Chapter IV: Monolithic Difference Amplifiers

Chapter IV
MONOLITHIC DIFFERENCE AMPLIFIERS
Difference (Subtractor) Amplifier Products
Monolithic difference amplifiers are a special category
of in-amps that are usually designed to be used in applications where large dc or ac common-mode voltages
are present. This includes many general current sensing
applications, such as motor control, battery chargers,
and power converters. In addition, there are numerous
high common-mode voltage automotive current sensing
applications, including: battery cell voltage monitoring,
transmission controls, fuel injection controls, engine
management, suspension controls, electronic steering,
electronic parking brake, and hybrid vehicle drive/hybrid
battery control. Because these amplifiers are typically
used to sense current by accurately amplifying the small
differential voltage across a shunt resistor in the load
path, they are often called current shunt amplifiers.
The AD8200 family of current shunt amplifiers is based
on a traditional difference amplifier input stage which
includes a resistor-divider configuration. These on-chip
precision resistors provide matching to within 0.01%,
which results in very good total error when compared
with a difference amplifier built from discrete op amps
and resistors. Unlike the AD8200 amplifiers, which can
withstand high common-mode input voltages by dividing
these voltages down at the input, the AD8210 amplifiers
tolerate the high common-mode input voltages by virtue
of the high breakdown voltages of their input transistors.
This provides numerous advantages over the AD8200
series amplifiers, including higher bandwidth, higher
input impedance, and lower overall noise amplification.
Combined, these advantages reduce total system error.
Table 4-1 provides a performance summary of Analog
Devices difference amplifier products.
Table 4-1. Latest Generation of Analog Devices Difference Amps Summarized1
Product Features
Power
Supply
Current
Typ
–3 dB
BW
Typ
(G = 10)
CMR
G = 10
(dB)
Min
Input
Offset
Voltage
Max
AD8202
AD8203
AD8205
AD8206
AD8210
AD8212
AD8213
AD8130
AD628
AD629
AD626
AMP03
250 A
250 A
1 mA
1 mA
500 A
200 A
1.3 mA11
12 mA
1.6 mA
0.9 mA
1.5 mA
3.5 mA
50 kHz
60 kHz7
50 kHz8
100 kHz3
500 kHz3
500 kHz
500 kHz
270 MHz
600 kHz15
500 kHz
100 kHz
3 MHz
803, 4, 5
805, 7
804, 5, 6
763, 9
1003, 5
90
100
8312, 13
7515
7712
5516
8512
1 mV6 10
1 mV6 10
2 mV6 15 typ
2 mV6 15 typ
1 mV6 5 typ
1 mV
10
1 mV
10
1.8 mV 3.5 mV
1.5 mV 4
1 mV
6
500 V 1
400 VNS
S.S., 28 V CMV, G = 20
S.S., 28 V CMV, G = 14
S.S., 65 V CMV, G = 50
S.S., 65 V CMV, G = 20
S.S., current shunt monitor
Adjustable gain; CMV up to 500 V10
Dual channel
270 MHz receiver
High CMV
High CMV, G = 1
High CMV
High BW, G = 1
NOTES
NS = not specified, NA = not applicable, S.S. = single supply.
1
Refer to ADI website at www.analog.com for latest products and specifications.
2
At 1 kHz. RTI noise = √(eni)2 + (eno/G)2.
3
Operating at a gain of 20.
4
For 10 kHz, <2 k source imbalance.
5
DC to 10 kHz.
6
Referred to input (RTI).
7
Operating at a gain of 14.
8
Operating at a gain of 50.
9
DC to 20 kHz.
10
With inexpensive external transistor.
11
Note that this is 0.65 mA per channel.
12
Operating at a gain of 1.
13
At frequency = 4 MHz.
14
At frequency  10 kHz.
15
Operating at a gain of 0.1.
16
f = 10 kHz, VCM = 6 V.
