AN682

AN682
Using Single Supply Operational Amplifiers in Embedded Systems
Author:
Bonnie Baker
Microchip Technology Inc.
INTRODUCTION
Beyond the primitive transistor, the operational amplifier (op amp) is the most basic building block for analog
applications. Fundamental functions such as gain, load
isolation, signal inversion, level shifting, adding and/or
subtracting signals are easily implemented with an op
amp. More complex circuits can also be implemented,
such as the instrumentation amplifier, a current-to-voltage converter, and filters, to name only a few. Regardless of the level of complexity of the op amp circuit,
knowing the fundamental operation and behavior of an
op amp will save a considerable amount of up-front
design time.
Formal classes on this subject can be very comprehensive and useful. However, many times they fall short in
terms of experience or common sense. For instance, a
common mistake that is made when designing with op
amps is neglecting to include bypass capacitors in the
circuit. Op amp theory often overlooks this practical
detail. If the bypass capacitor is missing, the amplifier
circuit can oscillate at a frequency that “theoretically”
doesn’t make sense. If textbook solutions are used, this
can be a difficult problem to solve.
This application note is divided into three sections. The
first section lists fundamental amplifier applications,
including design equations. These amplifier circuits
were selected with embedded system integration in
mind.
The second section uses these fundamental circuits to
build useful amplifier functions in embedded control
applications.
The third section identifies the most common singlesupply op amp circuit design mistakes. This list of mistakes has been gathered over many years of troubleshooting circuits with numerous designers in the
industry. The most common design pitfalls can easily
be avoided if the suggestions in this application note
are used.
 1998-2011 Microchip Technology Inc.
FUNDAMENTAL OP AMP CIRCUITS
The op amp is the analog building block that is analogous to the digital gate. By using the op amp in the
design, circuits can be configured to modify the signal
in the same fundamental way that the inverter and the
AND and OR gates do in digital circuits. In this section,
fundamental building blocks such as the voltage follower, non-inverting gain and inverting gain circuits are
discussed, followed by a rail splitter, difference
amplifier, summing amplifier and the current-to-voltage
converter.
Voltage Follower Amplifier
Starting with the most basic op amp circuit, the buffer
amplifier (shown in Figure 1) is used to drive heavy
loads, solve impedance matching problems, or isolate
high power circuits from sensitive, precise circuitry.
VDD
2
VIN
–
7
*
MCP601
3
+
VOUT
6
4
VOUT = VIN
*Bypass Capacitor, 1 µF
FIGURE 1:
voltage follower.
Buffer amplifier; also called a
The buffer amplifier shown in Figure 1 can be implemented with any single-supply, unity-gain, stable amplifier. In this circuit, as with all amplifier circuits, the op
amp must be bypassed with a capacitor. For singlesupply amplifiers that operate in bandwidths from DC to
megahertz, a 1 µF capacitor is usually appropriate.
Sometimes a smaller bypass capacitor is required, for
amplifiers that have bandwidths up to the 10s of megahertz. In these cases, a 0.1 µF capacitor would be
appropriate. If the op amp does not have a bypass
capacitor or the wrong value is selected, it may
oscillate.
DS00682D-page 1
AN682
The analog gain of the circuit in Figure 1 is +1 V/V.
Notice that this circuit has a positive overall gain, but
the feedback loop is tied from the output of the amplifier
to the inverting input. An all too common error is to
assume that an op amp circuit that has a positive gain
requires positive feedback. If positive feedback is used,
the amplifier will most likely drive to either rail at the
output.
This amplifier circuit will give good linear performance
across the bandwidth of the amplifier. The only restrictions on the signal will occur as a result of a violation of
the input common-mode and output swing limits. These
limitations are discussed in the third section of this
application note, Amplifier Design Pitfalls.
If this circuit is used to drive heavy loads, the amplifier
that is actually selected must be specified to provide
the required output currents. Another application where
this circuit may be used is to drive capacitive loads. Not
every amplifier is capable of driving capacitors without
becoming unstable. If an amplifier can drive capacitive
loads, the product data sheet will highlight this feature.
