AME AM457 Precision amplifier for bridge circuit Datasheet

Precision amplifier for bridge circuits
AM457
PRINCIPLE FUNCTION
Adjustable amplification of differential voltage signals (±5 to ±100mV FS)
to an adjustable, ratiometric output voltage of between
0.2 and Vcc-0.2Volt with a diagnostic unit
VCC = 5V ± 5%
Diagnostic unit
Differential input
voltages
( 5... 100mV FS)
AM457
e.g. VOUT = 0.2...VCC-0.2V
(adjustable)
e.g. 0.5...4.5V
ratiometric
Iout = 2mA
TYPICAL APPLICATIONS
• Precision amplifiers for bridge circuits with
low sensitivity:
for ceramic measurement cells
for DMS sensing elements
for GMR sensing elements
• Sensor amplification with functional control
analog microelectronics
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
Internet: www.analogmicro.de
Phone:
Fax:
Email:
+49 (0)6131/91 073 – 0
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May 2005
1/10
Rev. 2.2
Precision amplifier for bridge circuits
AM457
TABLE OF CONTENTS
Features
Block diagram
General description
Electrical specifications
Circuit
Description of functions
Example application
Block diagram and pinout
Delivery options
Further reading
3
3
3
4
5
6
7
9
10
10
TABLE OF FIGURES AND TABLES
Table 1: Specifications
Table 2: Circuit
Table 3: Pin configuration
4
5
9
Figure 1: Block diagram
Figure 2: Schematic circuit diagram for resistance bridges with low sensitivity
Figure 3: Block diagram of AM457
Figure 4: AM457 Pin out
Figure 5 : SOP 8 Package dimensions
analog microelectronics
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
Internet: www.analogmicro.de
Phone:
Fax:
Email:
+49 (0)6131/91 073 – 0
+49 (0)6131/91 073 – 30
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3
7
9
9
10
May 2005
2/10
Rev. 2.2
Precision amplifier for bridge circuits
AM457
GENERAL DESCRIPTION
FEATURES
• High input sensitivity
• Wide differential input voltage range
(±5...±100mV FS)
• Low offset
• Low offset drift
• Low input noise
• High CMRR: > 120dB
• Wide operating temperature range:
-40... +125°C
• Adjustable output voltage
• Rail-to-rail output stage:
VOUT = 0.2V ... Vcc – 0.2V
• Sink/source output
• Single ratiometric supply: VCC = 5V
• Integrated current source: ≤ 2mA
• Integrated diagnostic unit
• Separate function groups
• Integrated EMV protective circuitry
• Short-circuit-proofing
• Small-scale design
AM457 is a high-precision integrated amplifier which
has been developed to condition signals from small
differential input voltages within a range of a few
millivolts (±5 to ±100mV FS). The chief component
of the IC is a highly accurate, low-noise amplifier
circuit (AMP). An additional integrated operational
amplifier (OP) can be used to supply a sensing
element with current or record temperature for sensor
bridges powered by voltage, for example. The entire
system (sensing element plus amplifier) is monitored
by the integrated diagnostic unit which logs any
sensing element errors or defects, system
undervoltage, AMP overloads and excessive
temperature. The various function groups are not
mutually dependent on one another; the AMP can be
operated independently of the OP, for example. The
functionality of both component groups (AMP and
OP) is autonomous with regard to the diagnostic unit.
AM457 is a combination of high-impedance precision
amplifier, additional OP and extensive diagnostic unit,
enabling a self-monitoring sensor system to be
assembled using very few components.
BLOCK DIAGRAM
AM457
OUTOP
8 VCC
OP
7
IN+
AMP
_
INOP
VCC
9k
1k
Temp
Diagnostic
unit
5 DIAG
1 OUT
6
4
GND
Figure 1: Block diagram
analog microelectronics
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
Internet: www.analogmicro.de
Phone:
Fax:
Email:
+49 (0)6131/91 073 – 0
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May 2005
3/10
Rev. 2.2
Precision amplifier for bridge circuits
AM457
ELECTRICAL SPECIFICATIONS
Tamb = 25°C, VCC = 5V (unless otherwise stated)
Parameter
Symbol
Voltage Range
VCC
Quiescent Current
ICC
Conditions
Min.
4.5
Tamb = -40...+85°C
Typ.
Max.