4-1
VOS
Drift
(V/C)
Max
RTI
Noise2
(nV/√Hz)
(G = 10)
300 typ3
300 typ7
500 typ8
500 typ3
80 typ3
100 typ
70 typ
12.5 typ12, 14
300 typ15
550 typ12
250 typ
750 typ12
The AD8200 family of current sensing difference amplifiers has multiple gain options, which provide design
flexibility for the following important trade-offs:
63
! !
1.) The shunt resistance value vs. the power dissipated in
the circuit being measured
). 2!
' 2#
n). 2!
n
RG
RB
RC
RC
2&
2#
Figure 4-1. AD8202 connection diagram.
Any common-mode voltage applied to both inputs will
keep the bridge balanced and the A1 output at zero.
Because the resistor networks are carefully matched,
the common-mode signal rejection approaches this
ideal state. However, if the signals applied to the inputs
differ, the result is a difference at the input to A1. A1
responds by adjusting its output to drive R B, by way of
RG, to adjust the voltage at its inverting input until it
matches the voltage at its noninverting input.
100k
A1
RB
2&
2'
Figure 4-2 provides more details. The preamp incorporates a dynamic bridge (subtractor) circuit. Identical
networks (within the shaded areas) consisting of R A,
R B, RC, and RG attenuate input signals applied to Pins
1 and 8. Note that when equal amplitude signals are
asserted at inputs 1 and 8, and the output of A1 is equal
to the common potential (i.e., zero), the two attenuators
form a balanced-bridge network. When the bridge is
balanced, the differential input voltage at A1, and thus
its output, will be zero.
RA
RCM
/54
The AD8202 consists of a preamp and buffer arranged
as shown in Figure 4-1.
RA
#/-
Similarly, the AD8205 has a gain of 50, for use in
applications where it is most important to minimize the
power dissipation in the resistive shunt. This higher gain
is used with lower resistance shunts, which, of course,
have a lower output voltage. This slightly reduces the
signal-to-noise performance of the system.
1
!
n
2"
The automotive industry standard calls for a gain of 20,
which, in most cases, gives an excellent trade-off between
all three variables. However, there are conditions which
favor other gains. For example, the AD8203 operates at a
gain of 14. This allows for convenient scaling of the output to accommodate both 5 V and 3.3 V A/D converters,
while still using the same value resistive shunt.
–IN
'
3.) The shunt resistance value vs. the amplifier gain
needed
8
!$
!
2.) The shunt resistance value vs. the signal-to-noise
ratio
+IN
2'
2"
3
(TRIMMED)
A3
5
RF
AD8202
2
COM
Figure 4-2. AD8202 simplified schematic.
4-2
A2
RF
RCM
RG
4
By attenuating voltages at Pins 1 and 8, the amplifier
inputs are held within the power supply range, even if
the input levels of Pins 1 and 8 exceed the supply or
fall below common (ground). The input network also
attenuates normal (differential) mode voltages. RC
and RG form an attenuator that scales A1 feedback,
forcing large output signals to balance relatively small
differential inputs. The resistor ratios establish the
preamp gain at 10.
to the AD8203. The two products are almost identical,
except for their internal preset gains and their power
consumption.
Because the differential input signal is attenuated and
then amplified to yield an overall gain of 10, the
amplifier A1 operates at a higher noise gain, multiplying
deficiencies such as input offset voltage and noise with
respect to Pins 1 and 8.
In typical applications, the AD8205 is used to measure
current by amplifying the voltage across a current shunt
placed across the inputs.
To minimize these errors while extending the commonmode range, a dedicated feedback loop is employed to
reduce the range of common-mode voltage applied to
A1 for a given overall range at the inputs. By offsetting
the range of voltage applied to the compensator, the
input common-mode range is also offset to include
voltages more negative than the power supply. Amplifier A3 detects the common-mode signal applied to A1
and adjusts the voltage on the matched RCM resistors
to reduce the common-mode voltage range at the A1
inputs. By adjusting the common voltage of these resistors, the common-mode input range is extended while
the normal mode signal attenuation is reduced, leading
to better performance referred to input.