However, if an amplifier cannot drive capacitive loads,
the product data sheets will not explicitly say.
Another use for the buffer amplifier is to solve impedance matching problems. This would be applicable in a
circuit where the analog signal source has a relatively
high impedance, as compared to the impedance of the
following circuitry. If this occurs, there will be a voltage
loss with the signal, as a consequence of the voltage
divider between the source’s impedance and the following circuitry’s impedance. The buffer amplifier is a
perfect solution to the problem. The input impedance of
the non-inverting input of an amplifier can be as high as
1013 for CMOS amplifiers. In addition, the output
impedance of this amplifier configuration is usually less
than 10.
VDD
VDD
–
MCP601
VIN
Gaining Analog Signals
The buffer solves a lot of analog signal problems; however, there are instances in circuits where a signal
needs to be gained. Two fundamental types of amplifier
circuits can be used. With the first type, the signal is not
inverted, as shown in Figure 3. This type of circuit is
useful in single-supply(1) amplifier applications, where
negative voltages are usually not possible.
R1
R2
VDD
–
*
VOUT
MCP601
VIN
+
*Bypass Capacitor, 1 µF
FIGURE 3:
Op amp configured in a noninverting gain circuit.
The input signal to this circuit is presented to the highimpedance, non-inverting input of the op amp. The gain
applied by the amplifier circuit to the signal is equal to:
R2
R1
This type of amplification is difficult to do with any level
of accuracy in the best of situations. This precision
measurement can easily be disrupted by changing the
output current drive of the device that is doing the
amplification work. An increase in current drive will
cause self heating of the chip, which induces an offset
change. An analog buffer can be used to perform the
function of driving heavy loads, while the front-end
circuitry can be used to make precision measurements.
–
EQUATION 1:
R2
VOUT = 1 + ------  VIN
 R1 
*
*
VOUT
+
+
Buffer
Precision Amplifier
Typical values for these resistors in single supply circuits are above 2 k for R2. The resistor (R1) restrictions are dependent on the amount of gain desired
versus the amount of amplifier noise and input offset
voltage, as specified in the product data sheet of the op
amp.
*Bypass Capacitor, 1 µF
FIGURE 2:
Load isolation is achieved by
using a buffer amplifier.
Yet another use of this configuration is to separate a
heat source from the sensitive precision circuitry, as
shown in Figure 2. Imagine that the input circuitry to
this buffer amplifier is amplifying a 100 µV signal.
DS00682D-page 2
1.
For this discussion, single supply implies that the negative
supply pin of the operational amplifier is tied to ground and
the positive supply pin is tied to +5V. All discussion in this
application note can be extrapolated to other supply voltages
where the single supply exceeds 5V or dual supplies are
used.
 1998-2011 Microchip Technology Inc.
AN682
Once again, this circuit has some restrictions in terms
of the input and output range. The non-inverting input
is restricted by the common-mode range of the amplifier. The output swing of the amplifier is also restricted,
as stated in the product data sheet of the individual
amplifier. Most typically, the larger signal at the output
of the amplifier causes more signal clipping errors than
the smaller signal at the input. If undesirable clipping
occurs at the output of the amplifier, the gain should be
reduced.
An inverting amplifier configuration is shown in
Figure 4. With this circuit, the signal at the input resistor
(R1) is gained and inverted to the output of the
amplifier. The gain equation for this circuit is:
EQUATION 2:
Single Supply Circuits and Supply
Splitters
As was shown in the inverting gain circuit (Figure 4),
single supply circuits often need a level shift to keep the
signal between negative (usually ground) and positive
supply pins. This level shift can be designed with a single amplifier and a combination of resistors and capacitors, as shown in Figure 5. Many times a simple buffer
amplifier without compensation capacitors will accomplish this task. In other cases the level shift circuit will
see dynamic or transient load changes, like the reference to an Analog-to-Digital (A/D) converter. In these
applications, the level shift circuit must hold its voltage
constant. If it does change, a conversion error might be
observed.
