answer?
22
Analog and Interface Guide – Volume 2
Operational Amplifiers
Figure 1: A variety of applications across industry has this simple
sub-circuit embedded in the system. This circuit simply takes an
analog input signal and gains that signal to the output of the
amplifier. For instance, an input signal of +0.5V to +1 VDC would
become a +1V to +2 VDC signal at V
DC signal oscillate? Or, would a 50 kHz sinusoidal signal oscillate
or ring? The bandwidth of this amplifier is 2.8 MHz.
This simple amplifier circuit uses an amplifier in a gain of +2
V/V. The amplifier has an 100 kΩ resistor connected to its
inverting-input to ground and 100 kΩ resistor in the feedback
loop. It would be easy to assume that this circuit is stable.
However, tedious calculations will verify that this amplifier circuit
will ring. This is due to the parasitic capacitances around the
resistors and the high differential/common-mode capacitance
of the amplifier’s input stage. For this particular amplifier, the
input common-mode capacitance is 6 pF and the differential-
mode capacitance is 3 pF. These capacitances interact with
the feedback resistor causing a semi-unstable condition. If you
bench-test this circuit you will immediately see this condition on
the oscilloscope. Parasitics on the breadboard will aggravate this
instability.
Figure 2: By enhancing the circuit in Figure 1 with the parasitic
capacitances of the resistors and amplifier, a simple of a circuit
is not so simple. In the DC domain these capacitors will operated
as open circuits. In the AC domain, the capacitors will affect the
perfect square wave from the input to output. The perfect square
wave will have quite a ring at the V
R
1
50 kHz
1V
= 100 kΩ
R
V
V
P-P
1
IN
IN
= 100 kΩ
0.5 pF
6 pF
6 pF
–
+
R
Op Amp
2
R
= 100 kΩ
3 pF
Op Amp
2
= 100 kΩ
0.5 pF
OUT
OUT
V
node.
OUT
V
. The question is, would this
OUT
ADC
ADC