The output of the dynamic bridge taken from A1 is
connected to Pin 3 by way of a 100 k series resistor,
provided for low-pass filtering and gain adjustment.
The resistors in the input networks of the preamp and
the buffer feedback resistors are ratio-trimmed for
high accuracy.
The output of the preamp drives a gain-of-2 buffer
amplifier, A2, implemented with carefully matched
feedback resistors, R F.
AD8205 Difference Amplifier
The AD8205 is a single-supply difference amplifier that
uses a unique architecture to accurately amplify small
differential current shunt voltages in the presence of
rapidly changing common-mode voltages. It is offered
in both packaged and die form.
The gain of the AD8205 is 50 V/V, with an accuracy of
1.2%. This accuracy is guaranteed over the operating
temperature range of –40C to +125C. The die temperature range is –40C to +150C with a guaranteed
gain accuracy of 1.3%.
The input offset is less than 2 mV referred to the input at
25C, and 4.5 mV maximum referred to the input over
the full operating temperature range for the packaged
part. The die input offset is less than 6 mV referred to
the input over the die operating temperature range.
The AD8205 operates with a single supply from
4.5 V to 10 V (absolute maximum = 12.5 V). The
supply current is less than 2 mA.
High accuracy trimming of the internal resistors allows
the AD8205 to have a common-mode rejection ratio
better than 78 dB from dc to 20 kHz.The common-mode
rejection ratio over the operating temperature is 76 dB
for both the die and the packaged part.
The output offset can be adjusted from 0.05 V to 4.8 V
(V+ = 5 V) for unipolar and bipolar operation.
The AD8205 consists of two amplifiers (A1 and A2), a
resistor network, a small voltage reference, and a bias
circuit (not shown). See Figure 4-3.
The two-stage system architecture of the AD8202
(Figure 4-2) enables the user to incorporate a low-pass
filter prior to the output buffer. By separating the gain
into two stages, a full-scale, rail-to-rail signal from the
preamp can be filtered at Pin 3, and a half-scale signal
resulting from filtering can be restored to full scale
by the output buffer amp. The source resistance seen
by the inverting input of A2 is approximately 100 k,
to minimize the effects of A2’s input bias current.
Typically, this current is quite small, and errors
resulting from applications that mismatch the resistance
are correspondingly small. The simplified schematic and
theory of operation given for the AD8202 also applies
4-3
–IN
RA
+IN
RA
A1
RB
RB
RC
RC
250mV
GND
RF
RF
RD
RD
A2
VOUT
VREF1
AD8205
RE
RF
RREF
RREF
VREF2
Figure 4-3. AD8205 simplified schematic.
The set of input attenuators preceding A1 consists of RA,
RB, and RC, which reduces the common-mode voltage to
match the input voltage range of A1.The two attenuators
form a balanced-bridge network.When the bridge is balanced, the differential voltage created by a common-mode
voltage is 0 V at the inputs of A1. The input attenuation
ratio is 1/16.7. The combined series resistance of RA, RB,
and RC is approximately 200 k  20%.
By attenuating the voltages at Pin 1 and Pin 8, the A1
amplifier inputs are held within the power supply range,
even if Pin 1 and Pin 8 exceed the supply or fall below
common (ground). A reference voltage of 250 mV
biases the attenuator above ground. This allows
the amplifier to operate in the presence of negative
common-mode voltages.
The input network also attenuates normal (differential)
mode voltages. A1 amplifies the attenuated signal by 26.
The input and output of this amplifier are differential to
maximize the ac common-mode rejection.
A2 converts the differential voltage from A1 into a singleended signal and provides further amplification.The gain
of this second stage is 32.15.
The reference inputs, VREF1 and VREF2, are tied through
resistors to the positive input of A2, which allows the
output offset to be adjusted anywhere in the output
operating range. The gain is 1 V/V from the reference
pins to the output when the reference pins are used
in parallel. The gain is 0.5 V/V when they are used to
divide the supply.