R2
R2
VOUT = –  ------  VIN +  1 + ------  V BIAS
 R1 

R1 
VDD
The ranges for R1 and R2 are the same as in the
non-inverting circuit shown in Figure 3.
R1
C2
T
R3
R2
R1
*
–
VIN
VDD
–
VS
R2
VOUT
C1
+
VREF
*
VOUT
MCP601
VBIAS
*
MCP601
R4
VIN
ADC
+
*Bypass Capacitor, 1 µF
FIGURE 4:
Op amp configured in an
inverting gain circuit. in single supply environments, a VBIAS is required to insure the output
stays above ground.
In single supply applications, this circuit can easily be
misused. For example, let R2 equal 10 k R1 equal
1 k, VBIAS equal 0V, and the voltage at the input resistor R1 equal to 100 mV. With this configuration, the output voltage would be 1V. This would violate the output
swing range of the op amp. In reality, the output of the
amplifier would go as near to the ground as possible.
The inclusion of a DC voltage at VBIAS in this circuit
solves this problem. In the previous example, a voltage
of 225 mV applied to VBIAS would level shift the output
signal up 2.475V. This would make the output signal
equal to (2.475V  1V) or 1.475V at the output of the
amplifier. Typically, the average output voltage should
be designed to be equal to VDD/2.
 1998-2011 Microchip Technology Inc.
*Bypass Capacitor, 1 µF
R1 = 10 to 100
R2 = 10 to 100
FIGURE 5:
A supply splitter is constructed using one op amp. This type of function
is particularly useful in single supply circuits.
A solid level shift voltage can easily be implemented
using a voltage divider (R3 and R4), or a reference voltage source buffered by the amplifier. The transfer
function for this circuit is:
EQUATION 3:
R4
VOUT = V DD  ------------------- 
 R3 + R 4 
The circuit in Figure 5 has an elaborate compensation
scheme, to allow for the heavy capacitive load C1. The
benefit of this big capacitor is that it presents a very low
AC resistance to the reference pin of the A/D converter.
In the AC domain, the capacitor serves as a charge reservoir that absorbs any momentary current surges
which are characteristic of sampling A/D converter
reference pins.
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AN682
The Difference Amplifier
Summing Amplifier
The difference amplifier combines the non-inverting
amplifier and inverting amplifier circuits of Figure 3 and
Figure 4 into a signal block that subtracts two signals.
The implementation of this circuit is shown in Figure 6.
Summing amplifiers are used when multiple signals
need to be combined by addition or subtraction. Since
the difference amplifier can only process two signals, it
is a subset of the summing amplifier.
R1
R1
R2
V2
V4
VDD
–
R1
VDD
R1
R1
*
MCP601
V1
R2
V3
VOUT
+
R2
–
V1
V2
*
MCP601
VOUT
+
R1
R2
VREF
*Bypass Capacitor, 1 µF
*Bypass Capacitor, 1 µF
FIGURE 6:
Op amp configured in a
difference amplifier circuit.
FIGURE 7:
Op amp configured in a
summing amplifier circuit.
The transfer function for this amplifier circuit is:
The transfer function of this circuit is:
EQUATION 4:
EQUATION 5:
R2
VOUT = V1 – V2   --------  + VR E F
 R1 
R2
VOUT =  V 1 + V 2 – V3 – V 4   ------ 
 R1 
This circuit configuration will reliably take the difference
of two signals as long as the signal source impedances
are low. If the signal source impedances are high with
respect to R1, there will be a signal loss due to the voltage divider action between the source and the input
resistors to the difference amplifier. Additionally, errors
can occur if the two signal source impedances are mismatched. With this circuit, it is possible to have gains
equal to, or higher than one.
Any number of inputs can be used on either the inverting or non-inverting input sides, as long as there are an
equal number of both with equivalent resistors.
DS00682D-page 4
 1998-2011 Microchip Technology Inc.
