Chapter I: In-Amp Basic

Chapter I
IN-AMP BASICS
Introduction
BRIDGE SUPPLY
VOLTAGE
Instrumentation amplifiers (in-amps) are sometimes
misunderstood. Not all amplifiers used in instrumentation applications are instrumentation amplifiers, and by
no means are all in-amps used only in instrumentation
applications. In-amps are used in many applications,
from motor control to data acquisition to automotive.
The intent of this guide is to explain the fundamentals
of what an instrumentation amplifier is, how it operates,
and how and where to use it. In addition, several different categories of instrumentation amplifiers are
addressed in this guide.
0.01�F
1
2
3
4
Figure 1-1 shows a bridge preamp circuit, a typical in-amp
application.When sensing a signal, the bridge resistor values
change, unbalancing the bridge and causing a change in
differential voltage across the bridge. The signal output
of the bridge is this differential voltage, which connects
directly to the in-amp’s inputs. In addition, a constant dc
voltage is also present on both lines. This dc voltage will
normally be equal or common mode on both input lines. In
its primary function, the in-amp will normally reject the
common-mode dc voltage, or any other voltage common
to both lines, while amplifying the differential signal voltage,
the difference in voltage between the two lines.
7
AD8221
VOUT
6
–
0.01�F
Unlike an op amp, for which closed-loop gain is determined by external resistors connected between its
inverting input and its output, an in-amp employs an
internal feedback resistor network that is isolated from its
signal input terminals.With the input signal applied across
the two differential inputs, gain is either preset internally
or is user set (via pins) by an internal or external gain
resistor, which is also isolated from the signal inputs.
0.33�F
8
+
RG
IN-AMPS vs. OP AMPS: WHAT ARE THE
DIFFERENCES?
An instrumentation amplifier is a closed-loop gain
block that has a differential input and an output that
is single-ended with respect to a reference terminal.
Most commonly, the impedances of the two input
terminals are balanced and have high values, typically
109 , or greater. The input bias currents should also
be low, typically 1 nA to 50 nA. As with op amps, output
impedance is very low, nominally only a few milliohms,
at low frequencies.
+VS
5
–VS
REF
0.33�F
Figure 1-1. AD8221 bridge circuit.
In contrast, if a standard op amp amplifier circuit were
used in this application, it would simply amplify both the
signal voltage and any dc, noise, or other common-mode
voltages. As a result, the signal would remain buried under
the dc offset and noise. Because of this, even the best
op amps are far less effective in extracting weak signals.
Figure 1-2 contrasts the differences between op amp and
in-amp input characteristics.
Signal Amplification and Common-Mode Rejection
An instrumentation amplifier is a device that amplifies
the difference between two input signal voltages while
rejecting any signals that are common to both inputs.The
in-amp, therefore, provides the very important function
of extracting small signals from transducers and other
signal sources.
Common-mode rejection (CMR), the property of
canceling out any signals that are common (the same
potential on both inputs), while amplifying any signals
that are differential (a potential difference between the
inputs), is the most important function an instrumentation amplifier provides. Both dc and ac common-mode
rejection are important in-amp specifications. Any errors
due to dc common-mode voltage (i.e., dc voltage present
at both inputs) will be reduced 80 dB to 120 dB by any
modern in-amp of decent quality.
However, inadequate ac CMR causes a large, timevarying error that often changes greatly with frequency
and, therefore, is difficult to remove at the IA’s output.
Fortunately, most modern monolithic IC in-amps provide
excellent ac and dc common-mode rejection.
1-1
Common-mode gain (ACM), the ratio of change in
output voltage to change in common-mode input voltage, is related to common-mode rejection. It is the net
gain (or attenuation) from input to output for voltages
common to both inputs. For example, an in-amp with
a common-mode gain of 1/1000 and a 10 V commonmode voltage at its inputs will exhibit a 10 mV output
change. The differential or normal mode gain (AD) is
the gain between input and output for voltages applied
differentially (or across) the two inputs. The commonmode rejection ratio (CMRR) is simply the ratio of
the differential gain, AD, to the common-mode gain.
Note that in an ideal in-amp, CMRR will increase in
proportion to gain.
Common-mode rejection is usually specified for full
range common-mode voltage (CMV) change at a given
frequency and a specified imbalance of source impedance
(e.g., 1 k source imbalance, at 60 Hz).
Mathematically, common-mode rejection can be represented as
where:
AD is the differential gain of the amplifier;
VCM is the common-mode voltage present at the
amplifier inputs;
VOUT is the output voltage present when a common-mode
input signal is applied to the amplifier.
