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LTC2430IGN Datasheet(PDF) 29 Page - Linear Technology |
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LTC2430IGN Datasheet(HTML) 29 Page - Linear Technology |
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29 / 40 page  LTC2430/LTC2431 29 24301f an infinite bandwidth source and 216nV/ √Hz for a single 0.5MHz pole source. From these numbers, it is clear that particular attention must be given to the design of external amplification circuits. Such circuits face the simultaneous requirements of very low bandwidth (just a few Hz) in order to reduce the output referred noise and relatively high bandwidth (at least 500kHz) necessary to drive the input switched-capacitor network. A possible solution is a high gain, low bandwidth amplifier stage followed by a high bandwidth unity-gain buffer. When external amplifiers are driving the LTC2430/ LTC2431, the ADC input referred system noise calculation can be simplified by Figure 29. The noise of an amplifier driving the LTC2430/LTC2431 input pin can be modeled as a band-limited white noise source. Its bandwidth can be approximated by the bandwidth of a single pole lowpass filter with a corner frequency fi. The amplifier noise spec- tral density is ni. From Figure 29, using fi as the x-axis selector, we can find on the y-axis the noise equivalent bandwidth freqi of the input driving amplifier. This band- width includes the band limiting effects of the ADC internal calibration and filtering. The noise of the driving amplifier referred to the converter input and including all these effects can be calculated as N = ni • √freqi. The total system noise (referred to the LTC2430/LTC2431 input) can now be obtained by summing as square root of sum of squares the three ADC input referred noise sources: the LTC2430/ LTC2431 internal noise (2.8 µV), the noise of the IN+ driving amplifier and the noise of the IN– driving amplifier. APPLICATIO S I FOR ATIO Figure 28. Input Signal Bandwidth Using the Internal Oscillator Figure 26. Resolution (NoiseRMS ≤ 1LSB) vs Output Data Rate and VCC OUTPUT DATA RATE (READINGS/SEC) 0 15 16 18 19 20 22 10 50 70 2430 F26 17 21 40 90 100 20 30 60 80 VINCM = VREFCM VIN = 0V FO = EXT OSC REF– = GND TA = 25°C RES = LOG2(VREF/NOISERMS) VCC = VREF = 5V VCC = 2.7V VREF = 2.5V Figure 27. Resolution (INLMAX ≤ 1LSB) vs Output Data Rate and VCC OUTPUT DATA RATE (READINGS/SEC) 0 15 16 18 19 20 22 10 50 70 2430 F27 17 21 40 90 100 20 30 60 80 VINCM = VREFCM VIN = 0V FO = EXT OSC REF– = GND TA = 25°C RES = LOG2(VREF/INLMAX) VCC = VREF = 5V VCC = 2.7V VREF = 2.5V DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz) 0 –3 –2 –1 0 4 2431 F28 –4 –5 –6 1 2 3 5 FO = HIGH FO = LOW INPUT NOISE SOURCE SINGLE POLE EQUIVALENT BANDWIDTH (Hz) 1 10 100 1000 10 100 1k 10k 100k 1M 2431 G29 0.1 0.1 1 FO = LOW FO = HIGH Figure 29. Input Referred Noise Equivalent Bandwidth of an Input Connected White Noise Source |
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