Unit
5
5.5
V
1.5
2.5
mA
125
°C
Temperature Specifications
Operating
Tamb
-45
Storage
Tst
-55
Junction
TJ
Thermal Resistance
Θja
DIL8 plastic package
111
°C/W
Θja
SO8 narrow plastic package
181
°C/W
±0.1
±0.3
mV
Tamb= -45…105°C
±0.5
±1.2
µV/°C
±120
µV
200
nA
–600
pA/°C
±100
mV
150
°C
150
°C
Amplifier AMP
Offset Voltage
VOS
VOS vs. Temperature
dVOS/dT
VOS vs. Temperature
dVOS/dT
Tamb= 105…125°C
Input Bias Current
IB
VCM = 2.5V
IB vs. Temperature
dIB/dT
Tamb= -45…125°C
Differential Input Voltage
VIN
VIN =VIN+ – VIN-
30
–130
±5
Input Offset Current
IOS
VCM = 2.5V
±0.5
±5
nA
IOS vs. Temperature
dIOS/dT
Tamb= -45…125°C
±2.5
±30
pA/°C
Input Resistance
RIN
VCM / IB,typ (VCM = 2.5V)
100
MΩ
Input Capacitance
CIN
90
pF
Common Mode Input Range
CMIR
1
Common Mode Rejection Ratio
CMRR
120
135
dB
Open Loop Gain
G0
120
140
dB
Adjustable Gain
G
10
Output Voltage Range
VOUT
0.2
Output Current
IOUT
Output Load Capacitance
CL
Output Load Resistance
RL
Power Supply Rejection Ratio
PSRR
Gain Bandwidth Product
GBW
Sink and source
VCC – 1.3
VCC – 0.2
250
5
20
90
G = 10; C1 = 1nF; RL = 20kΩ
V
µA
1
Sink and source
V
nF
kΩ
110
dB
30
kHz
Including filters for EMC
protection
Non Linearity
Slew Rate
Input Voltage Noise
10-4
G ≤ 100
SR
en
C1 = 1nF; RL = 20kΩ
0.3
Tamb, ;Rs = 1kΩ; VCC = 5V
0.8
V/µs
30
nV/√Hz
Rs = Source Impedance
analog microelectronics
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
Internet: www.analogmicro.de
Phone:
Fax:
Email:
+49 (0)6131/91 073 – 0
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[email protected]
May 2005
4/10
Rev. 2.2
Precision amplifier for bridge circuits
AM457
Error Monitoring (Diagnosis)
Output Voltage on Error (high)
VDIAG
VCC – 0.2
Output Voltage without Error (low)
VDIAG
GND+ 0.2
V
Output Current on Error
IERR
VDIAG = 2.5V
1.5
3.5
6
µA
Output Current without Error
INOERR
VDIAG = 2.5V
-6
-3.5
-1.5
µA
V
Threshold for Temperature Error Monitor- TERR
ing
105
143
°C
Threshold for Output Current (AMP) Error
Monitoring
IIA,ERR
250
800
µA
Threshold for Supply Voltage Error
Monitoring
VCC,ERR
3.8
4.3
V
Threshold for High Input Voltage (AMP)
Error Monitoring
VIN,high,ERR
VCC – 1.8
VCC – 0.8
V
Threshold for Low Input Voltage (AMP)
Error Monitoring
VIN,low,ERR
0.4
0.9
V
With regard to the sensing element
Operational Amplifier (OP)
Offset Voltage
VOS
VOS vs. Temperature
dVOS/dT
Tamb= -45…105°C
±4
±8
mV
±10
±40
µV/°C
VOS vs. Temperature
dVOS/dT
Tamb= 105…125°C
Input Bias Current
IB
±320
µV
IB vs. Temperature
dIB/dT
Input Resistance
RIN
Input Capacitance
CIN
Common Mode Input Range
CMIR
0
0.5
Open Loop Gain
G0
I0ut =0.1mA
90
110
dB
G0.2mA
I0ut =2mA
75
90
dB
80
300
nA
Tamb= -45…125°C
100
400
pA/°C
VCM / IB,typ (VCM = 0.5V)
6.25
MΩ
30
pF
0
1
V
Output Voltage Range
VOUT
Output Source Current
IOUT
Output Load Capacitance
C1
Gain Bandwidth Product
GBW
GGAIN = 10; C1 = 1nF; RL = 1.25kΩ
0.9
1.4
MHz
Slew Rate
SR
GGAIN = 100; C1 = 1nF; RL = 25kΩ
0.2
0.7
V/µs
PNP Open Collector Output
1
VCC – 0.2
V
2
mA
5
nF
Table 1: Specifications
Currents flowing into the IC are negative. Positive input of the operational amplifier (fixed internally): 0.1⋅VCC
VCM Input Common Mode Voltage
CIRCUIT
Parameter
AMP Output Capacitor
Symbol
Conditions
C1
Min.
