AM422

VOLTAGE/CURRENT CONVERTER FEATURES • Wide Supply Voltage Range: 6...35V • Wide Operating Temperature Range: –40°C...+85°C • Adjustable Voltage Reference: 4.5 to 10V • Operational Amplifier Input: 0.5...4.5V, 0...5V, other • Adjustable Offset Current • Available as Three– (0/4...20mA) or Two–Wire Version (4...20mA) • Adjustable Output Current Range • Protection Against Reverse Polarity • Protected Current Output AM422 GENERAL DESCRIPTION The AM422 is a low cost monolithic voltage– to–current converter specially designed for analog signal transmission. The AM422 is available in a 3– or 2–wire version, which allows applications with flexible input voltage ranges to be used for a standard output current. Output current range and current offset level are freely adjustable by external resistors. The IC consists of three basic sections: an operational amplifier input stage for single ended input signals (0.5–4.5V, 0–10V, or other), a programmable 4.5 to 10V reference for transducer excitation, and a current output, freely adjustable in a wide current range (4–20mA, 0–20mA, other). With the broad spectrum of possible input signals the AM422 is a flexible and multipurpose voltage–to–current converter for single ended transducers or voltage transmission. APPLICATIONS • • • • Sensor Interface Industrial Process Control Programmable Current Source Current Source Reference DELIVERY • DIL8 packages (samples) • SOP8 packages • Dice on 5“ blue foil BLOCK DIAGRAM VSET 6 SET 8 1 VREF 2 G=1 Reference Voltage VCC VREF AM422 5 V VIN 4 Input Amplifier 2 RS I 7 GND 3 OUT Figure 1 analog microelectronics Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Internet: www.analogmicro.de Phone: +49 (0)6131/91 073 – 0 Fax: +49 (0)6131/91 073 – 30 E–Mail: info@analogmicro.de April 99 1/10 Rev. 2.1 VOLTAGE/CURRENT CONVERTER ELECTRICAL SPECIFICATIONS Tamb = 25°C, VCC = 24V, VREF = 5V, IREF = 1mA (unless otherwise noted) Parameter Voltage Range Quiescent Current Temperature Specifications Operating Storage Junction Thermal Resistance Tamb Tst TJ Θja Θja Voltage Reference Voltage VREF VREF Trim Range Current VREF vs. Temperature Line Regulation VR10 IREF* dVREF/dT dVREF/dV dVREF/dV Load Regulation dVREF/dI dVREF/dI Load Capacitance Set Stage Internal Gain Input Voltage Offset Voltage VOS vs. Temperature Input Bias Current IB vs. Temperature Input Stage Internal Gain Input Voltage Offset Voltage VOS vs. Temperature Input Bias Current IB vs. Temperature GIN VIN VOS dVOS/dT IB dIB/dT 0 ±0.5 ±1.6 8 7 0.5 GIA VSET VOS dVOS/dT IB dIB/dT ISET = 4mA, R0 = 25Ω 1 2.6 ±1 ±5 8 6 CL IREF ≈ 5mA 1.9 Tamb = – 40...+85°C VCC = 6V...35V VCC = 6V...35V, IREF ≈ 5mA VSET not connected VSET = GND, VCC ≥ 11V 4.75 9.5 4.5 0 ±90 30 60 0.05 0.06 2.2 5.00 10.0 DIL8 plastic package SO8 plastic package 110 180 –40 –55 Symbol VCC ICC Tamb = – 40...+85°C, IREF = 0mA Conditions Min. 6 Typ. AM422 Max. 35 1.5 Unit V mA 85 125 150 °C °C °C °C/W °C/W 5.25 10.5 VR10 10 ±140 80 150 0.10 0.15 5.0 V V V mA ppm/°C ppm/V ppm/V %/mA %/mA µF V ±3 mV µV/°C 20 15 nA pA/°C 1.15 ±2.5 ±5 20 18 V mV µV/°C nA pA/°C analog microelectronics April 99 2/10 VOLTAGE/CURRENT CONVERTER Parameter V/I Converter Internal Gain Trim Range Voltage Range at R0 FS Offset Voltage VOS vs. Temperature Output Offset Current IOUTOS vs. Temperature Output Offset Current IOUTOS vs. Temperature Output Control Current IOUTC vs. Temperature Output Voltage Range VR0FS VOS dVOS/dT IOUTOS dIOUTOS/dT IOUTOS dIOUTOS/dT IOUTC dIOUTC/dT VOUT VOUT Output Current