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LTC2990IMSTRPBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC2990IMSTRPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 24 page  LTC2990 0 2990f applicaTions inForMaTion target application, it is beneficial to configure the LTC2990 for Kelvin coded results to limit the number of math opera- tions required in the target processor. T Unsigned T K COMP ACT CAL K MEAS _ _ = ( ) η η 2 2 15 15 (5) T Unsigned T C COMP ACT CAL C MEAS _ _ . = ( ) + η η 2 273 15 115 2 2 273 15 2 4 15 4 • – . • ( ) (6) Sampling Currents Single-ended voltage measurements are directly sampled by the internal ADC. The average ADC input current is a function of the input applied voltage as follows: IIN(AVG) = (VIN – 1.49) • 0.17µA Inputs with source resistance less than 200Ω will yield full-scale gain errors due to source impedance of <1/2LSB for 14-bit conversions. The nominal conversion time is 1.5ms for single-ended conversions. Current Measurements The LTC2990 has the ability to perform 14-bit current measurements with the addition of a current sense resis- tor (see Figure 3). In order to achieve accurate current sensing a few de- tails must be considered. Differential voltage or current measurementsaredirectlysampledbytheinternalADC.The average ADC input current for each leg of the differential input signal during a conversion is (VIN – 1.49) • 0.34µA. The maximum source impedance to yield 14-bit results with, 1/2LSB full-scale error is ~50Ω. In order to achieve highaccuracy,4-point,orKelvinconnectedmeasurements of the sense resistor differential voltage are necessary. In the case of current measurements, the external sense resistor is typically small, and determined by the full-scale input voltage of the LTC2990. The full-scale differential voltage is 0.300V. The external sense resistance is then a functionofthemaximummeasurablecurrent,orREXT_MAX = 0.300/IMAX. For example, if you wanted to measure a current range of ±5A, the external shunt resistance would equal 0.300/5 = 60mΩ. Thereexistsawaytoimprovethesenseresistor’sprecision usingtheLTC2990.TheLTC2990measuresbothdifferential voltage and remote temperature. It is therefore, possible to compensate for the absolute resistance tolerance of the sense resistor and the temperature coefficient of the sense resistor in software. The resistance would be measured by running a calibrated test current through the discrete resistor. The LTC2990 would measure both the differential voltage across this resistor and the resistor temperature. From this measurement, RO and TO in the equation be- low would be known. Using the two equations, the host microprocessor could compensate for both the absolute tolerance and the TCR. RT = RO • [1 + α(T – TO)] where: α = +3930 ppm/°C for copper trace α = ±2 to ~+200ppm/°C for discrete R (7) I = (V1 – V2)/RT (8) Figure 3. Simplified Current Sense Schematic V1 V2 LTC2990 0V – VCC RSENSE ILOAD 2990 F03 |
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