Typ.
1
Compensation Capacitor
C2
100
DIAG Output Capacitor (optional)
C3
3.3
OP Output Capacitor (optional)
C4
Stabilization Capacitor (optional)
C5
EMV Protection Capacitor (optional)
C6
Load Resistor
RL
Table 2: Circuit Components
1
470
5
nF
pF
nF
nF
nF
pF
kΩ
Recommendation: Ceramic capacitor
analog microelectronics
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
Internet: www.analogmicro.de
Unit
5
100
20
Max.
Phone:
Fax:
Email:
+49 (0)6131/91 073 – 0
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May 2005
5/10
Rev. 2.2
Precision amplifier for bridge circuits
AM457
DESCRIPTION OF FUNCTIONS
AM457 is an integrated precision circuit for the high-impedance amplification and conditioning of
signals and monitoring of sensor bridges with low sensitivity, particularly for ceramic or DMS
sensing elements. The IC generates a rail-to-rail output signal of 0.2V to Vcc-0.2V. With suitable
dimensioning an output voltage of 0.5...4.5V can also be provided. As these values are standard the
following description refers to this output voltage.
Besides amplifying and conditioning signals the IC can also display a range of system errors
("system" in this context refers to the sensor bridge and the IC plus the voltage supply). AM457
works on the principle of ratiometry with a supply voltage of 5V ±5%.
The IC is distinguished by its low offset, extremely low thermal offset drift and its ability to cope
with a wide range of temperature, enabling it to be classified as a precision amplifier.
FUNCTIONS
AM457 is made up of three modular function blocks (see Figure 1) which can be accessed
individually or operated in suitable combinations. These are:
1. A precision amplifier (AMP) with a sink/source facility for the amplification and conditioning
of bridge signals within a range of ±5...±100mV FS including the output stage. The output stage
is protected against short circuits. The amplification and offset can be fixed using external
resistors.
2. An operational amplifier (OP) whose positive input is set internally to reference point 0.1⋅VCC.
The negative input is accessible externally, with which the OP is set as an inverter. The OP may
be used as a current source to power the bridge, for example, as an output line driver or to
record temperature when a sensing element is supplied by voltage. The latter application allows
the temperature dependency of the sensing element bridge resistors to be exploited, for example.
3. A diagnostic unit which monitors system features. With a signal source in bridge configuration
this displays the following faults and errors:
•
•
•
•
•
•
•
•
Lack of bridge voltage
Lack of bridge ground
Missing signal connection
Two missing signal connections
At least one of the four bridge resistors is of high impedance
Malfunction of the signal conditioning unit due to an AMP overload
Malfunction of the signal conditioning unit due to undervoltage
Detection of excessive temperature by the signal conditioning unit.
In the event of one or more disruptions output DIAG (pin 5) switches to a logic high (see the
electrical specifications). A low is otherwise indicated.
The precision amplifier (AMP) and additional operational amplifier (OP) can be operated
independently of one another without the diagnostic unit.
analog microelectronics
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
Internet: www.analogmicro.de
Phone:
Fax:
Email:
+49 (0)6131/91 073 – 0
+49 (0)6131/91 073 – 30
[email protected]
May 2005
6/10
Rev. 2.2
Precision amplifier for bridge circuits
AM457
EXAMPLE APPLICATION
Offset and span compensation for a Wheatstone bridge with discrete adjusting resistors.
AM457 is suitable for the amplification of all resistance bridge circuits (four-resistor bridges)
with low sensitivity.
To compensate for or calibrate the offset and span and to amplify the signal as necessary a network
was developed (see Figure 2) for a four-resistor resistance bridge circuit (such as a DMS sensing
element, for example) consisting of the resistance bridge, an AM457 IC, four resistors and a
maximum of six capacitors (see [1]). AM457's characteristics define the network and the IC
amplifies the bridge output signal to the required output values.
