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AD7730LBRZ Fiches technique(PDF) 48 Page - Analog Devices |
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AD7730LBRZ Fiches technique(HTML) 48 Page - Analog Devices |
48 / 53 page AD7730/AD7730L –48– OUTPUT NOISE AND RESOLUTION SPECIFICATION The AD7730L can be programmed to operate in either chop mode or nonchop mode. The chop mode can be enabled in ac-excited or dc-excited applications; it is optional in dc-excited applications, but chop mode must be enabled in ac-excited applications. These options are discussed in more detail in earlier sections. The chop mode has the advantage of lower drift numbers and better noise immunity, but the noise is approximately 20% higher for a given –3 dB frequency and output data rate. It is envisaged that the major- ity of weigh-scale users of the AD7730L will operate the part in chop mode to avail themselves of the excellent drift performance and noise immunity when chopping is enabled. The following tables outline the noise performance of the part in both chop and nonchop modes over all input ranges for a selection of output rates. Output Noise (CHP = 1) This mode is the primary mode of operation of the device. Table XXI shows the output rms noise for some typical output update rates and –3 dB frequencies for the AD7730 when used in chopping mode (CHP of Filter Register = 1) with a master clock frequency of 2.4576 MHz. These numbers are typical and are generated at a differential analog input voltage of 0 V. The output update rate is selected via the SF0 to SF11 bits of the Filter Register. Table XXII, meanwhile, shows the output peak-to-peak resolu- tion in counts for the same output update rates. The numbers in brackets are the effective peak-to-peak resolution in bits (rounded to the nearest 0.5 LSB). It is important to note that the numbers in Table XXII represent the resolution for which there will be no code flicker within a six-sigma limit. They are not calculated based on rms noise, but on peak-to-peak noise. The numbers are generated for the bipolar input ranges. When the part is operated in unipolar mode, the output noise will be the same as the equivalent bipolar input range. As a result, the numbers in Table XXI will remain the same for unipolar ranges while the numbers in Table II will change. To calculate the numbers for Table XXII for unipolar input ranges simply divide the peak-to-peak resolution number in counts by two or subtract one from the peak-to-peak resolution number in bits. Table XXI. Output Noise vs. Input Range and Update Rate (CHP = 1) Typical Output RMS Noise in nV Output –3 dB SF Settling Time Settling Time Input Range Input Range Input Range Input Range Data Rate Frequency Word Normal Mode Fast Mode = 80 mV = 40 mV = 20 mV = 10 mV 25 Hz 0.98 Hz 2048 920 ms 120 ms 245 140 105 70 50 Hz 1.97 Hz 1024 460 ms 60 ms 340 220 160 100 75 Hz 2.96 Hz 683 306 ms 40 ms 420 270 170 110 100 Hz* 3.95 Hz 512 230 ms 30 ms 500 290 180 130 200 Hz 7.9 Hz 256 115 ms 15 ms 650 490 280 165 *Power-On Default Table XXII. Peak-to-Peak Resolution vs. Input Range and Update Rate (CHP = 1) Peak-to-Peak Resolution in Counts (Bits) Output –3 dB SF Settling Time Settling Time Input Range Input Range Input Range Input Range Data Rate Frequency Word Normal Mode Fast Mode = 80 mV = 40 mV = 20 mV = 10 mV 25 Hz 0.98 Hz 2048 920 ms 120 ms 110k (17) 94k (16.5) 64k (16) 46k (15.5) 50 Hz 1.97 Hz 1024 460 ms 60 ms 80k (16.5) 60k (16) 42k (15.5) 33k (15) 75 Hz 2.96 Hz 683 306 ms 40 ms 62k (16) 50k (15.5) 39k (15) 31k (15) 100 Hz* 3.95 Hz 512 230 ms 30 ms 53k (15.5) 46k (15.5) 36k (15) 25k (14.5) 200 Hz 7.9 Hz 256 115 ms 15 ms 44k (15.5) 27k (15) 24k (14.5) 20k (14.5) *Power-On Default Output Noise (CHP = 0) Table XXIII shows the output rms noise for some typical output update rates and –3 dB frequencies for the AD7730L when used in nonchopping mode (CHP of Filter Register = 0) with a master clock frequency of 2.4576 MHz. These numbers are typical and are generated at a differential analog input voltage of 0 V. The output update rate is selected via the SF0 to SF11 bits of the Filter Regis- ter. Table XXIV, meanwhile, shows the output peak-to-peak resolution in counts for the same output update rates. The numbers in brackets are the effective peak-to-peak resolution in bits (rounded to the nearest 0.5 LSB). It is important to note that the numbers in Table XXIV represent the resolution for which there will be no code flicker within a six-sigma limit. They are not calculated based on rms noise, but on peak-to-peak noise. The numbers are generated for the bipolar input ranges. When the part is operated in unipolar mode, the output noise will be the same as the equivalent bipolar input range. As a result, the numbers in Table XXIII will remain the same for unipolar ranges while the numbers in Table XXIV will change. To calculate the number for Table XXIV for unipolar input ranges simply divide the peak- to-peak resolution number in counts by two or subtract one from the peak-to-peak resolution number in bits. REV. B |
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