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AD7711AN Datasheet(PDF) 11 Page - Analog Devices |
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AD7711AN Datasheet(HTML) 11 Page - Analog Devices |
11 / 28 page 2 AD7711 –11– REV. F Tables I and II show the output rms noise for some typical notch and –3 dB frequencies. The numbers given are for the bipolar input ranges with a VREF of +2.5 V. These numbers are typical and are generated with an analog input voltage of 0 V. The output noise from the part comes from two sources. The first is the electrical noise in the semiconductor devices used in the implementation of the modulator (device noise). The second occurs when the analog input signal is converted into the digital domain adding quanti- zation noise. The device noise is at a low level and is largely independent of frequency. The quantization noise starts at an even lower level but rises rapidly with increasing frequency to become the dominant noise source. Consequently, lower filter notch set- tings (below 60 Hz approximately) tend to be device noise dominated while higher notch settings are dominated by quantization noise. Changing the filter notch and cutoff frequency in the quantization noise dominated region results in a more dramatic im- provement in noise performance than it does in the device noise dominated region as shown in Table I. Furthermore, quantization noise is added after the PGA, so effective resolution is independent of gain for the higher filter notch frequencies. Meanwhile, device noise is added in the PGA and, therefore, effective resolution suffers a little at high gains for lower notch frequencies. At the lower filter notch settings (below 60 Hz), the no missing codes performance of the device is at the 24-bit level. At the higher settings, more codes will be missed until at 1 kHz notch setting, no missing codes performance is only guaranteed to the 12-bit level. However, since the effective resolution of the part is 10.5 bits for this filter notch setting, this no missing codes performance should be more than adequate for all applications. The effective resolution of the device is defined as the ratio of the output rms noise to the input full scale. This does not remain constant with increasing gain or with increasing bandwidth. Table II shows the same table as Table I except that the output is now expressed in terms of effective resolution (the magnitude of the rms noise with respect to 2 × V REF/GAIN, i.e., the input full scale). It is possible to do post filtering on the device to improve the output data rate for a given –3 dB frequency and also to further reduce the output noise (see Digital Filtering section). Table I. Output Noise vs. Gain and First Notch Frequency First Notch of Typical Output RMS Noise ( V) Filter and O/P –3 dB Gain of Gain of Gain of Gain of Gain of Gain of Gain of Gain of Data Rate 1 Frequency 124816 32 64 128 10 Hz 2 2.62 Hz 1.0 0.78 0.48 0.33 0.25 0.25 0.25 0.25 25 Hz 2 6.55 Hz 1.8 1.1 0.63 0.50 0.44 0.41 0.38 0.38 30 Hz 2 7.86 Hz 2.5 1.31 0.84 0.57 0.46 0.43 0.4 0.4 50 Hz 2 13.1 Hz 4.33 2.06 1.2 0.64 0.54 0.46 0.46 0.46 60 Hz 2 15.72 Hz 5.28 2.36 1.33 0.87 0.63 0.62 0.6 0.56 100 Hz 3 26.2 Hz 13 6.4 3.7 1.8 1.1 0.9 0.65 0.65 250 Hz 3 65.5 Hz 130 75 25 12 7.5 4 2.7 1.7 500 Hz 3 131 Hz 0.6 × 103 0.26 × 103 140 70 35 25 15 8 1 kHz 3 262 Hz 3.1 × 103 1.6 × 103 0.7 × 103 0.29 × 103 180 120 70 40 NOTES 1The default condition (after the internal power-on reset) for the first notch of filter is 60 Hz. 2For these filter notch frequencies, the output rms noise is primarily dominated by device noise and as a result is independent of the value of the reference voltage. Therefore, increasing the reference voltage will give an increase in the effective resolution of the device (i.e., the ratio of the rms noise to the input full scale is in- creased since the output rms noise remains constant as the input full scale increases). 3For these filter notch frequencies, the output rms noise is dominated by quantization noise and as a result is proportional to the value of the reference voltage. Table II. Effective Resolution vs. Gain and First Notch Frequency First Notch of Effective Resolution 1 (Bits) Filter and O/P –3 dB Gain of Gain of Gain of Gain of Gain of Gain of Gain of Gain of Data Rate Frequency 124816 32 64 128 10 Hz 2.62 Hz 22.5 21.5 21.5 21 20.5 19.5 18.5 17.5 25 Hz 6.55 Hz 21.5 21 21 20 19.5 18.5 17.5 16.5 30 Hz 7.86 Hz 21 21 20.5 20 19.5 18.5 17.5 16.5 50 Hz 13.1 Hz 20 20 20 20 19 18.5 17.5 16.5 60 Hz 15.72 Hz 20 20 20 19.5 19 18 17 16 100 Hz 26.2 Hz 18.5 18.5 18.5 18.5 18 17.5 17 16 250 Hz 65.5 Hz 15 15 15.5 15.5 15.5 15.5 15 14.5 500 Hz 131 Hz 13 13 13 13 13 12.5 12.5 12.5 1 kHz 262 Hz 10.5 10.5 11 11 11 10.5 10 10 NOTE 1Effective resolution is defined as the magnitude of the output rms noise with respect to the input full scale (i.e., 2 × V REF/GAIN). The above table applies for a VREF of +2.5 V and resolution numbers are rounded to the nearest 0.5 LSB. |
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