The ratios of Resistors RA, RB, RC, RD, and RF are
trimmed to a high level of precision to allow the
common-mode rejection ratio to exceed 80 dB. This
is accomplished by laser trimming the resistor ratio
matching to better than 0.01%.
The AD8210 operates with a single supply between
4.5 V to 5.5 V. The supply current is typically less
than 2 mA.
The AD8210 is comprised of two main blocks: a
differential amplifier and an instrumentation amplifier. A
load current flowing through the external shunt resistor
produces a voltage at the input terminals. The input
terminals are connected to the differential amplifier (A1)
by Resistors R1 and R2. A1 nulls the voltage appearing
across its own input terminals by adjusting (balancing)
the current through R1 and R2 with Transistors Q1 and
Q2. When the input signal to the AD8210 is zero, the
currents in R1 and R2 are equal. When the differential
signal is nonzero, the current increases through one
of the resistors and decreases in the other. The current
difference is proportional to the size and polarity of
the input signal. Since the differential input voltage is
converted into a current, common-mode rejection is not
dependent on resistor matching; therefore, both high
accuracy and performance are provided throughout the
wide common-mode voltage range.
The differential currents through QI and Q2 are
converted into a differential voltage due to R3 and R4.
A2 is configured as an instrumentation amplifier,
and this differential input signal is converted into a
single-ended output voltage by A2.The gain is internally
set with thin-film resistors to 20 V/V.
The output reference voltage is easily programmed by the
VREF1 and VREF2 pins. In a typical configuration, VREF1
is connected to VCC while VREF2 is connected to GND.
In this case, the output is centered at VCC/2 when the
input signal is zero.
The total gain of 50 is made up of the input attenuation
of 1/16.7 multiplied by the first stage gain of 26 and the
second stage gain of 32.15.
I SHUNT
R SHUNT
R1
The output stage is Class A with a PNP pull-up transistor
and a 300 A current sink pull-down.
The AD8206 is nearly identical to the AD8205, except
for gain and power consumption. Please see the AD8205
circuit description for AD8206 theory of operation.
The AD8210 is a current shunt monitor IC. Figure 4-4
shows the block diagram.
The gain of the AD8210 is 20 V/V, with an accuracy of
0.7%. This accuracy is guaranteed over the operating
temperature range of –408C to +1258C.
VOUT = (I SHUNT × R SHUNT) × 20
R2
VS
AD8210
A1
Q1
Q2
VREF1
A2
R3
VOUT
R4
VREF 2
GND
Figure 4-4. AD8210 block diagram.
4-4
The differential amplifier topology of the AMP03 serves
both to amplify the difference between two signals and
to provide extremely high rejection of the common-mode
input voltage. With a typical common-mode rejection of
100 dB, the AMP03 solves common problems encountered in instrumentation design. It is ideal for performing
either the addition or subtraction of two input signals
without using expensive externally matched precision
resistors. Because of its high CMRR over frequency,
the AMP03 is an ideal general-purpose amplifier for
data acquisition systems that must operate in a noisy
environment. Figures 4-8 and 4-9 show the AMP03’s
CMRR and closed-loop gain vs. frequency.
140
130
CMRR (dB)
120
110
+125°C
+25°C
100
90
–40°C
80
70
60
100
1k
10k
100k
120
FREQUENCY (Hz)
TA = 25C
VS = 15V
110
Figure 4-5. AD8210 CMRR vs. frequency and
temperature (common-mode voltage < 5 V).
100
90
80
130
CMRR (dB)
140
+25°C
120
70
60
50
CMRR (dB)
40
110
–40°C
30
+125°C
20
100
10
90
0
80
1
10
100
1k
10k
FREQUENCY (Hz)
100k
1M
Figure 4-8. AMP03 CMRR vs. frequency.