AN682
Current-to-Voltage Conversion
An op amp can be used to easily convert the signal
from a sensor that produces an output current, such as
a photodetector, into a voltage. This is implemented
with a single resistor and an optional capacitor in the
feedback loop of the amplifier, as shown in Figure 8.
C2
R2
D1
VDD
ID1
–
Light
*
MCP601
VOUT
+
VBIAS
Two circuits are shown in Figure 8. The top circuit is
designed to provide precision sensing from the photodetector. In this circuit the voltage across the detector
is nearly zero and equal to the offset voltage of the
amplifier. With this configuration, current that appears
across the resistor R2 is primarily a result of the light
excitation on the photodetector.
The photosensing circuit on the bottom of Figure 8 is
designed for higher speed sensing. This is done by
reverse biasing the photodetector, which reduces the
parasitic capacitance of the diode. There is more leakage through the diode, which causes a higher DC error.
R2
D1
Light
As light impinges on the photo diode, charge is generated, causing a current to flow in the reverse bias direction of the photodetector. If a CMOS op amp is used,
the high input impedance of the op amp causes the current from the detector (ID1) to go through the path of
lower resistance R2. Additionally, the op amp input bias
current error is low because it is CMOS (typically
< 200 pA). The non-inverting input of the op amp is referenced to ground, which keeps the entire circuit
biased to ground. These circuits will only work if the
common mode range of the amplifier includes zero.
VDD
ID1
–
*
MCP601
VOUT
+
VOUT = R2 ID1
*Bypass Capacitor, 1 µF
FIGURE 8:
Current-to-voltage converter
using an amplifier and one resistor. The top lightscanning circuit is appropriate for precision applications. The bottom circuit is appropriate for
high-speed applications.
 1998-2011 Microchip Technology Inc.
DS00682D-page 5
AN682
USING THE FUNDAMENTALS
Instrumentation Amplifier
Instrumentation amplifiers are found in a large variety
of applications, from medical instrumentation to process control. The instrumentation amplifier is similar to
the difference amplifier in that it subtracts one analog
signal from another, but it differs in terms of the quality
of the input stage. A classic, three op amp
instrumentation amplifier is illustrated in Figure 9.
VDD
V2
+ ½
MCP602
*
R3
EQUATION 6:
2R2 R 4
VOUT = V1 –V2  1 + ---------   ------  + VREF

RG   R3 
A second instrumentation amplifier is shown in
Figure 10. In this circuit, the two amplifiers serve the
functions of load isolation, and signal gain. The second
amplifier also differentiates the two signals.
R4
–
RG
VDD
R2
R2
RG
–½
MCP602
R3
+
– ½
V1
The reference voltage of the difference stage of this
instrumentation amplifier is capable of spanning a wide
range. Most typically this node is referenced to half of
the supply voltage in a signal supply application. A supply splitter, such as the circuit in Figure 5, can be used
for this purpose. The transfer function of this circuit is:
R1
VOUT
– ½
MCP602
R4
VREF
VDD
VREF
MCP602
+
R2
*
V2
R1
*
R2
+
–½
MCP602
*Bypass Capacitor, 1 µF
FIGURE 9:
An instrumentation amplifier
can be designed using three amplifiers. The input
op amps provide signal gain. The output op amp
converts the signal from two inputs to a singleended output with a difference amplifier.
With this circuit, the two input signals are presented to
the high-impedance non-inverting inputs of the amplifiers. This is a distinct advantage over the difference
amplifier configuration, when source impedances are
high or mismatched. The first stage also gains the two
incoming signals. This gain is simply adjusted with one
resistor, RG.
Following the first stage of this circuit is a difference
amplifier. The function of this portion of the circuit is to
reject the common mode voltage of the two input signals, as well as to differentiate them. The source
impedances of the signals into the input of the difference amplifier are low, equivalent and well controlled.
DS00682D-page 6
V1
+
VOUT
*Bypass Capacitor, 1 µF
FIGURE 10:
An instrumentation amplifier
can be designed using two amplifiers. This
configuration is best suited for higher gains
(gain > 3 V/V).
