Analog Design Journal Signal Chain High-side current sources for industrial applications By Ahmed Noeman System Engineer, Industrial Systems, Freising Introduction Table 1. Current-source performance specifications Parameter Applicability The use of current sources in industrial applications is widespread because the current-based signal provides higher noise immunity than voltage-based signals in a harsh industrial environment. Current is used to carry information signals in 4–20-mA loops and also for excitation of passive sensors like resistors. High-side current sources are generally trickier to design compared to lowside current sinks. This article introduces and compares different topologies used to implement a high-side current source for industrial applications, and includes evaluation of performance metrics for each topology. Unit Minimum and maximum possible output current mA Compliance range Minimum and maximum output voltage at which current is valid V Maximum supply Maximum acceptable supply V Inaccuracy before carrying any calibration % Temperature drift Error due to temperature change (range >100°C) ppm/°C Load regulation Change in output current vs. change in output voltage Output noise Total output noise over certain bandwidth nA PSRR Power supply rejection ratio dB Accuracy Initial accuracy Current source parameters and characterization Bandwidth Table 1 shows current-source performance metrics, their definitions, and measurement units. Although it is possible to eliminate initial inaccuracies through calibration, it is not possible to compensate for temperature drifts and load regulation; thus, those drifts will determine the overall accuracy of the current source. If the current source is powering a sensor, the power supply rejection ratio (PSRR) and output impedance will determine the maximum bandwidth at which the current source is working. This is because the PSRR and output impedance deteriorate with increasing frequency and add error components to the measured signal. In addition to static performance, dynamic performance must also be considered to ensure the stability of the current source against perturbation, especially if the current source will switch during operation or the load voltage will experience a step transient. The dynamic performance can be checked by testing the settling time upon step inputs. Although some of the parameters in Table 1 can only be predicted through calculations of component parametrics, it is also possible to verify many of them through simulation if component models are available. Simulation is a pretty easy step for verifying a specification within a given set of conditions. Figure 1 shows a conceptual current source with supply-voltage, output-voltage and control-voltage sources. The majority of current source parameters can be Texas Instruments Definition Output current range (IOUT) Output impedance AC output impedance seen from (ZO) the output node Power efficiency Dynamic %/ V Settling time: • Load step • Control step • Supply ramp Ω Output current divided by total current consumed by source % Settling time (of the output current) and stability upon a step in the load voltage or a step of the control input (configurable) ns Figure 1. Conceptual current source with different voltage sources VSUP + – VCC VCTRL ZO VC + – IOUT VOUT + – 1 VO ADJ 2Q 2019 Analog Design Journal Signal Chain A shunt regulator and a MOSFET verified by conducting various sweeps of those supplies, which are summarized in Table 2. Precision shunt regulators are among the most popular options for obtaining a voltage reference (VREF). Providing the shunt regulator with a minimum current (IQ) will ensure that the VREF is applied on the set resistor (RSET) as shown in Figure 2, where the circuit implementation uses the TLV431BQ regulator. The parameters calculation and simulation results are presented in Table 3. The output current is calculated as IOUT = VREF/RSET. This