The term CMR is a logarithmic expression of the
common-mode rejection ratio (CMRR).That is, CMR =
20 log10 CMRR.
To be effective, an in-amp needs to be able to amplify
microvolt-level signals while rejecting common-mode
voltage at its inputs. It is particularly important for the
in-amp to be able to reject common-mode signals over the
bandwidth of interest. This requires that instrumentation amplifiers have very high common-mode rejection
over the main frequency of interest and its harmonics.
IN-LINE CURRENT MEASUREMENT
I
THE VERY HIGH VALUE, CLOSELY MATCHED INPUT
RESISTANCES CHARACTERISTIC OF IN-AMPS
MAKE THEM IDEAL FOR MEASURING LOW
LEVEL VOLTAGES AND CURRENTS—WITHOUT
LOADING DOWN THE SIGNAL SOURCE.
REFERENCE
VOLTAGE
R1
R2
V 
CMRR = AD  CM 
 VOUT 
R
R–
V
R+
IN-AMP
R3
R4
OUTPUT
REFERENCE
R–
OUTPUT
R+
VOLTAGE
MEASUREMENT
FROM A BRIDGE
IN-AMP
REFERENCE
THE INPUT RESISTANCE OF A TYPICAL IN-AMP
IS VERY HIGH AND IS EQUAL ON BOTH INPUTS.
CREATES A NEGLIGIBLE ERROR VOLTAGE.
R– = R+ = 109
TO 10 12
IN-AMP INPUT CHARACTERISTICS
R2
RIN = R1 ( 1k TO 1M )
GAIN = R2/R1
RIN = R+ (106 TO 10 12 )
GAIN = 1 + (R2/R1)
R1
R–
R–
TYPICAL
R+ OP AMP
OUTPUT
R+
TYPICAL
OP AMP
OUTPUT
A MODEL SHOWING THE INPUT
RESISTANCE OF A TYPICAL OP AMP
IN THE OPEN-LOOP CONDITION
A MODEL SHOWING THE INPUT RESISTANCE OF A
TYPICAL OP AMP OPERATING AS AN INVERTING
AMPLIFIER—AS SEEN BY THE INPUT SOURCE
(R–) = (R+) = 106
OP AMP INPUT CHARACTERISTICS
Figure 1-2. Op amp vs. in-amp input characteristics.
1-2
TO 10 15
For techniques on reducing errors due to out-of-band
signals that may appear as a dc output offset, please refer
to the RFI section of this guide.
At unity gain, typical dc values of CMR are 70 dB to more
than 100 dB, with CMR usually improving at higher gains.
While it is true that operational amplifiers connected as
subtractors also provide common-mode rejection, the
user must provide closely matched external resistors
(to provide adequate CMRR). On the other hand,
monolithic in-amps, with their pretrimmed resistor
networks, are far easier to apply.
Common-Mode Rejection: Op Amp vs. In-Amp
Op amps, in-amps, and difference amps all provide
common-mode rejection. However, in-amps and diff
amps are designed to reject common-mode signals so
that they do not appear at the amplifier’s output. In
contrast, an op amp operated in the typical inverting
or noninverting amplifier configuration will process
common-mode signals, passing them through to the
output, but will not normally reject them.
Figure 1-3a shows an op amp connected to an input
source that is riding on a common-mode voltage. Because
of feedback applied externally between the output and
the summing junction, the voltage on the “–” input is
forced to be the same as that on the “+” input voltage.
Therefore, the op amp ideally will have zero volts across
its input terminals. As a result, the voltage at the op amp
output must equal VCM, for zero volts differential input.
Even though the op amp has common-mode rejection, the
common-mode voltage is transferred to the output along
with the signal. In practice, the signal is amplified by the
op amp’s closed-loop gain, while the common-mode
voltage receives only unity gain. This difference in gain
does provide some reduction in common-mode voltage
as a percentage of signal voltage. However, the commonmode voltage still appears at the output, and its presence
reduces the amplifier’s available output swing. For many
reasons, any common-mode signal (dc or ac) appearing
at the op amp’s output is highly undesirable.
VOUT = (VIN  GAIN) VCM
GAIN = R2/R1
CM GAIN = 1
V– = VCM
VCM
R1
VCM
VIN
R2
ZERO V
VOUT
V+ = VCM
Figure 1-3a. In a typical inverting or noninverting amplifier circuit using an op amp,
both the signal voltage and the common-mode voltage appear at the amplifier output.