Range FS Output Resistance Load Capacitance Protection Functions Voltage Limitation at R0 Temperature Limitation Protection against reverse polarity Current in case of reverse polarity System Parameters Nonlinearity ideal input 0.05 VLIMR0 VLIMR0 TLIMIT Ground vs. VS vs. IOUT Ground = 35V, VS = IOUT = 0 3.8 VR0 = VIN/2, SET = VREF/2 VIN = 0, VR0 = VSET/2 – VREF/2 580 580 110 635 640 130 IOUTFS ROUT CL GVI adjustable by R0 0.75 400 1.00 1.00 500 ±2 ±7 –35 55 14 22 5 –9 0 0 20 0.5 0 1.0 Symbol Conditions Min. Typ. AM422 Max. Unit 1.25 580 ±6 ±20 –50 80 22 35 mV mV µV/°C µA nA/°C µA nA/°C µA nA/°C VCC – 6 10 V V mA MΩ 500 nF β F ≥ 100 β F ≥ 100 3–wire operation 3–wire operation 2–wire operation 2–wire operation 2–wire operation, VR0/100mV 2–wire operation VOUT = RL IOUT, VCC < 16V VOUT = RL IOUT, VCC ≥ 16V IOUT = VR0/R0, 3–wire operation 690 700 150 35 mV mV °C V mA 0.15 %FS * In 2–wire operation a maximum current of IOUTmin – ICC is valid Currents flowing into the IC are negative BOUNDARY CONDITIONS Parameter Sense Resistor Symbol R0 R0 Stabilisation Resistor R5 R5 Load Resistance Sum Offset Resistors VREF Capacitance Output Capacitance D1 Breakdown Voltage T1 Forward Current Gain RL R3 + R4 C1 C2 VBR only for 2–wire operation Conditions IOUTFS = 20mA c = 20mA/IOUTFS IOUTFS = 20mA c = 20mA/IOUTFS limitation only for 3–wire operation Min. 20 c ⋅ 20 35 c ⋅ 35 0 20 1.9 90 35 50 2.2 100 50 150 Typ. 25 c ⋅ 25 40 c ⋅ 40 Max. 29 c ⋅ 29 45 c ⋅ 45 500 200 5.0 250 Unit Ω Ω Ω Ω Ω kΩ µF nF V βF analog microelectronics April 99 3/10 VOLTAGE/CURRENT CONVERTER FUNCTIONAL DIAGRAMS Voltage Reference RA AM422 3−Wire System VS Input Amplifier VIN RIN V IOUT I RL Ground Figure 2 Voltage Reference RA 2−Wire System VS Input Amplifier VIN RIN V IOUT I RL Ground Figure 3 FUNCTIONAL DESCRIPTION The IC AM422 is an integrated voltage–to–current converter for a broad spectrum of possible single ended input signals. With variations of a few external components the output current can be adjusted over a wide range. In addition to the resistors R0 – R5 and the capacitor C1 (C2), the circuitry needs only an output transistor T1 and a diode D1. The external transistor decreases the power dissipation of the IC and the diode is protecting the transistor against reverse polarity. Typical values for the external components are listed in the Application Notes. In principle the AM422 can be used as a two–wire or as a three–wire voltage–to–current interface. The schematic of the entire system for the three–wire output is demonstrated on Figure 2. The external reference point Ground has to be identical to the ground pin 7 (GND) and the IC supply voltage becomes VCC = VS. Opposed to this, the ground pin 7 in the two–wire mode (Figure 3) (GND) analog microelectronics April 99 4/10 VOLTAGE/CURRENT CONVERTER AM422 has to be connected between the resistors R5 and RL. In this case the IC supply voltage VCC depends on the voltage VS and the load resistor RL and can be calculated: VCC = VS − IOUT ⋅ RL Basically the IC AM422 is composed of 3 functional sections as they shown in Figure 1: 1. An operational amplifier input which allows the adjustment of the output current range with the two external resistors R1 and R2. With the variation of the resistors the input voltage range and the output current range can be set. 2. A voltage controlled current output, with a wide current range is adjustable with external resistors RSET, R3 and R4. The resistors fix the output offset current, which depends on the reference voltage and corresponds to a minimal output current. The output current IOUT is supplied by the external transistor T1, driven by the output of the IC pin 3 (OUT). 