Application notes AN1015 current loop output

Transcription

Application notes AN1015
AM462 – How to convert a 0...5V input signal into a 4...20mA
current loop output
The task on hand
An input signal of 0 to 5V is to be converted with sufficient accuracy (<0,5%FS) into an output
current of 4 to 20mA for 2-wire application (current loop).
The following application notes aim to show how a practicable circuit based on AM462 can be
dimensioned and which points must be taken into account in order to gain the required accuracy.
Following a general outline of the current loop circuit and the dimensional formulas the level of
accuracy achieved in the output signal (offset and span) shall be described with reference to
various example measurement curves.
Application description AN1015 also applies without restriction to the current output of AM460
and AM452.
Analog interface
Despite the large number of industrial buses now available most industrial measurement technology
applications require that the measurement signal be converted into a suitable standard analog signal
for further processing (signal transmission). The reasons for this lie in the simple handling of signal
transmission, in the technical limitations of digital buses and/or in the considerable economic
advantages of analog solutions.
In principle both voltage and current signals are suitable for analog signal transmission. Over the
years, however, a quasi-standard has become widely accepted which has a 'default' setting of
0...5(10)V for voltage signals and 0(4)...20mA for current signals. In the industrial environment we
thus come across plenty of measuring instrumentation with the relevant output values. For
adaptation purposes these must often be able to convert voltage into current and vice versa.
Translating a current signal into a voltage signal calls for little more than the application of Ohm's
law. If a voltage signal is to be converted into current, however, unexpected difficulties often arise.
If electromagnetic perturbation is to be reckoned with (from welding apparatus, electric motors or
external transmitters, for example) and/or if long cables are needed, 4...20mA current transmission
is without doubt the most advantageous form of information transfer [1]. A 0...5V signal source is
often provided which has to be converted into an adequate current.
The following explains how this can be realized using the AM462 IC.
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
Phone: +49 (0)6131/91 0730-0
Fax:
+49 (0)6131/91 073-30
Internet: http://www.analogmicro.de
Email: info@analogmicro.de
July 2008 - Rev 1.0 - Page 1/8
current loop output
Current loop transmission (e.g. 4...20mA)
V S= 11...35V
0..5V
Output
AM462
0(4)...20mA
Figure 1: Task on hand
In industrial practice there is a difference between 2- and 3-wire current transmission. Due to its
greater significance the following shall describe the 2-wire version for 4...20mA, also known as a
current loop. The difference between both methods is outlined in Figures 2 and 3.
4...20 mA
Sensing
element
Voltage
supply
Current
source
Transducer
RL
Voltage
measurement
Ground
Figure 2: Industrial 2-wire application
Sensing
element
Current
source
Voltage
supply
4...20 mA
RL
Voltage
measurement
Ground
Figure 3: Industrial 3-wire application
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July 2008 - Rev 1.0 - Page 2/8
current loop output
Description of AM462
AM462 [2] is a modular V/I transducer IC which has been specially designed for the conversion of
voltage signals referenced to ground into current signals. It is suitable for both 2- and 3-wire
operation (see Figures 2 and 3). AM462's various functions are described below in Figure 4 which
also shows the few external components which are needed to operate AM462.
C1
1
R3
15
13
R4
VS
16
AM462
2
OP2
G SET
Voltage reference
VBG
VINP
V/I Converter
G VI
10
9
T1
D1
8
3
OP1
4
Input voltage
referenced to ground
2-wireconnection
5
R2
R1
C2
R0
11
VOUTAD
6
14
VINDAI
}
IC-ground: GND
Systemground: Ground
Different
potentials
Connections which set unused components to a defined operating point
R5
IOUT
RL
GND
Ground
Figure 4: Schematic circuit diagram for 2-wire operation with AM462
AM462 consists of several individually accessible functional modules (OPs, V/I converter and a
reference) which can be connected up externally or operated separately.
1. Operational amplifier OP1 enables a positive voltage signal in a range of 0…VCC-5V to be