circuit provides a cost-effective solution with a reasonable error that is 1.2% (0.8% + 0.4%) over the temperature range, and a moderate initial accuracy of 0.7% that can be compensated with resistor trimming. This circuit is suitable for applications with 7- to 8-bit resolution. The 100-Ω resistors and 100-nF capacitor around U1 are necessary for circuit stability. This circuit can work with a high supply voltage with only some limited dynamic performance. Table 2. Current-source performance simulation or prediction Parameter Simulation? Prediction/Verification (Conditions) Yes Typically calculated from component parameters, but can be simulated later For programmable sources: DC transfer function; VC swept from 0 to VC_max Compliance range and load regulation Yes DC transfer function analysis: VO swept from 0 to VCC (fixed VCC and VC) IOUT change is regulation; limits of VO at which IOUT exceeds variation is compliance Maximum supply No Typically not modeled; calculated from the maximum supply limit in the data sheet Initial accuracy No Requires accurate models and statistical analysis; generally more convenient to calculate Temperature drift No Not commonly supported with models; requires calculation Yes Noise analysis: One AC source out of the three supplies (proper DC value for VCC, VC, VO) IOUT total noise at specific relevant frequency (fC) represents system bandwidth Output current range Output noise PSRR Yes Figure 2. Current source using a shunt regulator VCC IQ 100 Ω M1 AC transfer function analysis: AC source at VCC; IOUT (AC signal) in decibels is PSRR 100 nF U1 Output impedance Yes AC transfer function analysis: AC source at VO; 1/IOUT (AC) linear is ZO vs. frequency RSET Power efficiency Yes DC operating point analysis: IOUT/ICC (for specific VCC, VO and VC) VOUT Yes Transient analysis: • Supply ramp: VCC ramped from zero; IOUT settling time and behavior is monitored • VC step: VC unit step (minimum to maximum); IOUT settling to final value • VO step: VO unit step over compliance range limits; IOUT settling to final value Settling time • Load step • Control step • Supply ramp RLIM 100 Ω VREF M1 U1 VREF RLIM RSET IOUT VCC VOUT IQ CSD18541F5 TLV431BQ 1.24 V 604 kΩ, 1% 124 Ω, 0.1% 10.01 mA 24 V 12 V 14 µA IOUT Table 3. Current-source specifications using a shunt regulator Parameter Calculation and/or Simulation Results Output current range Set by M1 max IDS, and M1 max power; IOUT_max = Pmax_M1/Vdsmax_M1 = 0.5/23 = 21 mA At lower IOUT, the initial error due to IQ becomes significant The next step is to examine some topologies for current sources. To establish a basis for useful comparison, the application is limited to a high-side current source that can work off a 24-V industrial supply, assume a 10-mA output and 10-kHz bandwidth for noise, calculate the output impedance at DC and 1 kHz, and assume a midsupply (12 V) as the default output voltage. Compliance range VOUT_max = VCC – (VREF + VGSTH_M1 + IQ × RLIM) = 24 – (1.24 + 1.75 + 1.8) ≈ 20 V Maximum supply Vmax_M1 = 60 V, then IOUT_max = 8 mA Initial accuracy (1 + ∆VREF)/(1 – ∆RSET) + IQ/IOUT = (1 + 0.005)/0.999 + 0.001 = 0.7% Temperature drift ∆VREF/VREF = 11 mV/1.24 V = 0.8% over temperature range Constant-current sources Load regulation 0.4% over compliance range, or 2 µA/V Output noise 5.2 nA over 10 kHz PSRR –75 dB at 10 kHz Output impedance 588 kΩ at DC, 46.5 kΩ at 1 kHz Power efficiency 100% Settling time Supply ramp: 114 µs (with large overshoot) Load step (9 V): 700 µs (with large undershoot) Many applications require a constant-current source that is stable over supply drift, temperature drift, and output variations. The basic principle is to use an accurate voltage reference applied over a precision resistor to create an accurate current. Texas Instruments 2 ADJ 2Q 2019 Analog Design Journal Signal Chain Shunt regulator and op amp Table 4. Current-source specifications using a shunt regulator and op amp It is