【Shishuo Design】What?! The error of the 4mA to 20mA current loop is less than 0.2%?!
Precision matched resistors provide accurate voltage-to-current conversion
This circuit uses a two-stage amplifier that takes advantage of the unique matching characteristics of the LT5400. Next, we will analyze these two stages of amplifiers in detail.
The output (usually from a DAC), typically 1V to 5V, is applied to the noninverting input of op amp IC1A. This voltage sets the current through R1 to exactly VIN/R1 via FET Q2. The same current is pulled down through R2, so the voltage at the bottom of R2 is the 24V loop supply voltage minus the input voltage. This portion of the circuit has three major sources of error: the matching of R1 and R2, the offset voltage of IC1A, and the leakage current of Q2.
The exact values of R1 and R2 are not critical, but they must be accurately matched to each other. The LT5400A-grade version achieves this with an error of ±0.01%; the LT1490A has an offset voltage of less than 700V between 0°C and 70°C.
This voltage produces an error of 0.07% at an input voltage of 1V. The leakage current of the NDS7002A is 10nA, although it is typically much smaller. This leakage current represents an error of 0.001%.
The voltage across R3 is kept equal to the voltage across R2 by pulling current through Q1. Because the voltage across R2 is equal to the input voltage, the current through Q1 is exactly the input voltage divided by R3. By connecting a precise 250Ω shunt resistor in parallel with R3, this current will accurately track the input voltage. The error sources for this stage are: the value of R3, the offset voltage of IC1 R, and the leakage current of Q1.
Resistor R3 sets the output current directly, so its value is critical to the accuracy of the circuit. This circuit uses a commonly used 250Ω shunt resistor to complete the current loop. The Riedon SF-2 device in the figure has an initial accuracy of 0.1%, and the temperature drift is very low. Similar to the first stage, the offset voltage produces no more than 0.07% error. The leakage current of Q1 is less than 100nA, and the maximum error produced is 0.0025%.
Without any trimming, the total output error is better than 0.2%. The current sense resistor R3 is the dominant error source. If a higher quality device (such as a Vishay PLT series device) is used, an accuracy of 0.1% can be achieved. The current loop output is subject to considerable stress in use. Diodes D1 and D2 from the output to the 24V loop supply and ground help protect Q1; R6 provides some isolation. Higher isolation can be achieved by increasing the value of R6 at the expense of some conditioned voltage at the output. If the maximum output voltage requirement is less than 10V, then the value of R6 can be increased to 100Ω to provide higher isolation from output stress. If the design requires increased protection, a transient voltage suppressor can be added to the output, of course, this will result in some loss of output accuracy due to leakage current.
This design uses only two of the four matched resistors in the LT5400 package. The other two resistors can be used for other circuit functions, such as a precision inverter, or another 4mA to 20mA converter. In addition, additional resistors can be introduced in parallel with R1 and R2. This approach reduces the statistical error introduced by the resistors by a factor of the square root of two.
LT5400
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Excellent matching performance
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Grade A: 0.01% matching accuracy
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Grade B: 0.025% matching accuracy
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0.2ppm/°C Matched Temperature Drift
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±75V operating voltage (±80V absolute maximum)
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8ppm/°C Absolute Resistance Temperature Drift
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Long term stability: < 2ppm (at 2000 hours)
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–55°C to 150°C operating temperature range
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8-pin MSOP package
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