70
50
60
100
1k
10k
100k
TA = 25C
VS = 15V
40
Figure 4-6. AD8210 CMRR vs. frequency and
temperature (common-mode voltage > 5 V).
The AMP03 is a monolithic, unity-gain, 3 MHz differential amplifier. Incorporating a matched thin-film
resistor network, the AMP03 features stable operation
over temperature without requiring expensive external
matched components. The AMP03 is a basic analog
building block for differential amplifier and instrumentation applications (Figure 4-7).
AMP03
–IN 2
+IN 3
25k6
25k6
25k6
25k6
5
SENSE
7
–VCC
6
OUTPUT
4
–VEE
1
REFERENCE
CLOSED-LOOP GAIN (dB)
FREQUENCY (Hz)
30
20
10
0
–10
–20
–30
100
1k
10k
100k
FREQUENCY (Hz)
1M
Figure 4-9. AMP03 closed-loop gain
vs. frequency.
Figure 4-7. AMP03 functional block diagram.
4-5
10M
Figure 4-10 shows the small signal pulse response of
the AMP03.
TA = 25C
VS = 15V
The uncommitted amplifier is a high open-loop gain,
low offset, low drift op amp, with its noninverting input
connected to the internal 10 kV resistor. Both inputs are
accessible to the user.
Careful layout design has resulted in exceptional
common-mode rejection at higher frequencies. The
inputs are connected to Pin 1 (+IN) and Pin 8 (–IN),
which are adjacent to the power pins, Pin 2 (–VS) and
Pin 7 (+VS). Because the power pins are at ac ground,
input impedance balance and, therefore, common-mode
rejection are preserved at higher frequencies.
100
90
0
50mV
7
Figure 4-10. AMP03 small signal pulse response.
–IN 8
The AD628 is a high common-mode voltage difference
amplifier, combined with a user-configurable output
amplifier (see Figure 4-11 and Figure 4-12). Differential
mode voltages in excess of 120 V are accurately scaled by
a precision 11:1 voltage divider at the input. A reference
voltage input is available to the user at Pin 3 (VREF).
The output common-mode voltage of the difference
amplifier is the same as the voltage applied to the reference pin. If the uncommitted amplifier is configured for
gain, connecting Pin 3 to one end of the external gain
resistor establishes the output common-mode voltage
at Pin 5 (OUT).
RG
8
100k6
10k6
–IN
–IN
G = +0.1
A2
10k6
A1
5
1
OUT
CFILT
+IN
A2
+IN 1
OUT
5
–IN
100k6
10k6
–VS
2
VREF
3
RG
6
REXT3
REFERENCE
VOLTAGE
REXT2
REXT1
Figure 4-12. AD628 circuit connections.
Gain Adjustment


R
GTOTAL = 0.1 × 1 + EXT 1 
 REXT 2 
10k6
VREF
10k6
A1
+IN
100k6
4
AD628
G = +0.1
+IN
3
4
10k6
–IN
+IN
+IN
100k6
The AD628 system gain is provided by an architecture
consisting of two amplifiers. The gain of the input stage
is fixed at 0.1; the output buffer is user-adjustable as
GA2 = 1 + REXT1/REXT2.
6
–IN
CFILT
+VS
1s
Figure 4-11. AD628 simplified schematic.
The output of the difference amplifier is internally
connected to a 10 kV resistor trimmed to better than
60.1% absolute accuracy. The resistor is connected
to the noninverting input of the output amplifier and
is accessible to the user at Pin 4 (CFILT). A capacitor
can be connected to implement a low-pass filter, a
resistor can be connected to further reduce the output
voltage, or a clamp circuit can be connected to limit
the output swing.
At 2 nA maximum, the input bias current of the buffer
amplifier is very low, and any offset voltage induced at
the buffer amplifier by its bias current may normally
be neglected (2 nA 3 10 kV = 20 mV). However, to
absolutely minimize bias current effects, REXT1 and
REXT2 can be selected so that their parallel combination is 10 kV. If practical resistor values force the
parallel combination of REXT1 and REXT2 below 10 kV,
a series resistor (REXT3) can be added to make up for
the difference. Table 4-2 lists several values of gain and
corresponding resistor values.