possible to eliminate some of the drawbacks of the circuit shown in Figure 2. Figure 3 shows a current source that uses a shunt regulator and an operational amplifier (op amp) to buffer the voltage reference. In this circuit, IOUT = VREF/RSET and the 80-kΩ resistor ensures that the shunt regulator gets the minimum required current to turn on. This circuit can achieve a wide compliance range, veryhigh PSRR and ZO, and excellent load regulation. The op-amp offset drift and reference drift contribute directly to the overall accuracy. This topology can achieve very high accuracy when using precision components. Table 4 lists the performance metrics. Parameter Figure 3. Current source using a shunt regulator and op amp 10 nF VREF U1 IQ1 VCC 80 kΩ IQ2 – U2 + RSET IOUT Texas Instruments U1 U2 VREF RSET IOUT VCC VOUT TLV431BQ OPA187 1.24 V 124 Ω, 0.1% 10.0 mA 24 V 12 V Calculation and/or Simulation Results Output current range Set by U2 max IOUT and U2 max power; IOUT_max < 30 mA At lower IOUT, the relative error becomes significant Compliance range VOUT_max = VCC – VREF – VOUT_max_U2 = 24 – 1.24 – 0.5 ≈ 22.25 V Simulation results 21.25 V VOUT_min ≈ 0.25 V (IQ goes below limit for U1 to function) Maximum supply Vmax_U2 = 36 V Initial accuracy (1 + ∆VREF + VOS_U2)/(1 – ∆RSET) = (1 + 0.005 + 0.00001)/0.999 = 1% Temperature drift ∆VREF + ∆VOS_U2/VREF = (11 mV + 5 µV)/1.24 V = 0.9% over temperature range Load regulation 0% over compliance range Output noise 16.4 nA over 10 kHz PSRR –95 dB at 10 kHz Output impedance 6.5 MΩ at DC, 76 kΩ at 1 kHz Power efficiency IOUT/ICC = IOUT/(IOUT + IQ1 + IQ2) = 10/10.25 = 97% Settling time Supply ramp: 800 µs (with some overshoot) Load step (12 V): 250 µs (with undershoot) VOUT 3 ADJ 2Q 2019 Analog Design Journal Signal Chain Programmable current sources Table 5. Modified Howland current source specifications Parameter To complete the current-source evaluation, there are two topologies for implementing a programmable high-side current source with the control voltage referenced to ground. They are the modified Howland circuit and cascaded op amps. Modified Howland circuit As shown in Figure 4, a Howland circuit has been modified to have a buffer in the feedback loop. The output current of this circuit is calculated with Equation 1. IOUT AV = × VC RSET Calculation and/or Simulation Results Output current range Set by U1 max IOUT and max power, IOUT_max < 60 mA At lower IOUT, the relative error increases Compliance range VOUT_max is set by VIN_U2 and VOUT_max_U1 Simulation : 22.4-V VOUT_min is determined by U1 minimum input (practical value = 0.2 V) Maximum supply Vmax_U1 = 36 V Initial accuracy [1 + VOS_U1/VC + 2 × VOS_U2 /VC]/(1 – ∆RSET) = (excluding VC) (1 + 0.00002 + 0.00005)/0.9999 ≈ 0.02% (1) Temperature drift where the AV gain is equal to R2/R1. For the device chosen (the INA592), the gain equals 0.5, resulting in a 5-mA/V conversion gain. This circuit is quite interesting, as it is capable of driving bipolar (sink or source) current. The output impedance is proportional to the mismatch of R1s and R2s; implementing the circuit with a precision difference amplifier ensures the best matching for resistors. The circuit performance is sensitive to source resistance, as the input impedance equals R1||R2 = 4 kΩ in the given circuit. As shown in Tabel 5, this circuit offers very good accuracy with a wide compliance range and excellent dynamic performance. The trade-off is a slight increase in noise and lower efficiency. ∆VOS_U1/VC + 2 × ∆VOS_U2 /VC = 40 µ + 250 µ = 0.03% over temperature Load regulation 0.1% over compliance range Output noise 17.8 nA over 10 kHz PSRR –60 dB at 10 kHz Output impedance 3 MΩ at DC, 109 kΩ at 1 kHz Power efficiency IOUT/ICC = IOUT/(IOUT + IQ1 + IQ2) = 10/12.4 = 80% Settling time Supply ramp: 6.8 µs (no overshoot) Load step (12 V): Large undershoot during transition Output current step (10 mA): Instant settling Figure 4. Modified Howland current