amplified. OP1 gain GGAIN can be adjusted using external resistors R1 and R2. An overvoltage
protector is integrated to safeguard the device; this limits OP1's output voltage to the set value of
the reference voltage. The OP1 transfer function is calculated as follows:
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July 2008 - Rev 1.0 - Page 3/8
current loop output
VOUTAD = VINP ⋅ GGAIN with GGAIN = 1 +
R1
R2
(1)
where VINP is the input voltage at OP1.
2. The internal voltage-to-current converter (V/I converter) provides a voltage-controlled current
signal at the IC output which is activated by external transistor T1. This generates the actual
output current IOUT. For power dissipation purposes the transistor is connected externally and is
also protected against reverse polarity by an additional diode D1. An offset current of ISET can be
set using pin 16 (SET) with the aid of the internal voltage reference and an external voltage
divider, for example, as shown in Figure 4. This has an effect on the output signal. External
sensing resistor R0 permits the output current to be finely adjusted. The following applies to
output current IOUT:
V
⋅G
V ⋅G
(2)/(3)
I OUT = INDAI VI + I SET with I SET = SET SET
R0
R0
where VINDAI is the voltage at pin 6 (INDAI) and VSET the voltage at pin 16 (SET).
3. AM462's reference voltage source permits external components, such as sensors,
microprocessors and the like, to be supplied with voltage. The value of reference voltage VREF
can be set to between 5V and 10V using pin 13 (VSET).
4. The additional operational amplifier stage OP2 can be used as a current or voltage source to
power external components. The positive OP2 input is connected up internally to bandgap
voltage VBG so that the output current or voltage can be adjusted across a wide range with the
help of one or two external resistors.
Dimensioning
In order to convert the 0…5V input signal into a 4..20mA output current OP1 is required to act as a
high impedance input with GGAIN = 1. The V/I converter is also needed to activate the external
transistor and the reference to adjust the offset (4mA).
Using these modules of the AM462 and just a few external components the circuit illustrated in
Figure 5 can be produced.
Based on equations (1) and (2)/(3) the transfer function for this circuit is expressed as:
I OUT = VINP ⋅
GGAIN ⋅ GVI
G
+ I SET = V INP ⋅ GAIN + I SET
8 R0
R0
(4)
with I SET =
VREF ⋅ G SET
R4
V ⋅
R4
⋅
= REF ⋅
R0
R3 + R4
2 R0 R3 + R4
(5)
With ∆I OUT = 16mA, ∆VINP = 5V, GGAIN = 1 and ISET = 0, the following is accrued from (4):
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current loop output
GGAIN
→ R0 = 39Ω
(6)
8 R0
With the calculated value for R0, with VRef = 10V and ISET = 4mA and taking the boundary
conditions (See [2]) into account: R3 + R4 ≥20 kΩ ≤200kΩ and using formula (5) resistors R3 and
R4 can be calculated. It is prudent here to observe the values of the E12 series and to avoid an
unnecessary current load of the reference by selecting a suitably high impedance voltage divider
consisting of R3 and R4. Observing the secondary condition R3 + R4 = 84.7kΩ the following is
calculated:
∆I OUT = ∆VINP ⋅
R3 = 82kΩ and R4 = 2.7kΩ,
with which both the offset and span are now calibrated.
.
Measurements
Condition ISET = 4mA ensures that the V/I converter output triggers the external transistor in such a
way that with a signal input of 0V an output current IOUT of 4mA is achieved. Here it is prerequisite
that the power supply of the IC and the current flowing from the reference and from OP2 (if in use)
is by no means greater than 4mA. This particularly applies when taking the connected signal source
into consideration. In 2-wire operation "connected" means that the signal source is powered by the
reference and must be connected up to the IC ground (GND) together with AM462 (see Figure 5).
C1
AM462
1
R4
R3
15
13
GS ET
2
OP2
Voltage reference
10 V
VBG
0 ... 5 V
VINP
VS
16
R0
11
V/I Converter
GVI
10
9
T1
8
3
D1
OP1
5
4
C2
6
IC-ground: GND
R5
14
}
Different
potentials!
GND
IOUT
RL
Ground
Figure 5: Circuit for converting a 0...5V input signal into an output
current of 4..20mA in 2-wire mode
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July 2008 - Rev 1.0 - Page 5/8
current loop output
A secondary condition of the dimensioning setup suggested herein is that the reference be set to
10V, with pin 13 (VSET) connected to the IC ground (GND). This results in AM462 needing a
supply voltage of ≥11V in this circuitry.