1-3
Figure 1-3b shows a 3-op amp in-amp operating under
the same conditions. Note that, just like the op amp
circuit, the input buffer amplifiers of the in-amp pass the
common-mode signal through at unity gain. In contrast,
the signal is amplified by both buffers. The output signals
from the two buffers connect to the subtractor section of
the IA. Here the differential signal is amplified (typically
at low gain or unity) while the common-mode voltage is
attenuated (typically by 10,000:1 or more). Contrasting
the two circuits, both provide signal amplification (and
buffering), but because of its subtractor section, the inamp rejects the common-mode voltage.
Figure 1-3c is an in-amp bridge circuit. The in-amp
effectively rejects the dc common-mode voltage
appearing at the two bridge outputs while amplifying
the very weak bridge signal voltage. In addition, many
modern in-amps provide a common-mode rejection approaching 80 dB, which allows powering of the bridge
from an inexpensive, nonregulated dc power supply. In
contrast, a self constructed in-amp, using op amps and
0.1% resistors, typically only achieves 48 dB CMR, thus
requiring a regulated dc supply for bridge power.
BUFFER
VOUT = VIN (GAIN)
VCM
VCM
VCM
VIN
SUBTRACTOR
RG
VOUT
VIN TIMES
GAIN
VCM = 0
VCM
BUFFER
VCM
3-OP AMP
IN-AMP
Figure 1-3b. As with the op amp circuit above, the input buffers of an in-amp circuit
amplify the signal voltage while the common-mode voltage receives unity gain. However, the common-mode voltage is then rejected by the in-amp’s subtractor section.
VSUPPLY
VCM
BRIDGE
SENSOR
VIN
VCM
IN-AMP
VOUT
INTERNAL OR EXTERNAL
GAIN RESISTOR
Figure 1-3c. An in-amp used in a bridge circuit. Here the dc common-mode
voltage can easily be a large percentage of the supply voltage.
1-4
Figure 1-3d shows a difference (subtractor) amplifier
being used to monitor the voltage of an individual cell
that is part of a battery bank. Here the common-mode
dc voltage can easily be much higher than the amplifier’s
supply voltage. Some monolithic difference amplifiers,
such as the AD629, can operate with common-mode
voltages as high as 270 V.
Difference Amplifiers
Figure 1-4 is a block diagram of a difference amplifier.
This type of IC is a special-purpose in-amp that normally
consists of a subtractor amplifier followed by an output
buffer, which may also be a gain stage. The four resistors
used in the subtractor are normally internal to the IC,
and, therefore, are closely matched for high CMR.
Many difference amplifiers are designed to be used in
applications where the common-mode and signal voltages
may easily exceed the supply voltage. These diff amps
typically use very high value input resistors to attenuate
both signal and common-mode input voltages.
WHERE are in-amps and Difference
amps used?
Data Acquisition
In-amps find their primary use amplifying signals from
low level output transducers in noisy environments. The
amplification of pressure or temperature transducer
signals is a common in-amp application. Common bridge
applications include strain and weight measurement using
load cells and temperature measurement using resistive
temperature detectors, or RTDs.
DIFFERENCE AMPLIFIER
380k
VCM
380k
VIN
VOUT
380k
VCM
380k
Figure 1-3d. A difference amp is especially useful in applications such as battery
cell measurement, where the dc (or ac) common-mode voltage may be greater
than the supply voltage.
+15V
0.1�F
C1
0.1�F
DIFFERENTIAL
INPUT
SIGNAL
4
10k�
8
100k�
1
10k�
A1
100k�
2
+VS
A2
VREF
RG
3
5
6
0.1�F
RG
RF
C2
–15V
Figure 1-4. A difference amplifier IC.
1-5
VOUT
TO ADC
VOUT
–IN
10k�
–VS
+IN
+IN
VCM
7
AD628
–IN
VIN
CFILTER
Medical Instrumentation
High Speed Signal Conditioning
In-amps are widely used in medical equipment such as
EKG and EEG monitors, blood pressure monitors, and
defibrillators.
Because the speed and accuracy of modern video data
acquisition systems have improved, there is now a
growing need for high bandwidth instrumentation amplifiers, particularly in the field of CCD imaging equipment
where offset correction and input buffering are required.
Double-correlated sampling techniques are often used
in this area for offset correction of the CCD image. Two
sample-and-hold amplifiers monitor the pixel and reference
levels, and a dc-corrected output is provided by feeding
their signals into an instrumentation amplifier.