R4
C2
AM457
R1
7
VCC 8
OP
OUTOP
1k
C4
3
C6
2
6
IN+
IN-
AMP
Temp
9k
VCC
Diagnostic
unit
DIAG 5
OUT
R3
VDIAG
1
INOP
4
VCC=5V
C5
GND
C1
VOUT
GND
Figure 2: Schematic circuit diagram for resistance bridges with low sensitivity
The formulae used here to calibrate the offset and span of a bridge circuit are calculated using a
model which is based on the classic Wheatstone bridge circuit of four resistors.
On the basis of this model three simple equations were approximated for the setting of the resistors;
they are also suitable for calibration using a minimum of data pertinent to the sensing element and
without any need for further correction.
analog microelectronics
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
Internet: www.analogmicro.de
Phone:
Fax:
Email:
+49 (0)6131/91 073 – 0
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May 2005
7/10
Rev. 2.2
Precision amplifier for bridge circuits
AM457
The formulae used are:
R1 = R 2 =
2 ⋅R B
(kΩ)
5 ⋅d RS
Span adjustment
R1
(kΩ)
100
( −8 ⋅d RO + d RS ) ⋅R 4
(kΩ)
R3 =
8 ⋅d RO +9 ⋅d RS
R4 =
(1)
Auxiliary resistor (2)
Offset adjustment (3)
whereby the following applies to the individual sensing elements:
RB – Bridge resistance in ohms
dRS – Span/supply voltage in mV/V
dRO – Offset/supply voltage in mV/V
In order to calculate the discrete adjusting resistors R1, R2, R3 and R4 the offset, span and bridge
resistance of the sensing element to be calibrated are first determined. The sensing element is then
connected up to an AM457 to form the circuit illustrated in Figure 2. The resistors are dimensioned
and applied according to the given formulae. The values of the capacitors are selected according to
Table 1. (Capacitors C3…C6 are designated as decoupling capacitors; their use is therefore optional.)
Using the calculated resistances and the given capacitors the offset and span (0.5...4.5V) should
now be set. Discrepancies between the resistors used and the theoretical value enter the offset error
proportionally.
The calibration suggested in this article is suitable and has a sufficient accuracy for all sensing
elements
•
•
•
•
which consist of four bridge resistors
which permit calibration with discrete resistors
which have a sensitivity of ±5…±100mV FS
where the span signal is greater than the offset.
Depending on the level of accuracy required, when using sensing elements which have additional
resistors (such as ceramic sensing elements for offset correction, for example) the discrete adjusting
resistors must be corrected due to the discrepancy between the model and the sensing element (see
[2]).
analog microelectronics
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
Internet: www.analogmicro.de
Phone:
Fax:
Email:
+49 (0)6131/91 073 – 0
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May 2005
8/10
Rev. 2.2
Precision amplifier for bridge circuits
AM457
BLOCK DIAGRAM AND PINOUT
AM457
OP
7
8
VCC
1k
VCC
9k
OUTOP
5
Diagnostic unit
IN+
IN-
3
DIAG
Temp
1
AMP
2
OUT
6
4
GND
Figure 3: Block diagram of AM457
PIN
1
2
3
4
5
6
7
8
NAME
OUT
ININ+
GND
DIAG
INOP
OUTOP
VCC
EXPLANATION
OUT
Output AMP
Negative Input AMP
Positive Input AMP
IC Ground
Output Diagnosis
Negative Input OP
Output OP
Supply Voltage
ININ+
1
2
3
GND
4
8
7
6
5
VCC
OUTOP
INOP
DIAG
Figure 4: AM457 Pin out
Table 3: Pin configuration
analog microelectronics
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
Internet: www.analogmicro.de
Phone:
Fax:
Email:
+49 (0)6131/91 073 – 0
+49 (0)6131/91 073 – 30
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May 2005
9/10
Rev. 2.2
Precision amplifier for bridge circuits
AM457
DELIVERY OPTIONS
AM457 is available as:
• An SOP 8
• Dice on 5" blue foil (on request)
A1
e
D
A
w
b
L
E
H
Figure 5: SOP 8 package dimensions
FURTHER READING
www.analogmicro.de
[1]
Application notes AN1011: the AM457 amplifier IC for DMS sensing elements
[2]
Application notes AN1012: the AM457 amplifier IC for ceramic sensing elements
analog microelectronics
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
Internet: www.analogmicro.de
Phone:
Fax:
Email:
+49 (0)6131/91 073 – 0
+49 (0)6131/91 073 – 30
[email protected]
May 2005
10/10
Rev. 2.2