FEATURES
High Common-Mode Voltage Range –2 V to +24 V at a
5 V Supply Voltage
Operating Temperature Range
Die: –40C to +150C
8-Lead SOIC: –40C to +125C
Supply Voltage Range: 4.7 V to 12 V
Low-Pass Filter (One Pole or Two Pole)
EXCELLENT AC AND DC PERFORMANCE
15 V/C Max Offset Drift
20 ppm/C Max Gain Drift
80 dB CMRR Min DC to 10 kHz
PLATFORMS
Transmission Control
Diesel Injection Control
Engine Management
Semi-Active Suspension Control
Vehicle Dynamics Control
+IN
–IN
FUNCTIONAL BLOCK DIAGRAM
NCA1A2
200k200k
NC = NO CONNECT
Difference Amplifier
AD8200
SOIC (R) Package
DIE Form
+V
S
AD8200
10k
10k
GND
G = X10
+IN
A1
–IN
100k
G = X2
+IN
A2
–IN
OUT
GENERAL DESCRIPTION
The AD8200 is a single-supply difference amplifier for amplifying
and low-pass filtering small differential voltages in the presence
of a large common-mode voltage. The input CMV range extends
from –2 V to +24 V at a typical supply voltage of 5 V.
The AD8200 is offered in die and packaged form. Both package
options are specified over wide temperature ranges, making the
AD8200 well suited for use in many automotive platforms. The
SOIC package is specified over a temperature range of –40°C to
+125°C. The die is specified from –40°C to +150°C.
BATTERY
INDUCTIVE
CLAMP
DIODE
14V
4 TERM
SHUNT
POWER
DEVICE
COMMONNC = NO CONNECT
LOAD
+IN
AD8200
–IN
NC
GND
5V
+V
OUT
S
A1
OUTPUT
A2
Figure 1. High-Line Current Sensor
REV. 0
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
Automotive platforms demand precision components for better
system control. The AD8200 provides excellent ac and dc performance that keeps errors to a minimum in the user’s system.
Typical offset and gain drift in the SOIC package are 6 µV/°C
and 10 ppm/°C, respectively. The device also delivers a minimum CMRR of 80 dB from dc to 10 kHz.
The AD8200 features an externally accessible 100 kΩ resistor at
the output of the preamp A1, which can be used for low-pass
filter applications, and for establishing gains other than 20.
Operating Range4.7124.712V
Quiescent Current vs. TempV
= 0.1 V dc0.2510.251mA
O
PSRRVS = 4.7 V to 12 V75807580dB
TEMPERATURE RANGE
For Specified Performance–40+125–40+150°C
NOTES
1
Source Imbalance < 2 Ω.
2
The AD8200 preamplifier exceeds 80 dB CMRR at 10 kHz. However, since the signal is available only by way of a 100 k Ω resistor, even the small amounts of pinto-pin capacitance between Pins 1, 8 and 3, 4 may couple an input common-mode signal larger than the greatly attenuated preamplifier output. The effect of pin-topin coupling may be neglected in all applications using filter capacitors at Node 3.
Lead Temperature Range (Soldering 60 sec) . . . . . . . . 300°C
*Stresses beyond those listed under Absolute Maximum Ratings may cause
permanent damage to the device. This is a stress rating only; the functional
operation of the device at these or any other conditions above those indicated in
the operational sections of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect device reliability.
AD8200R–40°C to +125°CPlastic SOICSO-8
AD8200CHIPS–40°C to +150°CDIE Form
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection. Although
the AD8200 features proprietary ESD protection circuitry, permanent damage may occur on
devices subjected to high-energy electrostatic discharges. Therefore, proper ESD precautions are
recommended to avoid performance degradation or loss of functionality.
The AD8200 consists of a preamp and buffer arranged as shown
in Figure 3. Like-named resistors have equal values.
The preamp incorporates a dynamic bridge (subtractor) circuit.
Identical networks (within the shaded areas), consisting of R
R
, RC, and RG, attenuate input signals applied to Pins 1 and 8.
B
,
A
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.
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
, by way of RG, to adjust the voltage at its inverting
B
input until it matches the voltage at its noninverting input.
By attenuating voltages at Pins 1 and 8, the amplifier inputs are
held within the power supply range, even if Pin 1 and Pin 8 input
levels exceed the supply, or fall below Common (Ground.) The
input network also attenuates normal (differential) mode voltages. R
and RG form an attenuator that scales A1 feedback,
C
forcing large output signals to balance relatively small differential inputs. The resistor ratios establish the preamp gain at ten.