Monitor and Control Electronics
Diff amps may be used to monitor voltage or current in
a system and then trigger alarm systems when nominal
operating levels are exceeded. Because of their ability to
reject high common-mode voltages, diff amps are often
used in these applications.
Software-Programmable Applications
An in-amp may be used with a software-programmable
resistor chip to allow software control of hardware
systems.
Audio Applications
Because of their high common-mode rejection,
instrumentation amplifiers are sometimes used for audio
applications (as microphone preamps, for example), to
extract a weak signal from a noisy environment, and to
minimize offsets and noise due to ground loops. Refer
to Table 6-4 (page 6-26), Specialty Products Available
from Analog Devices.
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Video Applications
High speed in-amps may be used in many video and cable
RF systems to amplify or process high frequency signals.
Power Control Applications
In-amps can also be used for motor monitoring (to
monitor and control motor speed, torque, etc.) by measuring the voltages, currents, and phase relationships
of a 3-phase ac-phase motor. Diff amps are used in
applications where the input signal exceeds the
supply voltages.
IN-AMPS: AN EXTERNAL VIEW
Figure 1-5 provides a functional block diagram of an
instrumentation amplifier.
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Figure 1-5. Differential vs. common-mode input signals.
1-6
Since an ideal instrumentation amplifier detects only the
difference in voltage between its inputs, any commonmode signals (equal potentials for both inputs), such as
noise or voltage drops in ground lines, are rejected at the
input stage without being amplified.
Of course, power must be supplied to the in-amp. As
with op amps, the power would normally be provided
by a dual-supply voltage that operates the in-amp over
a specified range. Alternatively, an in-amp specified for
single-supply (rail-to-rail) operation may be used.
Either internal or external resistors may be used to set
the gain. Internal resistors are the most accurate and
provide the lowest gain drift over temperature.
An instrumentation amplifier may be assembled using one
or more operational amplifiers, or it may be of monolithic
construction. Both technologies have their advantages
and limitations.
One common approach is to use a single external resistor,
working with two internal resistors, to set the gain. The
user can calculate the required value of resistance for a
given gain, using the gain equation listed in the in-amp’s
spec sheet. This permits gain to be set anywhere within a
very large range. However, the external resistor can seldom
be exactly the correct value for the desired gain, and it
will always be at a slightly different temperature than the
IC’s internal resistors. These practical limitations always
contribute additional gain error and gain drift.
Sometimes two external resistors are employed. In general,
a 2-resistor solution will have lower drift than a single
resistor as the ratio of the two resistors sets the gain, and
these resistors can be within a single IC array for close
matching and very similar temperature coefficients (TC).
Conversely, a single external resistor will always be a TC
mismatch for an on-chip resistor.
The output of an instrumentation amplifier often has
its own reference terminal, which, among other uses,
allows the in-amp to drive a load that may be at a
distant location.
Figure 1-5 shows the input and output commons being
returned to the same potential, in this case to power
supply ground. This star ground connection is a very effective means of minimizing ground loops in the circuit;
however, some residual common-mode ground currents
will still remain. These currents flowing through RCM
will develop a common-mode voltage error, VCM. The
in-amp, by virtue of its high common-mode rejection,
will amplify the differential signal while rejecting VCM
and any common-mode noise.
In general, discrete (op amp) in-amps offer design flexibility at low cost and can sometimes provide performance
unattainable with monolithic designs, such as very high
bandwidth. In contrast, monolithic designs provide
complete in-amp functionality and are fully specified
and usually factory trimmed, often to higher dc precision
than discrete designs. Monolithic in-amps are also much
smaller, lower in cost, and easier to apply.
WHAT OTHER PROPERTIES DEFINE A HIGH
QUALITY IN-AMP?
Possessing a high common-mode rejection ratio, an
instrumentation amplifier requires the properties
described below.
High AC (and DC) Common-Mode Rejection
At a minimum, an in-amp’s CMR should be high over
the range of input frequencies that need to be rejected.
This includes high CMR at power line frequencies and
at the second harmonic of the power line frequency.
Low Offset Voltage and Offset Voltage Drift
As with an operational amplifier, an in-amp must have
low offset voltage. Since an instrumentation amplifier
consists of two independent sections, an input stage and
an output amplifier, total output offset will equal the sum
of the gain times the input offset plus the offset of the
output amplifier (within the in-amp). Typical values for
input and output offset drift are 1 V/C and 10 V/C,
respectively. Although the initial offset voltage may be
nulled with external trimming, offset voltage drift cannot
be adjusted out. As with initial offset, offset drift has two
components, with the input and output section of the
in-amp each contributing its portion of error to the total.