3. A programmable voltage reference (VSET = N.C. or VSET = GND) can be used as an excitation for constant voltage sensors or as supply for other external devices. The output current is based on two partial currents: an adjustable offset current and a current relying on the input signal VIN. The transfer function of the AM422 is then I OUT (VSET ,VIN ) = I SET (VSET ) + I IN (VIN ) (2) (1) For the adjustment of the AM422 two cases have to be differentiated. For input voltage ranges without an offset voltage (0...5V, 0...10V) the adjustment of the output current range is as follows: The minimum output current has to be set with an input voltage VIN = 0V. The output offset current becomes I SET (VIN = 0) = V V ( R + RSET ) − R3 1 ⋅ VSET − REF  = REF ⋅ 4 R0  2 R0 2 ( R3 + R4 + RSET ) (3) With R3 = R4 and simplifications the set resistor RSET is RSET ≈ 4 R0 R4 I SET VREF VINmax  R2    2 R0  R1 + R2  (4) The output current range has to be set with the transfer function of the IC and can be calculated by ∆I OUT = I OUTmax − I SET = The relationship of R1/R2 becomes R1 VINmax = −1 R2 2 R0 ( I OUTmax − I SET ) (6) (5) The adjustment of the IC using an input offset voltage (for example 0.5...4.5V) can be described as follows. For a desired change of the output current of ∆IOUT ∆I OUT = ∆V PIN 4 2 R0 ⇒ ∆V PIN 4 = 2 R0 ∆I OUT (7) analog microelectronics April 99 5/10 VOLTAGE/CURRENT CONVERTER the input voltage VIN has to change to  R2  ∆V PIN 4 = ∆VIN    R1 + R2  With the value of the relationship of the resistors R1 and R2 R1 ∆VIN − ∆V PIN 4 = R2 ∆V PIN 4 the additional offset current can be calculated as I SET = I OUTmin − I INmin = I OUTmin − VINmin  R2    2 R0  R1 + R2  AM422 (8) (9) (10) The value of the set resistor can then be found using equation 4 In respect to the load resistor RL, the value of the supply voltage VS has to be considered with care. The following relation is generally valid (see equation 1): VS ≥ IOUTmax RL +VCCmin . (11) The resulting operating range is indicated in Figure 4. Sample calculations and typical values for the external components are listed in the Application Notes (beginning page 8). RL [Ω] V − VCCmin RL ≤ S IOUTmax VCCmin = 6V RLmax = 500Ω IOUTmax = 20mA 500 300 Operating Area 0 0 6 12 16 24 35 VS [V] Figure 4 analog microelectronics April 99 6/10 VOLTAGE/CURRENT CONVERTER PINOUT PIN NAME VCC RS OUT VIN SET VSET GND VREF AM422 DESIGNATION Supply Voltage Sense Resistor Output Voltage Input Set Voltage Reference Voltage Select IC Ground Reference Voltage Output VCC RS OUT VIN 1 2 3 4 8 7 6 5 VREF GND VSET SET 1 2 3 4 Figure 5 5 6 7 8 DELIVERY The AM422 is available in 2– (AM422–2) or 3–wire version (AM422–1). The different versions are pin compatible. The AM422 is available as: • 8 pin DIL packages (samples) • SO 8 packages • Dice on 5“ blue foil PACKAGE DIMENSIONS SOP8 4,98 ± 0,1 1,45 ± 0,1 0,2 ± 0,05 0,2 ± 0,1 0,42 ± 0,07 1,27 ≤ 0,635 ≥ 0,3 0°-10° 6,2 ± 0,2 4,0 + 0,2 - 0,1 ≤ 2,00 8 1 4 Figure 6 analog microelectronics April 99 7/10 VOLTAGE/CURRENT CONVERTER TYPICAL THREE–WIRE APPLICATION (0 – 5/10V) R3 VREF 6 8 AM422 C1 RSET 5 AM422−1 VSET VREF 2 VS R4 G=1 VOFFSET 5/10 V Reference 1 R0 V VIN 4 2 Amplifier R1 R2 7 I 3 T1 D1 R5 IOUT RL Ground Figure 7 Used in a three–wire circuit (AM422–1) ground pin 7 (GND) is connected to Ground (Figure 7). The relationship of R1/R2, using