Because the differential input signal is attenuated, and then
amplified to yield an overall gain of ten, 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
R
R
R
R
G
–IN
R
A
A
A1
R
R
CM
R
B
B
R
R
G
C
C
CM
A3
100k
(TRIMMED)
AD8200
A2
R
F
R
F
TEK RUN: 2.5MS/s AVERAGE
1
V
, RL = 10k
OUT
MAGNIFIED V
V
3
2
CH1 1VCH 2 10mV M 20s CH1 1.36V
CH3 100mV
IN
OUT
TPC 8. Settling Time
To minimize these errors while extending the common-mode
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 R
resistors to reduce the
CM
common-mode voltage range at the A1 inputs. By adjusting the
common voltage of these resistors, the common-mode input
range is extended while, at the same time, 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 kΩ 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-two buffer-amplifier
A2, implemented with carefully matched feedback resistors R
.
F
The two-stage system architecture of the AD8200 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 kΩ, to minimize the effects of
A2’s input bias current. However, this current is quite small and
errors resulting from applications that mismatch the resistance
are correspondingly small.
APPLICATIONS
The AD8200 difference amplifier is intended for applications
where it is required to extract a small differential signal in the
presence of large common-mode voltages. The input resistance
is nominally 200 kΩ, and the device can tolerate common-mode
voltages higher than the supply voltage and lower than ground.
The open collector output stage will source current to within
20 mV of ground.
REV. 0
COM
Figure 3. Simplified Schematic
–5–
AD8200
CURRENT SENSING
High-Line, High-Current Sensing
Basic automotive applications making use of the large commonmode range are shown in Figures 1 and 2. The capability of the
device to operate as an amplifier in primary battery supply circuits is shown in Figure 1, Figure 2 illustrates the ability of the
device to withstand voltages below system ground.
Low Current Sensing
The AD8200 can also be used in low current sensing applications, such as a 4–20 mA current loop shown in Figure 4. In
such applications, the relatively large shunt resistor can degrade
the common-mode rejection. Adding a resistor of equal value in
the low-impedance side of the input corrects for this error.
+IN
–IN
NC
AD8200
GND
5V
OUT
+V
S
A1
NC = NO CONNECT
OUTPUT
A2
10
1%
+
10
1%
Figure 4. 4–20 mA Current Loop Receiver
GAIN ADJUSTMENT
The default gain of the preamplifier and buffer are ×10 and ×2
respectively, resulting in a composite gain of ×20. With the
addition of external resistor(s) or trimmer(s), the gain may be
lowered, raised, or finely calibrated.
Gains Less than 20
See Figure 5. Since the preamplifier has an output resistance of
100 kΩ, an external resistor connected from Pins 3 and 4 to
GND will decrease the gain by a factor R
+V
S
NC+IN
S
V
DIFF
2
V
V
CM
DIFF
2
AD8200
100k
10k10k
EXT
OUT+V
A2A1GND–IN
/(100 kΩ + R
OUT
20R
R
EXT
= 100k
EXT
+ 100k
20 – GAIN
GAIN =
R
EXT
EXT
GAIN
).
Gains Greater than 20
Connecting a resistor from the output of the buffer amplifier to
its noninverting input, as shown in Figure 6, will increase the
/(R
gain. The gain is now multiplied by the factor R
100 kΩ); for example, it is doubled for R
EXT
= 200 kΩ. Overall
EXT
EXT
–
gains as high as 50 are achievable in this way. Note that the
accuracy of the gain becomes critically dependent on resistor
value at high gains. Also, the effective input offset voltage at
Pins 1 and 8 (about six times the actual offset of A1) limits the
part’s use in very high-gain, dc-coupled applications.
+V
S
+V
NC+IN
V
DIFF
2
AD8200
V
V
CM
DIFF
2
NC = NO CONNECT
100k
GND
OUT
S
10k10k
R
EXT
A2A1–IN
GAIN =
R
EXT
OUT
R
EXT
= 100k
20R
– 100k
GAIN – 20
EXT
GAIN
Figure 6. Adjusting for Gains Greater than 20
GAIN TRIM
Figure 7 shows a method for incremental gain trimming using
a trimpot and external resistor R
EXT
.
The following approximation is useful for small gain ranges:
∆G ≈ (10 MΩ÷ R
Thus, the adjustment range would be ±2% for R
±10% for R
V
EXT
CM
= 1 MΩ, etc.