As gain is increased, the offset drift of the input stage
becomes the dominant source of offset error.
1-7
A Matched, High Input Impedance
Low Noise
The impedances of the inverting and noninverting input
terminals of an in-amp must be high and closely matched
to one another. High input impedance is necessary to
avoid loading down the input signal source, which could
also lower the input signal voltage.
Because it must be able to handle very low level input
voltages, an in-amp must not add its own noise to that of
the signal. A minimum input noise level of 10 nV/√Hz @
1 kHz (gain > 100) referred to input (RTI) is desirable.
Micropower in-amps are optimized for the lowest possible
input stage current and, therefore, typically have higher
noise levels than their higher current cousins.
Values of input impedance from 109  to 1012  are
typical. Difference amplifiers, such as the AD629, have
lower input impedances, but can be very effective in high
common-mode voltage applications.
Low Input Bias and Offset Current Errors
Again, as with an op amp, an instrumentation amplifier has bias currents that flow into, or out of, its input
terminals; bipolar in-amps have base currents and FET
amplifiers have gate leakage currents. This bias current
flowing through an imbalance in the signal source
resistance will create an offset error. Note that if the
input source resistance becomes infinite, as with ac
(capacitive) input coupling, without a resistive return
to power supply ground, the input common-mode
voltage will climb until the amplifier saturates. A high
value resistor connected between each input and ground
is normally used to prevent this problem. Typically, the
input bias current multiplied by the resistor’s value in
ohms should be less than 10 mV (see Chapter V). Input
offset current errors are defined as the mismatch between
the bias currents flowing into the two inputs. Typical
values of input bias current for a bipolar in-amp range
from 1 nA to 50 nA; for a FET input device, values of
1 pA to 50 pA are typical at room temperature.
Low Nonlinearity
Input offset and scale factor errors can be corrected by
external trimming, but nonlinearity is an inherent performance limitation of the device and cannot be removed by
external adjustment. Low nonlinearity must be designed
in by the manufacturer. Nonlinearity is normally specified
as a percentage of full scale, whereas the manufacturer
measures the in-amp’s error at the plus and minus fullscale voltage and at zero. A nonlinearity error of 0.01%
is typical for a high quality in-amp; some devices have
levels as low as 0.0001%.
Simple Gain Selection
Gain selection should be easy. The use of a single
external gain resistor is common, but an external resistor will affect the circuit’s accuracy and gain drift with
temperature. In-amps, such as the AD621, provide a
choice of internally preset gains that are pin-selectable,
with very low gain TC.
Adequate Bandwidth
An instrumentation amplifier must provide bandwidth
sufficient for the particular application. Since typical unitygain, small-signal bandwidths fall between 500 kHz and
4 MHz, performance at low gains is easily achieved, but at
higher gains bandwidth becomes much more of an issue.
Micropower in-amps typically have lower bandwidth than
comparable standard in-amps, as micropower input stages
are operated at much lower current levels.
1-8
Differential to Single-Ended Conversion
Differential to single-ended conversion is, of course, an
integral part of an in-amp’s function: A differential input
voltage is amplified and a buffered, single-ended output
voltage is provided.There are many in-amp applications
that require amplifying a differential voltage that is riding
on top of a much larger common-mode voltage. This
common-mode voltage may be noise, or ADC offset,
or both. The use of an op amp rather than an in-amp
would simply amplify both the common mode and the
signal by equal amounts. The great benefit provided by
an in-amp is that it selectively amplifies the (differential)
signal while rejecting the common-mode signal.
Rail-to-Rail Input and Output Swing
Modern in-amps often need to operate on single-supply
voltages of 5 V or less. In many of these applications, a
rail-to-rail input ADC is often used. So-called rail-to-rail
operation means that an amplifier’s maximum input or
output swing is essentially equal to the power supply
voltage. In fact, the input swing can sometimes exceed
the supply voltage slightly, while the output swing is often
within 100 mV of the supply voltage or ground. Careful
attention to the data sheet specifications is advised.
Power vs. Bandwidth, Slew Rate, and Noise
As a general rule, the higher the operating current of
the in-amp’s input section, the greater the bandwidth
and slew rate and the lower the noise. But higher operating
current means higher power dissipation and heat. Batteryoperated equipment needs to use low power devices,
and densely packed printed circuit boards must be
able to dissipate the collective heat of all their active
components. Device heating also increases offset drift
and other temperature-related errors. IC designers
often must trade off some specifications to keep power
dissipation and drift to acceptable levels.
1-9
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