The circuit reference voltage is supplied to the first op
amp in the signal chain. Typically, this voltage is half of
the supply voltage in a single supply environment. The
transfer function of this circuit is:
EQUATION 7:
R1 2R1
V OUT = V1 –V 2  1 + ------ + ----------  + V REF

R2 R G 
 1998-2011 Microchip Technology Inc.
AN682
Floating Current Source
Filters
A floating current source can come in handy when driving a variable resistance, like a Resistive Temperature
Device (RTD). This particular configuration produces
an appropriate 1 mA source for an RTD-type sensor;
however, it can be tuned to any current.
Band-pass and low-pass filters are very useful in eliminating unwanted signals prior to the input of an A/D
converter. The low-pass filter shown in Figure 12 has
two poles that can be configured for a Butterworth filter
response. Butterworth filters have a flat magnitude
response in the pass-band with good all-around
performance.
R1
R1
VDD
–
½
MCP602
* 2 (VREF - 2VR1)
R3
R4
100 k
909 k
+
VDD
RL = 2.5 k
–
–
½
VREF = 2.5V + VR1
MCP602
R1
R1
+
IOUT
R1
VREF - 2VR1
RTD
R1 = 25 k
*Bypass Capacitor, 1 µF
FIGURE 11:
A floating current source can
be constructed using two op amps and a
precision voltage reference.
With this configuration, the voltage of VREF is reduced
via the first resistor (R1) by the voltage VR1. The voltage
applied to the non-inverting input of the top op amp is
VREF  VR1. This voltage is gained to the amplifier’s output by two to equal 2(VREF  VR1). Meanwhile, the output for the bottom op amp is presented with the voltage
VREF  2VR1. Subtracting the voltage at the output of
the top amplifier from the non-inverting input of the bottom amplifier gives:
2(VREF VR1) (VREF 2VR1), which equals VREF.
The transfer function of the circuit is:
EQUATION 8:
V REF
I OUT = ------------RL
VIN
R2
*
MCP601
VOUT
+
54.9 k 97.6 k C1
470 pF
C2
100 pF
Second Order: 10 kHz, Low-Pass Sallen Key Filter
*Bypass Capacitor, 1 µF
FIGURE 12:
Low-pass, two-pole, active
filters are easily designed with one op amp. The
resistors and capacitors can be adjusted to implement other filter types, such as Bessel and Chebyshev.
On the down side, there is some overshoot and ringing
with a step response through this filter. This may or
may not be an issue, depending on the application circuit requirements. The gain of this filter is adjustable
with R3 and R4.
Notice the similarities in this gain equation and the
non-inverting amplifier shown in Figure 3.
This type of filter is also referred to as an anti-aliasing
filter, which is used to eliminate circuit noise in the frequency band above half of Nyquist of the sampling system. In this manner, these high-frequency noises, that
would typically alias back into the signal path, are
removed.
The DC gain of the circuit in Figure 12 is:
EQUATION 9:
V OUT
R4
------------- = 1 + ------ 

V IN
R3 
 1998-2011 Microchip Technology Inc.
DS00682D-page 7
AN682
The band-pass filter shown in Figure 13 is configured
with a zero and two poles, to accommodate speech
applications. The single zero high-pass filter portion of
this circuit is constructed with C1 and R1 in parallel with
R2. Notice that R1 and R2 also create a supply splitter
voltage at the non-inverting inputs of both of the amplifiers. This insures that both op amps operate in their linear region. The second amplifier, U2, in conjunction
with the components R3, R4, C3 and C4 set a two pole
corner frequency.
This filter eliminates high-frequency noise that may be
aliased back into the signal path. The signal gain of this
circuit is:
EQUATION 10:
R3
R2
V OUT = V IN  ------   -------------------- 
R 4 R1 + R2
For more information about low-pass filters, refer to
AN699 – “Anti-Aliasing Analog Filters for Data Acquisitions Systems”.