4-6
Table 4-2. Nearest Standard 1% Resistor Values
for Various Gains (See Figure 4-12)
A2 Gain
(V/V)
REXT1
(V)
REXT2
(V)
REXT3
(V)
0.1
0.2
0.25
0.5
1
2
5
10
1
2
2.5
5
10
20
50
100
10 k
20 k
25.9 k
49.9 k
100 k
200 k
499 k
1 M

20 k
18.7 k
12.4 k
11 k
10.5 k
10.2 k
10.2 k
0
0
0
0
0
0
0
0
50
30
1k
10k
100k
1M
10M
FREQUENCY (Hz)
Figure 4-14. AD628 small signal frequency
response, VOUT = 200 mV p-p, G = +0.1, +1, +10,
and +100.
60
50
40

 × 1
G = +100
30
120
20
G = +10
10
0
G = +1
–10
110
–20
100
CMRR (dB)
G = +0.1
–40
100
130
VS = 15V
90
–40
10
70
40
100
1k
10k
1k
10k
100k
1M
Figure 4-15. AD628 large signal frequency
response, VOUT = 20 V p-p, G = +0.1, +1, +10,
and +100.
50
10
100
FREQUENCY (Hz)
VS = 2.5V
60
G = +0.1
–30
80
30
G = +1
0
–30
GAIN (dB)
EXT 4
10
–20
Using a divider and setting A2 to unity gain yields
GW / DIVIDER
G = +10
20
–10
To set the system gain to less than 0.1, an attenuator
can be created by placing a resistor, R EXT4 , from
Pin 4 (C FILT ) to the reference voltage. A divider
would be formed by the 10 kV resistor, which is in
series with the positive input of A2 and R EXT4 . A2
would be configured for unity gain.

REXT 4
= 0.1 × 
 10 kΩ + R
G = +100
40
GAIN (dB)
Total Gain
(V/V)
60
100k
FREQUENCY (Hz)
Figure 4-13. AD628 CMRR vs. frequency.
For extensive coverage of AD628 applications circuits,
refer to Chapter 6 of this guide.
The AD626 is a single- or dual-supply differential
amplifier consisting of a precision balanced attenuator,
a very low drift preamplifier (A1), and an output
buffer amplifier (A2). It has been designed so that
small differential signals can be accurately amplified
and filtered in the presence of large common-mode
voltages (much greater than the supply voltage) without
the use of any other active components.
4-7
+VS
+IN
R1
200k
FILTER
AD626
C1
5pF
R12
100k
A1
–IN
R2
200k
R3
41k
R11
10k
A2
C2
5pF
R6
500
R4
41k
R5
4.2k
R17
95k
R15
10k
R9
10k
R7
500
R10
10k
R8
10k
R14
555
GAIN = 100
GND
OUT
R13
10k
–VS
Figure 4-16. AD626 simplified schematic.
Figure 4-16 shows the main elements of the AD626.
The signal inputs at Pins 1 and 8 are first applied to dual
resistive attenuators, R1 through R4, whose purpose is
to reduce the peak common-mode voltage at the input
to the preamplifier—a feedback stage based on the very
low drift op amp A1. This allows the differential input
voltage to be accurately amplified in the presence of
large common-mode voltages—six times greater than
that which can be tolerated by the actual input to A1. As
a result, the input common-mode range extends to six
times the quantity (VS – 1 V).The overall common-mode
error is minimized by precise laser trimming of R3 and
R4, thus giving the AD626 a common-mode rejection
ratio of at least 10,000:1 (80 dB). The output of A1 is
connected to the input of A2 via 100 k (R12) resistor
to facilitate the low-pass filtering of the signal of interest.