V
DIFF
2
V
DIFF
2
NC = NO CONNECT
+IN
–IN
NC
AD8200
GND
) %
EXT
= 5 MΩ;
EXT
5V
OUT
+V
S
A1
A2
R
EXT
OUT
GAIN TRIM
20k MIN
R
EXT
NC = NO CONNECT
Figure 5. Adjusting for Gains Less than 20
The overall bandwidth is unaffected by changes in gain using
this method, although there may be a small offset voltage due to
the imbalance in source resistances at the input to the buffer. In
many cases this can be ignored, but if desired, can be nulled by
inserting a resistor equal to 100 kΩ minus the parallel sum of
and 100 kΩ, in series with Pin 4. For example, with R
R
EXT
EXT
= 100 kΩ (yielding a composite gain of ×10), the optional offset
nulling resistor is 50 kΩ (see Figure 11.)
–6–
Figure 7. Incremental Gain Trim
REV. 0
AD8200
40LOG (f2/f1)
f
1
ATTENUATION
f
2
f
2
2
/f
1
FREQUENCY
A 1-POLE FILTER, CORNER f1, AND
A 2-POLE FILTER, CORNER f
2
, HAVE
THE SAME ATTENUATION –40LOG (f
2/f1
)
AT FREQUENCY f
2
2
/f
1
20dB/DECADE
40dB/DECADE
Internal Signal Overload Considerations
When configuring gain for values other than 20, the maximum
input voltage with respect to the supply voltage and ground
must be considered, since either the preamplifier or the output
buffer will reach its full-scale output (approximately V
– 0.2 V)
S
with large differential input voltages. The input of the AD8200
is limited to (V
– 0.2) ÷ 10, for overall gains ≤10, since the
S
preamplifier, with its fixed gain of ×10, reaches its full-scale
output before the output buffer. For gains greater than 10, the
swing at the buffer output reaches its full-scale first and limits
the AD8200 input to (VS – 0.2) ÷ G, where G is the overall gain.
LOW-PASS FILTERING
In many transducer applications it is necessary to filter the signal to remove spurious high-frequency components, including
noise, or to extract the mean value of a fluctuating signal with a
peak-to-average ratio (PAR) greater than unity. For example, a
full-wave rectified sinusoid has a PAR of 1.57, a raised cosine
has a PAR of 2, and a half-wave sinusoid has a PAR of 3.14.
Signals having large spikes may have PARs of 10 or more.
When implementing a filter, the PAR should be considered so
the output of the AD8200 preamplifier (A1) does not clip before
A2, since this nonlinearity would be averaged and appear as an
error at the output. To avoid this error, both amplifiers should
be made to clip at the same time. This condition is achieved
when the PAR, is no greater than the gain of the second amplifier (2 for the default configuration). For example, if a PAR of 5
is expected, the gain of A2 should be increased to 5.
Low-pass filters can be implemented in several ways using the
features provided by the AD8200. In the simplest case, a singlepole filter (20 dB/decade) is formed when the output of A1 is
connected to the input of A2 via the internal 100 kΩ resistor by
strapping Pins 3 and 4, and a capacitor added from this node to
ground, as shown in Figure 8. If a resistor is added across the
capacitor to lower the gain, the corner frequency will increase; it
should be calculated using the parallel sum of the resistor and
100 kΩ.
5V
OUT
5V
V
DIFF
2
V
V
CM
DIFF
2
NC = NO CONNECT
+IN
AD8200
–IN
NC
GND
OUT
+V
S
A2
A1
255k
C
F
= 1Hz – F
C
OUT
C
Figure 9. 2-Pole Low-Pass Filter
A 2-pole filter (with a roll-off of 40 dB/decade) can be implemented using the connections shown in Figure 9. This is a
Sallen-Key form based on a ×2 amplifier. It is useful to remember that a 2-pole filter with a corner frequency f
filter with a corner at f
frequency (f
(f
). This is illustrated in Figure 10. Using the standard resis-
2/f1
2
/f1). The attenuation at that frequency is 40 Log
2
have the same attenuation at the
1
and a 1-pole
2
tor value shown, and equal capacitors (Figure 9), the corner
frequency is conveniently scaled at 1 Hz-µF (0.05 µF for a 20 Hz
corner). A maximally flat response occurs when the resistor is
lowered to 196 kΩ and the scaling is then 1.145 Hz-µF. The
output offset is raised by about 5 mV (equivalent to 250 V at
the input pins).
V
V
CM
DIFF
2
V
DIFF
2
NC = NO CONNECT
+IN
–IN
NC
+V
AD8200
GND
A1
S
OUT
A2
F
C IN FARADS
C
1
=
C
2C10
5
Figure 8. A Single-Pole, Low-Pass Filter Using the Internal
Ω
Resistor
100 k
If the gain is raised using a resistor, as shown in Figure 8, the
corner frequency is lowered by the same factor as the gain is
raised. Thus, using a resistor of 200 kΩ (for which the gain
would be doubled) the corner frequency is now 0.796 Hz-µF,
(0.039 µF for a 20 Hz corner frequency.)