VDD
R3
VDD
C3
R1
C1
VIN
–
½
MCP602
*
+
R4
REF
R5
–
½
MCP602
C4
IN+
ADC
IN–
+
R2
PIC12C509
*Bypass Capacitor, 1 µF
FIGURE 13:
Band-pass filters can be implemented with one op amp designed to perform the highpass function, and a second amplifier to perform the low-pass function.
24.9 k
24.9 k
¼ –
MCP604
+
1 mA
2.49 k
– ¼
MCP604
100 
+
100 k
REF
10 k
¼
MCP604
– ¼
4.7 µF
PIC12C509
–
100 k
+IN
ADC
+
MCP604
+
VREF = 2.5 V
24.9 k
24.9 k
2.67 k
13 k
3.3 µF
Pt100
2.2 µF
Lead Compensation
FIGURE 14:
DS00682D-page 8
Gain = 6V/V
Complete single supply temperature measurement circuit.
 1998-2011 Microchip Technology Inc.
AN682
Putting it Together
AMPLIFIER DESIGN PITFALLS
The circuit shown in Figure 14 utilizes four operational
amplifiers along with a 12-bit A/D converter, to implement a complete single-supply temperature measurement circuit. The temperature sensor is an RTD that
requires current excitation. The current excitation is
supplied by the circuit described in Figure 11. The gain
and anti-aliasing filter is implemented with the circuit
shown in Figure 13.
This section lists the common problems associated
with using an op amp with a power supply and an input
signal on a PC Board. It is divided into four categories:
The voltage signal from the RTD is sensed by an amplifier, used in a combination of non-inverting and
inverting configurations.
Hopefully, the most common problems with op amp
implementation have been addressed within this
application note.
The output of this amplifier is then sent to an amplifier
configured as a two-pole, low-pass filter in a gain of
+6 V/V. A gain of six was chosen in order to comply with
the input range of the A/D converter. Assuming the
sampling frequency of the A/D converter is 75 kHz,
which is also know as the Nyquist frequency, the cut-off
frequency of the anti-aliasing filter (U4) is set to 10 kHz.
This allows plenty of bandwidth for the filter to attenuate the signal prior to half of Nyquist. The A/D converter
is a 12-bit Successive Approximation Register (SAR)
converter that is interfaced to the PIC12C509
microcontroller.
•
•
•
•
General Suggestions
1.
2.
3.
4.
5.
6.
 1998-2011 Microchip Technology Inc.
General Suggestions
Input Stage Problems
Bandwidth Issues
Single Supply Rail-to-Rail
Be careful of the supply pins. Don’t make them
too high per the amplifier specification sheet,
and don’t make them too low. High supplies will
damage the part. In contrast, low supplies will
not bias the internal transistors and the amplifier
won’t work or it may not operate properly.
Make sure the negative supply (usually ground)
is actually tied to a low-impedance potential.
Additionally, make sure the positive supply is the
voltage you expect when it is referenced to the
negative supply pin of the op amp. Placing a
voltmeter across the negative and positive
supply pins verifies that you have the right
relationship between the pins.
Ground cannot be trusted, especially in digital
circuits. Plan your grounding scheme carefully. If
the circuit has a lot of digital circuitry, consider
separate ground and power planes. It is very
difficult, if not impossible, to remove digital
switching noise from an analog signal.
Decouple the amplifier power supplies with
bypass capacitors as close to the amplifier as
possible. For CMOS amplifiers, a 0.1 µF capacitor is usually recommended. Also decouple the
power supply with a 10 µF capacitor.
Use short lead lengths to the inputs of the amplifier. If you have a tendency to use the white perf
boards for prototyping, be aware that they can
cause noise and oscillation. There is a good
chance that these problems won’t be a problem
with the PCB implementation of the circuit.
Amplifiers are static sensitive! If they are damaged, they may fail immediately or exhibit a soft
error (like offset voltage or input bias current
changes) that will get worse over time.
DS00682D-page 9
AN682
Input Stage Problems
REFERENCES
1.