The AD626 is easily configured for gains of 10 or 100. For
a gain of 10, Pin 7 is simply left unconnected; similarly,
for a gain of 100, Pin 7 is grounded. Gains between 10
and 100 are easily set by connecting a resistor between
Pin 7 and analog GND. Because the on-chip resistors
have an absolute tolerance of 20% (although they are
ratio matched to within 0.1%), at least a 20% adjustment
range must be provided. The nominal value for this gain
setting resistor is equal to
R=
 50, 000 Ω 
− 555 Ω
 GAIN − 10 
500mV
20s
100
90
10
0%
Figure 4-17. T
he large signal pulse
response of the AD626. G = 10.
Figure 4-17 shows the large signal pulse response of
the AD626.
The AD629 is a unity-gain difference amplifier designed
for applications that require the measurement of signals
with common-mode input voltages of up to 270 V.
The AD629 keeps error to a minimum by providing
excellent CMR in the presence of high common-mode
input voltages. Finally, it can operate from a wide power
supply range of 2.5 V to 18 V.
The AD629 can replace costly isolation amplifiers
in applications that do not require galvanic isolation. Figure 4-18 is the connection diagram of the
AD629. Figure 4-19 shows the AD629’s CMR vs.
frequency.
4-8
–IN
2
+IN 3
21.1k�
AD629
380k�
380k�
380k�
PD
8
7
20k�
VIN
+VS
6
OUTPUT
5
REF B
95
90
85
CMR (dB)
VOUT
6
REF
4
FB
5
2
Figure 4-20. AD8130 block diagram.
100
80
75
70
65
60
55
1k
FREQUENCY (Hz)
7
8
Figure 4-20 is a block diagram of the AD8130. Its design
uses an architecture called active feedback, which differs
from that of conventional op amps. The most obvious
differentiating feature is the presence of two separate
pairs of differential inputs compared to a conventional
op amp’s single pair. Typically for the active feedback
architecture, one of these input pairs is driven by a
differential input signal, while the other is used for the
feedback. This active stage in the feedback path is where
the term active feedback is derived. The active feedback
architecture offers several advantages over a conventional
op amp in several types of applications. Among these are
excellent common-mode rejection, wide input commonmode range, and a pair of inputs that are high impedance
and totally balanced in a typical application.
Figure 4-18. AD629 connection diagram.
100
1
–VS
NC = NO CONNECT
50
20
+VS
3
–
+
–VS 4
NC
10k
–30
20k
–40
Figure 4-19 AD629 common-mode rejection
vs. frequency.
High Frequency Differential Receiver/Amplifiers
Although not normally associated with difference amplifiers, the AD8130 series of very high speed differential
receiver/amplifiers represent a new class of products
that provide effective common-mode rejection at VHF
frequencies. The AD8130 has a –3 dB bandwidth of
270 MHz, an 80 dB CMR at 2 MHz, and a 70 dB CMR
at 10 MHz.
COMMON-MODE REJECTION (dB)
REF A 1
–50
–60
–70
–80
VS = 2.5V
–90
VS = 5V, 12V
–100
–110
–120
10k
100k
1M
FREQUENCY (Hz)
10M
Figure 4-21. AD8130 CMR vs. frequency.
4-9
100M
In addition, while an external feedback network
establishes the gain response as in a conventional
op amp, its separate path makes it totally independent of
the signal input. This eliminates any interaction between
the feedback and input circuits, which traditionally causes
problems with CMRR in conventional differential-input
op amp circuits.
3
2
VS = 2.5V
1
GAIN (dB)
0
–1
VS = 5V
–2
VS = 12V
–3
–4
–5
–6
–7
1
10
100
400
FREQUENCY (MHz)
Figure 4-22. AD8130 frequency response
vs. gain and supply voltage.
Figure 4-21 shows the CMR vs. frequency of the
AD8130. Figure 4-22 shows its gain vs. frequency for
various supply voltages.
4-10
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