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: [email protected] July 2008 - Rev 1.0 - Page 1/8 Application notes AN1015 AM462 – How to convert a 0...5V input signal into a 4...20mA 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 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: [email protected] July 2008 - Rev 1.0 - Page 2/8 Application notes AN1015 AM462 – How to convert a 0...5V input signal into a 4...20mA 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: 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: [email protected] July 2008 - Rev 1.0 - Page 3/8 Application notes AN1015 AM462 – How to convert a 0...5V input signal into a 4...20mA 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): 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: [email protected] July 2008 - Rev 1.0 - Page 4/8 Application notes AN1015 AM462 – How to convert a 0...5V input signal into a 4...20mA 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 Systemground: Ground 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 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: [email protected] July 2008 - Rev 1.0 - Page 5/8 Application notes AN1015 AM462 – How to convert a 0...5V input signal into a 4...20mA 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 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: [email protected] July 2008 - Rev 1.0 - Page 6/8 Application notes AN1015 AM462 – How to convert a 0...5V input signal into a 4...20mA 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 } Systemground: Ground Different potentials! GND R5 - Voltage supply Ground IOUT Figure 8: AM462 with protective circuitry 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: [email protected] July 2008 - Rev 1.0 - Page 7/8 Application notes AN1015 AM462 – How to convert a 0...5V input signal into a 4...20mA 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 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: [email protected] July 2008 - Rev 1.0 - Page 8/8
© Copyright 2024