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Datenblatt-Suchmaschine für elektronische Bauteile |
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Datenblatt-Suchmaschine für elektronische Bauteile |
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AD7714YN Datenblatt(PDF) 21 Page - Analog Devices |
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AD7714YN Datenblatt(HTML) 21 Page - Analog Devices |
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21 / 40 page  2 AD7714 REV. C –21– Table XIV. Input Sampling Frequency vs. Gain Gain Input Sampling Freq (fS) 1fCLK IN/64 (38.4 kHz @ fCLK IN = 2.4576 MHz) 22 × f CLK IN/64 (76.8 kHz @ fCLK IN = 2.4576 MHz) 44 × f CLK IN/64 (153.6 kHz @ fCLK IN = 2.4576 MHz) 88 × f CLK IN/64 (307.2 kHz @ fCLK IN = 2.4576 MHz) 16 8 × f CLK IN/64 (307.2 kHz @ fCLK IN = 2.4576 MHz) 32 8 × f CLK IN/64 (307.2 kHz @ fCLK IN = 2.4576 MHz) 64 8 × f CLK IN/64 (307.2 kHz @ fCLK IN = 2.4576 MHz) 128 8 × f CLK IN/64 (307.2 kHz @ fCLK IN = 2.4576 MHz) Burnout Current The AD7714 contains two 1 µA currents, one source current from AVDD to AIN(+) and one sink from AIN(–) to AGND. The currents are either both on or off depending on the BO bit of the Mode Register. These currents can be used in checking that a transducer has not burned out nor gone open-circuit before attempting to take measurements on that channel. If the cur- rents are turned on, allowed flow in the transducer, a measure- ment of the input voltage on the analog input taken and the voltage measured is full scale, it indicates that the transducer has gone open-circuit; if the voltage measured is zero, it indicates that the transducer has gone short-circuit. For normal opera- tion, these burnout currents are turned off by writing a 0 to the BO bit. For the source current to work correctly, the applied voltage on AIN(+) should not go within 500 mV of AVDD. For the sink current to work correctly, the applied voltage on the AIN(–) input should not go within 500 mV of AGND. Bipolar/Unipolar Inputs The analog inputs on the AD7714 can accept either unipolar or bipolar input voltage ranges. Bipolar input ranges do not imply that the part can handle negative voltages on its analog inputs, since the analog input cannot go more negative than –30 mV to ensure correct operation of the part. The input channels are either fully differential or pseudo-differential (all other channels referenced to AIN6). In either case, the input channels are arranged in pairs with an AIN(+) and AIN(–). As a result, the voltage to which the unipolar and bipolar signals on the AIN(+) input are referenced is the voltage on the respective AIN(–) input. For example, if AIN(–) is +2.5 V and the AD7714 is configured for unipolar operation with a gain of 2 and a VREF of +2.5 V, the input voltage range on the AIN(+) input is +2.5 V to +3.75 V. If AIN(–) is +2.5 V and the AD7714 is configured for bipolar mode with a gain of 2 and a VREF of +2.5 V, the analog input range on the AIN(+) input is +1.25 V to +3.75 V (i.e., 2.5 V ± 1.25 V). If AIN(–) is at AGND, the part cannot be con- figured for bipolar ranges in excess of ±30 mV. Bipolar or unipolar options are chosen by programming the B/U bit of the Filter High Register. This programs the selected chan- nel for either unipolar or bipolar operation. Programming the channel for either unipolar or bipolar operation does not change any of the input signal conditioning; it simply changes the data output coding and the points on the transfer function where calibrations occur. REFERENCE INPUT The AD7714’s reference inputs, REF IN(+) and REF IN(–), provide a differential reference input capability. The common- mode range for these differential inputs is from AGND to AVDD. The nominal reference voltage, VREF (REF IN(+) –REF IN(–)), for specified operation is +2.5 V for the AD7714-5 and +1.25 V for the AD7714-3. The part is functional with VREF voltages down to 1 V but with degraded performance as the output noise will, in terms of LSB size, be larger. REF IN(+) must always be greater than REF IN(–) for correct operation of the AD7714. Both reference inputs provide a high impedance, dynamic load similar to the analog inputs in unbuffered mode. The maxi- mum dc input leakage current is ±1 nA over temperature and source resistance may result in gain errors on the part. In this case, the sampling switch resistance is 5 k Ω typ and the refer- ence capacitor (CREF) varies with gain. The sample rate on the reference inputs is fCLK IN/64 and does not vary with gain. For gains of 1 to 8, CREF is 8 pF; for a gain of 16, it is 5.5 pF, for a gain of 32, it is 4.25 pF, for a gain of 64, it is 3.625 pF and for a gain of 128, it is 3.3125 pF. The output noise performance outlined in Tables I through IV is for an analog input of 0 V and is unaffected by noise on the reference. To obtain the same noise performance as shown in the noise tables over the full input range requires a low noise reference source for the AD7714. If the reference noise in the bandwidth of interest is excessive, it will degrade the perfor- mance of the AD7714. In applications where the excitation voltage for the bridge transducer on the analog input also de- rives the reference voltage for the part, the effect of the noise in the excitation voltage will be removed as the application is ratiometric. Recommended reference voltage sources for the AD7714-5 and AD7714Y grade with AVDD = 5 V include the AD780, REF43 and REF192 while the recommended reference sources for the AD7714-3 and AD7714Y with AVDD = 3 V include the AD589 and AD1580. It is generally recommended to decouple the output of these references to further reduce the noise level. DIGITAL FILTERING The AD7714 contains an on-chip low-pass digital filter which processes the output of the part’s sigma-delta modulator. There- fore, the part not only provides the analog-to-digital conversion function but it also provides a level of filtering. There are a number of system differences when the filtering function is provided in the digital domain rather than the analog domain and the user should be aware of these. First, since digital filtering occurs after the A-to-D conversion process, it can remove noise injected during the conversion process. Analog filtering cannot do this. Also, the digital filter can be made programmable far more readily than an analog filter. Depending on the digital filter design, this gives the user the capability of programming cutoff frequency and output update rate. On the other hand, analog filtering can remove noise superim- posed on the analog signal before it reaches the ADC. Digital filtering cannot do this and noise peaks riding on signals near full scale have the potential to saturate the analog modulator and digital filter, even though the average value of the signal is within limits. To alleviate this problem, the AD7714 has over- range headroom built into the sigma-delta modulator and digital filter which allows overrange excursions of 5% above the analog input range. If noise signals are larger than this, consideration should be given to analog input filtering, or to reducing the input channel voltage so that its full scale is half that of the analog input channel full scale. This will provide an overrange capability greater than 100% at the expense of reducing the dynamic range by 1 bit (50%). |
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