• Sergio Franco – “Design with Operational
Amplifiers and Analog Integrated Circuits”,
McGraw Hill, 2001
• Thomas Frederiksen – “Intuitive Operational
Amplifiers: From Electron to Op Amp”, McGraw
Hill, 1988
• Williams, Jim – “Analog Circuit Design”,
Butterworth-Heinemann, 1991
• Bonnie Baker – “AN699 – Anti-aliasing Analog
Filters for Data Acquisition Systems”, Microchip
Technology Inc., DS00699, 1999
• Bonnie Baker – “AN722 – Operational Amplifier
Topologies and DC Specifications”, Microchip
Technology Inc., DS00722, 1999
• Bonnie Baker – “AN723 – Operational Amplifier
AC Specifications and Applications”, Microchip
Technology Inc., DS00723, 2000
2.
3.
Know what input range is required from your
amplifier. If either inputs of the amplifier go
beyond the specified input range, the output will
typically be driven to one of the power supply
rails.
If you have a high gain circuit, be aware of the
offset voltage of the amplifier. That offset is
gained with the rest of your signal, and it might
dominate the results at the output of the
amplifier.
Do not use rail-to-rail input stage amplifiers,
unless it is necessary. By the way, they are only
needed when a buffer amplifier circuit is used or
possibly an instrumentation amplifier configuration. Any circuit with gain will drive the output of
the amplifier into the rail before the input has a
problem.
Bandwidth Issues
1.
2.
Account for the bandwidth of the amplifier when
sending signals through the circuit. You may
have designed an amplifier for a gain of 10 and
find that the AC output signal is much lower than
expected. If this is the case, you may have to
look for an amplifier with a wider bandwidth.
Instability problems can usually be solved by
adding a capacitor in parallel with the feedback
resistor around the amplifier. This does mean
typically and not always. If an amplifier circuit is
unstable, a quick stability analysis will show the
problem and, probably, the solution.
Single Supply Rail-to-Rail
1.
2.
3.
Op amp output drivers are capable of driving a
limited amount of current to the load.
Capacitive loading an amplifier is risky business.
Make sure the amplifier is specified to handle
any loads that you may have.
It is very rare that a single-supply amplifier will
truly swing rail-to-rail. In reality, the output of
most of these amplifiers can only come within 50
to 200 mV from each rail. Check the product
data sheets of your amplifier.
DS00682D-page 10
 1998-2011 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
applications and the like is provided only for your convenience
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
MICROCHIP MAKES NO REPRESENTATIONS OR
WARRANTIES OF ANY KIND WHETHER EXPRESS OR
IMPLIED, WRITTEN OR ORAL, STATUTORY OR
OTHERWISE, RELATED TO THE INFORMATION,
INCLUDING BUT NOT LIMITED TO ITS CONDITION,
QUALITY, PERFORMANCE, MERCHANTABILITY OR
FITNESS FOR PURPOSE. Microchip disclaims all liability
arising from this information and its use. Use of Microchip
devices in life support and/or safety applications is entirely at
the buyer’s risk, and the buyer agrees to defend, indemnify and
hold harmless Microchip from any and all damages, claims,
suits, or expenses resulting from such use. No licenses are
conveyed, implicitly or otherwise, under any Microchip
intellectual property rights.
Trademarks
The Microchip name and logo, the Microchip logo, dsPIC,
KEELOQ, KEELOQ logo, MPLAB, PIC, PICmicro, PICSTART,
PIC32 logo, rfPIC and UNI/O are registered trademarks of
Microchip Technology Incorporated in the U.S.A. and other
countries.
FilterLab, Hampshire, HI-TECH C, Linear Active Thermistor,
MXDEV, MXLAB, SEEVAL and The Embedded Control
Solutions Company are registered trademarks of Microchip
Technology Incorporated in the U.S.A.