equation 2, becomes R1 VINmax = −1 R2 2 R0 ( I OUTmax − I SET ) The current ISET is used to set the output current offset and can be calculated by using equation 3 I SET (VIN = 0) = VREF ( R4 + RSET ) − R3 ⋅ R0 2 ( R3 + R4 + RSET ) With R3 = R4 the set resistor RSET becomes (equation 4) RSET ≈ 4 R0 R4 I SET VREF Example 1: Output current range 4...20mA In this case the values of the external devices (VIN = 0K5V , VREF = 5V ) are as follows R0 = 25Ω R3 = R4 = 33kΩ RSET ≈ 2.64kΩ R5 = 40Ω R1/R2 ≈ 5.25 RL = 0...500Ω C1 = 2.2µF Example 2: Output current range 0...20mA In this case the values of the external devices (VIN = 0K10V , VREF = 5V ) are as follows R0 = 25Ω R3 = R4 = 33kΩ RSET = 0Ω R5 = 40Ω R1/R2 ≈ 9 RL = 0...500Ω C1 = 2.2µF analog microelectronics April 99 8/10 VOLTAGE/CURRENT CONVERTER TYPICAL THREE–WIRE APPLICATION (0.5 – 4.5V) C1 R3 RSET 5 AM422 VREF 6 8 AM422−1 VSET VREF 2 R4 Sensor 0.5−4.5V G=1 VOFFSET VS 5/10 V Reference 1 R0 V VIN 4 2 Amplifier R1 R2 7 I 3 T1 D1 R5 IOUT RL Ground Figure 8 Example 3: Output current range IOUT = 4...20mA, input voltage range VIN = 0.5...4.5V The transfer function of the output current IOUT is (equation 2) V  R2  I OUT = I SET + I IN = I SET + IN   2 R0  R1 + R2  For the 0.5 to 4.5V application (Figure 8) a change of the input voltage from ∆VIN = 4V should be displayed over a change of the output current ∆IOUT = 16mA. With the voltage change at pin 4 (∆VPIN4 = 800mV, equation 7) the relationship of R1/R2, using equation 8, becomes then ∆V PIN 4 R2 800mV = = ∆VIN R1 + R2 4V With that relation an offset current ISET from I SET = I OUTmin − I INmin = I OUTmin − 2mA = 2mA has to be adjusted (equation 10). With R3 = R4 the set resistor RSET can be calculated (equation 4) RSET ≈ 4 R0 R4 I SET VREF R3 = R4 = 33kΩ R1 ≈ 68kΩ RSET ≈ 1.32kΩ R2 ≈ 18kΩ R5 = 40Ω RL = 0...500Ω ⇒ R1 ∆VIN − ∆V PIN 4 = =4 ∆V PIN 4 R2 The values of the external devices than can be calculated as follows (VREF = 5V ) R0 = 25Ω R1/R2 ≈ 4 C1 = 2.2µF analog microelectronics April 99 9/10 VOLTAGE/CURRENT CONVERTER TYPICAL TWO–WIRE APPLICATION (0 – 1V) R3 VREF 6 8 AM422 C1 RSET 5 AM422−2 VSET VREF 2 VS R4 G=1 VOFFSET 5/10 V Reference 1 R0 C2 V VIN 4 2 Amplifier R1 R2 7 I 3 T1 D1 R5 IOUT RL Ground Figure 9 Used in a two–wire circuit (AM422–2) ground pin 7 (GND, ⊥) is connected between R5 and the load resistor (Figure 9). The relationship R1/R2, using equation 2, becomes R1 VINmax = −1 R2 2 R0 ( I OUTmax − I SET ) The current ISET is used to set the output current offset and can be calculated by using equation 3 I SET (VIN = 0) = VREF ( R4 + RSET ) − R3 ⋅ R0 2 ( R3 + R4 + RSET ) With R3 = R4 the set resistor RSET becomes (equation 4) RSET ≈ 4 R0 R4 I SET VREF Example 4: Output current range 4...20mA In this case the values of the external devices (VIN = 0...1V) are as follows R0 = 25Ω R3 = R4 = 33kΩ RSET ≈ 2.64kΩ R5 = 40Ω R1/R2 ≈ 0.25 RL = 0...500Ω C1 = 2.2µF C2 = 100nF The information provided herein is believed to be reliable; however, Analog Microelectronics assumes no responsibility for inaccuracies or omissions. Analog Microelectronics assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user's own risk. Prices and specifications are subject to change without notice. No patent rights or licences to any of the circuits described herein are implied or granted to any third party. Analog Microelectronics does not authorise or warrant any Analog Microelectronics product use in life support devices and/or systems. analog microelectronics April 99 10/10