REV. 0
Figure 10. Comparative Responses of 1- and 2-Pole LowPass Filters
–7–
AD8200
HIGH LINE CURRENT SENSING WITH LPF AND GAIN
ADJUSTMENT
Figure 11 is another refinement of Figure 1, including gain
adjustment and low-pass filtering.
BATTERY
CLAMP
DIODE
14V
NC = NO CONNECT
4 TERM
SHUNT
POWER
DEVICE
INDUCTIVE
LOAD
COMMON
+IN
–IN
5V
+V
NC
OUT
S
AD8200
A2
GND
A1
V
OS/IB
NULL
C
5% CALIBRATION RANGE
= 0.796Hz – F
F
C
(0.22F FOR f = 3.6 Hz)
OUTPUT
4V/AMP
191k
20k
Figure 11. High-Line Current Sensor Interface. Gain = ×40,
Single-Pole, Low-Pass Filter
A power device that is either ‘ON’ or ‘OFF’ controls the current
in the load. The average current is proportional to the duty cycle
of the input pulse, and is sensed by a small value resistor. The
average differential voltage across the shunt is typically 100 mV,
although its peak value will be higher by an amount that depends
on the inductance of the load and the control frequency. The
common-mode voltage, on the other hand, extends from roughly
1 V above ground, when the switch is ‘ON,’ to about 1.5 V
above the battery voltage, when the device is ‘OFF,’ and the
clamp diode conducts. If the maximum battery voltage spikes
up to 20 V, the common-mode voltage at the input can be as
high as 21.5 V.
To produce a full-scale output of 4 V, a gain ×40 is used, adjustable by ±5% to absorb the tolerance in the shunt. There is
sufficient headroom to allow 10% overrange (to 4.4 V). The
roughly triangular voltage across the sense resistor is averaged
by a single-pole, low-pass filter, here set with a corner frequency
= 3.6 Hz, which provides about 30 dB of attenuation at 100 Hz.
A higher rate of attenuation can be obtained using a two-pole
filter having f
= 20 Hz, as shown in Figure 12. Although this
C
circuit uses two separate capacitors, the total capacitance is less
than half that needed for the single-pole filter.
DRIVING CHARGE REDISTRIBUTION A/D
CONVERTERS
When driving CMOS ADCs, such as those embedded in popular
microcontrollers, the charge injection (⌬Q) can cause a significant deflection in the output voltage of the AD8200. Though
generally of short duration, this deflection may persist until after
the sample period of the ADC has expired, due to the relatively
high open-loop output impedance of the AD8200. Including an
R-C network in the output can significantly reduce the effect.
The capacitor helps to absorb the transient charge, effectively
lowering the high-frequency output impedance of the AD8200.
For these applications, the output signal should be taken from the
midpoint of the R
LAG–CLAG
combination as shown in Figure 13.
Since the perturbations from the analog-to-digital converter are
small, the output impedance of the AD8200 will appear to be
low. The transient response will, therefore, have a time constant
governed by the product of the two LAG components, C
R
. For the values shown in Figure 13, this time constant is
LAG
LAG
×
programmed at approximately 10 µs. Therefore, if samples are
taken at several tens of microseconds or more, there will be
negligible charge “stack-up.”
5V
+IN
–IN
AD8200
A2
10k
10k
R
1k
LAG
C
LAG
0.01F
PROCESSOR
A/D
Figure 13. Recommended Circuit for Driving CMOS A/D
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
C02054–4.5–10/00 (rev. 0)
BATTERY
CLAMP
DIODE
14V
NC = NO CONNECT
4 TERM
SHUNT
POWER
DEVICE
INDUCTIVE
LOAD
COMMON
+IN
–IN
5V
+V
NC
AD8200
A1
GND
OUT
S
A2
127k
C
= 1Hz – F
F
C
(0.05F FOR f
C
Figure 12. Illustration of 2-Pole Low-Pass Filtering
432k
50k
= 20Hz)
C
OUTPUT
–8–
0.1574 (4.00)
0.1497 (3.80)
PIN 1
0.0098 (0.25)
0.0040 (0.10)
SEATING
0.1968 (5.00)
0.1890 (4.80)
85
0.0500 (1.27)
BSC
0.0192 (0.49)
0.0138 (0.35)
PLANE
8-Lead SOIC Package
(SO-8)
0.2440 (6.20)
0.2284 (5.80)
41
0.0688 (1.75)
0.0532 (1.35)
0.0098 (0.25)
0.0075 (0.19)
0.0196 (0.50)
0.0099 (0.25)
8
0.0500 (1.27)
0
0.0160 (0.41)
PRINTED IN U.S.A.
45
REV. 0
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