Analog-for-the-Digital Age, Application Maestro, chipKIT,
chipKIT logo, CodeGuard, dsPICDEM, dsPICDEM.net,
dsPICworks, dsSPEAK, ECAN, ECONOMONITOR,
FanSense, HI-TIDE, In-Circuit Serial Programming, ICSP,
Mindi, MiWi, MPASM, MPLAB Certified logo, MPLIB,
MPLINK, mTouch, Omniscient Code Generation, PICC,
PICC-18, PICDEM, PICDEM.net, PICkit, PICtail, REAL ICE,
rfLAB, Select Mode, Total Endurance, TSHARC,
UniWinDriver, WiperLock and ZENA are trademarks of
Microchip Technology Incorporated in the U.S.A. and other
countries.
SQTP is a service mark of Microchip Technology Incorporated
in the U.S.A.
All other trademarks mentioned herein are property of their
respective companies.
© 1998-2011, Microchip Technology Incorporated, Printed in
the U.S.A., All Rights Reserved.
Printed on recycled paper.
ISBN: 978-1-61341-141-4
Microchip received ISO/TS-16949:2009 certification for its worldwide
headquarters, design and wafer fabrication facilities in Chandler and
Tempe, Arizona; Gresham, Oregon and design centers in California
and India. The Company’s quality system processes and procedures
are for its PIC® MCUs and dsPIC® DSCs, KEELOQ® code hopping
devices, Serial EEPROMs, microperipherals, nonvolatile memory and
analog products. In addition, Microchip’s quality system for the design
and manufacture of development systems is ISO 9001:2000 certified.
 1998-2011 Microchip Technology Inc.
DS00682D-page 11
Worldwide Sales and Service
AMERICAS
ASIA/PACIFIC
ASIA/PACIFIC
EUROPE
Corporate Office
2355 West Chandler Blvd.
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Tel: 480-792-7200
Fax: 480-792-7277
Technical Support:
http://www.microchip.com/
support
Web Address:
www.microchip.com
Asia Pacific Office
Suites 3707-14, 37th Floor
Tower 6, The Gateway
Harbour City, Kowloon
Hong Kong
Tel: 852-2401-1200
Fax: 852-2401-3431
India - Bangalore
Tel: 91-80-3090-4444
Fax: 91-80-3090-4123
India - New Delhi
Tel: 91-11-4160-8631
Fax: 91-11-4160-8632
Austria - Wels
Tel: 43-7242-2244-39
Fax: 43-7242-2244-393
Denmark - Copenhagen
Tel: 45-4450-2828
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Tel: 91-20-2566-1512
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Tel: 33-1-69-53-63-20
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Tel: 81-45-471- 6166
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Tel: 49-89-627-144-0
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Tel: 678-957-9614
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Tel: 774-760-0087
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Tel: 630-285-0071
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Tel: 905-673-0699
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Tel: 61-2-9868-6733
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Tel: 86-10-8569-7000
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Tel: 86-23-8980-9588
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Tel: 86-571-2819-3180
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Tel: 63-2-634-9065
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Tel: 886-3-6578-300
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Taiwan - Kaohsiung
Tel: 886-7-213-7830
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Tel: 86-755-8203-2660
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Taiwan - Taipei
Tel: 886-2-2500-6610
Fax: 886-2-2508-0102
China - Wuhan
Tel: 86-27-5980-5300
Fax: 86-27-5980-5118
Thailand - Bangkok
Tel: 66-2-694-1351
Fax: 66-2-694-1350
Spain - Madrid
Tel: 34-91-708-08-90
Fax: 34-91-708-08-91
UK - Wokingham
Tel: 44-118-921-5869
Fax: 44-118-921-5820
China - Xian
Tel: 86-29-8833-7252
Fax: 86-29-8833-7256
China - Xiamen
Tel: 86-592-2388138
Fax: 86-592-2388130
China - Zhuhai
Tel: 86-756-3210040
Fax: 86-756-3210049
DS00682D-page 12
Italy - Milan
Tel: 39-0331-742611
Fax: 39-0331-466781
Korea - Daegu
Tel: 82-53-744-4301
Fax: 82-53-744-4302
05/02/11
 1998-2011 Microchip Technology Inc.
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