source using a difference amplifier VCC IQ1 IN– SENSE R1 R2 – VC OUT U1 RSET IQ2 + + – R1 IN+ R2 VCC + U2 – REF IOUT VOUT Texas Instruments 4 U1 U2 R1 R2 RSET VC IOUT VCC VOUT INA592 OPA196 12 kΩ 6 kΩ 100 Ω, 0.01% 2V 10.0 mA 24 V 12 V ADJ 2Q 2019 Analog Design Journal Signal Chain Cascaded op amps Table 6. Current-source specifications using cascaded op amps The topology shown in Figure 5 uses a buffer to copy the control voltage (VC) over to V1, applied on R1 and resulting in I1 current. Then, another buffer copies the V2 voltage to V3 over R3, which establishes the output current level. The output current is calculated with Equation 2. Parameter Calculation and/or Simulation Results Set by M2 max power, IOUT_max = Pmax_M2/VDS_max_M2 = 2.3/22 ≈100 mA Note that higher current requires lower R1,R2 Lower IOUT is set by U2IN_max (for this case, 0 mA) Higher current reduces compliance range Output current range R 2 × VC (2) R1 × R3 which results in a 5-mA/V conversion gain. I1 is set to be 0.2 × IOUT. A lower I1 current means lower power but higher noise. Error analysis shows that equal offsets of U1 and U2 will eliminate the op-amp offset error. Using the dual op-amp package ensures offset tracking between the two op amps. Feedback capacitors and output resistors (200 ohms for U1 and 300 ohms for U2) are necessary to maintain circuit stability. The trade-off here is between current range and compliance range. For a 1:20 current range, voltage V3 will vary with the same ratio, so to maintain a few volts over R3 requires that U2 accept a close-to-supply input at a low current. This circuit offers a wide current range, a wide compliance range, excellent accuracy, low noise, and very high output impedance, along with great dynamic performance as shown in Table 6. IOUT = VOUT_max is set by U2IN max rather than U1OUT max, VCC – VIN_max_U2 = 22 Compliance range VOUT_min is determined by U2 minimum input, practically value = 0.0 V Maximum supply Vmax_U1 = 36 V Initial accuracy (excluding VC) (1 + ∆R2)/(1 – ∆R1)/(1 – ∆R3) = 0.03% for 0.01% resistors, or 0.3% for 0.1% resistors Temperature drift By tracking offsets, there is only resistor drift variation Load regulation 0% over compliance range Output noise 9.7 nA over 10 kHz PSRR –77 dB at 10 kHz Output impedance 142 MΩ at DC, 1.35 MΩ at 1 kHz Power efficiency IOUT/ICC = IOUT/(IOUT + I1 + IQ1 + IQ2) = 10/14 = 71% Settling time Supply ramp: 180 µs (with no overshoot) Load step (12 V): no settling observed Output current step (10 mA): 140 µs with no overshoot (depends on feedback RC) Figure 5. Current source using cascaded op amps VCC VCC I1 VCC R3 R2 10 kΩ IQ1 V3 V2 2.2 nF + U1 M1 – 200 Ω VC + – VCC IQ2 1 nF – V1 10 kΩ 330 Ω U2 + M2 IOUT R1 U1,U2 R1 R2 R3 VC IOUT I1 VCC VOUT M1 M2 OPA2192 1 kΩ, 0.01% 1 kΩ, 0.01% 200 Ω, 0.01% 2V 10.0 mA 2 mA 24 V 12 V CSD18541F5 NTF2955 VOUT References It is worth noting that there are many other topologies for current sources that vary in performance and applications, including floating voltage regulators, current drivers like TI’s XTR300, current-output digital-to-analog converters like the DAC7760, and of course, current references like the REF200. 1. Linden T. Harrison, “Current Sources and Voltage References,” Newnes, 2005. 2. Collin Wells and David F. Chan, “High-Side Voltage-toCurrent (V-I) Converter,” Texas Instruments Precision Designs: Verified Design (SLAU502), June 2013. Conclusion Related Web sites There are different performance metrics for industrial current sources and the various topologies require different evaluation and calculation methods. This article compared the performance of four different topologies used to implement industrial current sources. Texas Instruments Product information: OPA187, INA592, OPA196, OPA2192, XTR300 DAC7760, REF200 5 ADJ 2Q 2019 Analog Design Journal TI Worldwide Technical Support TI Support Thank you for your business. 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