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LTC2421 Datasheet(PDF) 24 Page - Linear Technology |
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LTC2421 Datasheet(HTML) 24 Page - Linear Technology |
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24 / 32 page  24 LTC2421/LTC2422 24212f In addition to the input current spikes, the input ESD pro- tection diodes have a temperature dependent leakage cur- rent. This leakage current, nominally 1nA ( ±100nA max), results in a fixed offset shift of 10 µV for a 10k source resistance. The effect of input leakage current is evident for CIN = 0 in Figures 19 and 22. A leakage current of 3nA results in a 150 µV (30ppm) error for a 50k source resistance. As RSOURCE gets larger, the switched capacitor input current begins to dominate. Reference Current (VREF) Similar to the analog input, the reference input has a dy- namic input current. This current has negligible effect on the offset. However, the reference current at VIN = VREF is similar to the input current at full-scale. For large values of reference capacitance (CVREF > 0.01µF), the full-scale er- ror shift is 0.03ppm/ Ω of external reference resistance independent of the capacitance at VREF, see Figure 23. If the capacitance tied to VREF is small (CVREF < 0.01µF), an input resistance of up to 80k (20pF parasitic capacitance at VREF) may be tolerated, see Figure 24. Unlike the analog input, the integral nonlinearity of the device can be degraded with excessive external RC time constants tied to the reference input. If the capacitance at node VREF is small (CVREF < 0.01µF), the reference input can tolerate large external resistances without reduction in INL, see Figure 25. If the external capacitance is large (CVREF > 0.01µF), the linearity will be degraded by APPLICATIO S I FOR ATIO RSOURCE (Ω) 1 –50 –40 –30 –20 –10 0 10 10 100 1k 10k 24212 F22 100k VCC = 5V VREF = 5V VIN = 5V TA = 25°C CIN = 0.01µF CIN = 100pF CIN = 1000pF CIN = 0pF Figure 22. Full-Scale Error vs RSOURCE (Small C) Figure 23. Full-Scale Error vs RVREF (Large C) Figure 24. Full-Scale Error vs RVREF (Small C) RESISTANCE AT VREF (Ω) 0 40 50 60 600 800 24212 F23 30 20 200 400 1000 10 0 –10 CVREF = 22µF CVREF = 10µF CVREF = 1µF CVREF = 0.1µF CVREF = 0.01µF CVREF = 0.001µF VCC = 5V VREF = 5V VIN = 5V TA = 25°C RESISTANCE AT VREF (Ω) 1 300 400 500 1k 10k 24212 F24 200 100 10 100 100k 0 –100 –200 VCC = 5V VREF = 5V VIN = 5V TA = 25°C CVREF = 0.01µF CVREF = 100pF CVREF = 1000pF CVREF = 0pF Figure 25. INL Error vs RVREF (Small C) RESISTANCE AT VREF (Ω) 1 30 40 50 1k 10k 24212 F25 20 10 10 100 100k 0 –10 –20 VCC = 5V VREF = 5V TA = 25°C CVREF = 0.01µF CVREF = 1000pF CVREF = 0pF CVREF = 100pF |
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