Output current Iout dependent on the input signal 0...5V
5
Vin/V
4
3
2
1
0
2
4
6
8
10
12
Iout/mA
14
16
20
18
22
Figure 6: Output current as a function of the
input voltage
Figure 6 gives a typical measurement curve of IOUT = f(VINP) @ RT and RL = 500Ω RL. In order to
accentuate the setting or calibration error, this has been referenced to the full scale output signal
(20mA) and is depicted in Figure 7.
Perc entage of er ror at RT re ferenced to full sc ale
0,40
Error /%FS
0,35
0,30
0,25
0,20
0,15
2
4
6
8
10
12
14
16
18
20
22
Iout/mA
Figure 7: Calibration error referenced to the full
scale signal (FS = 20mA
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July 2008 - Rev 1.0 - Page 6/8
current loop output
From the dimensioning condition equation (4) we can see that R0 is responsible for the gain and
from equation (5) that the voltage divider consisting of R3 and R4 is responsible for the offset.
Compared to the two adjusting resistors R3 and R4 the influence of resistor R0 (in relation to the full
scale value) on the accuracy of output current IOUT is larger by a factor of 5 (20mA/4mA). This
means that for RO a high precision resistor should be chosen which is as temperature resistant as
possible.
From Figure 7 we can see, however, that the largest error percentage is to be found around the
offset. This error results not from the calibration of the offset by resistors R3 and R4 but is rooted in
the offset voltages of OP1 and the V/I converter which most markedly come to light at low input
voltages.
A calibration error of ca. 0.25%FS at RT can be accrued from the measurement curve in
Figure 7.
Practical implementation
Figure 8 shows a circuit with a diode bridge which permits reverse polarity of the supply voltage
and a diode which protects the circuit against an overvoltage pulse. It must be noted that the voltage
drop across the diode bridge (ca. 1.4V) must be taken into account in the system supply VS.
Together with the line resistors capacitor C1 acts as a low-pass filter which guards the back-end
circuitry against transient perturbations.
C1
AM462
2
OP2
VBG
0 ... 5 V
VINP
1
R3
15
13
R4
VS
16
R0
11
G SET
Voltage reference
10 V
V/I Converter
G VI
10
9
T1
8
C2
3
OP1
4
D1
+
RL
5
IC-ground: GND
6
14
}
Different
potentials!
GND
R5
-
Voltage
supply
Ground
IOUT
Figure 8: AM462 with protective circuitry
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July 2008 - Rev 1.0 - Page 7/8
current loop output
Comments
Input voltage VINP could also be directly connected up to the input of the V/I converter pin 6
(INDAI, see Figure 5). In doing so the offset and offset drift of OP1 would be circumvented and the
calibration error reduced. However, the circuitry has been consciously built around OP1 for two
main reasons. On the one hand its internal high impedance input resistors do not burden the signal
source. On the other hand, with OP1 acting as an impedance converter, the effect of the input
resistor (integrated voltage divider 1/8) on the V/I converter is neutralized.
In the case of the signal input at pin 6 (INDAI) the V/I converter input resistor would have to be
considered to have 110…200kΩ, plus a temperature effect of 0.2%K. This causes additional errors
with signal sources with a large internal resistance. The low impedance output of OP1 enables this
error to be avoided.
Conclusion
The above application notes AN1015 have tackled the practical example of how a signal of 0...5V
can be converted into a current for 2-wire operation. A suitable circuit was dimensioned and used to
show how an input voltage can be translated into a current of 4...20mA with an accuracy of <1%FS
without the need for complex assembly or compensation. Measurement curves indicate the scale of
the calibration errors and which typical degrees of accuracy were achieved for this specific
application.
The back end of AM462 has been configured in such a way that it is protected against industrial
disturbance factors and can be operated regardless of the polarity of the power supply.
Further reading
www.analogmicro.de
[1] - PR1012. AM462: Voltage-to-current converter IC for 2-wire current loop applications
[2] - AM462 Data sheet
Phone: +49 (0)6131/91 0730-0
Fax:
+49 (0)6131/91 073-30
July 2008 - Rev 1.0 - Page 8/8

Application notes AN1015 current loop output

Transcription

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Keywords: op amp, high-voltage op amp, operational amplifier, high voltage operational... converter, current-loop system, 4-20mA