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AD7715 Datasheet(PDF) 21 Page - Analog Devices |
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AD7715 Datasheet(HTML) 21 Page - Analog Devices |
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21 / 41 page  AD7715 Data Sheet Rev. E | Page 20 of 40 In unbuffered mode, the analog inputs look directly into the input sampling capacitor, CSAMP. The dc input leakage current in this unbuffered mode is 1 nA maximum. As a result, the analog inputs see a dynamic load that is switched at the input sample rate (see Figure 5). This sample rate depends on master clock frequency and selected gain. CSAMP is charged to AIN(+) and discharged to AIN(−) every input sample cycle. The effective on-resistance of the switch, RSW, is typically 7 kΩ. HIGH IMPEDANCE >1GΩ RSW (7kΩ TYP) CSAMP (10pF) VBIAS SWITCHING FREQUENCY DEPENDS ON fCLKIN AND SELECTED GAIN AIN(+) AIN(–) Figure 5. Unbuffered Analog Input Structure CSAMP must be charged through RSW and through any external source impedances every input sample cycle. Therefore, in unbuffered mode, source impedances mean a longer charge time for CSAMP, and this may result in gain errors on the part. Table 24 shows the allowable external resistance/capacitance values, for unbuffered mode, such that no gain error to the 16-bit level is introduced on the part. Note that these capacitances are total capacitances on the analog input, external capacitance plus 10 pF capacitance from the pins and lead frame of the device. Table 24. External R, C Combination for No 16-Bit Gain Error (Unbuffered Mode Only) Gain External Capacitance (pF) 10 50 100 500 1000 5000 1 152 kΩ 53.9 kΩ 31.4 kΩ 8.4 kΩ 4.76 kΩ 1.36 kΩ 2 75.1 kΩ 26.6 kΩ 15.4 kΩ 4.14 kΩ 2.36 kΩ 670 Ω 32 16.7 kΩ 5.95 kΩ 3.46 kΩ 924 Ω 526 Ω 150 Ω 128 16.7 kΩ 5.95 kΩ 3.46 kΩ 924 Ω 526 Ω 150 Ω In buffered mode, the analog inputs look into the high impedance inputs stage of the on-chip buffer amplifier. CSAMP is charged via this buffer amplifier such that source impedances do not affect the charging of CSAMP. This buffer amplifier has an offset leakage current of 1 nA. In this buffered mode, large source impedances result in a small dc offset voltage developed across the source impedance but not in a gain error. Input Sample Rate The modulator sample frequency for the AD7715 remains at fCLKIN/128 (19.2 kHz @ fCLKIN = 2.4576 MHz) regardless of the selected gain. However, gains greater than 1 are achieved by a combination of multiple input samples per modulator cycle and a scaling of the ratio of reference capacitor to input capacitor. As a result of the multiple sampling, the input sample rate of the device varies with the selected gain (see Table 25). In buffered mode, the input is buffered before the input sampling capacitor. In unbuffered mode, where the analog input looks directly into the sampling capacitor, the effective input impedance is 1/CSAMP × fS where CSAMP is the input sampling capacitance and fS is the input sample rate. Table 25. Input Sampling Frequency vs. Gain Gain Input Sampling Frequency (fS) 1 fCLK IN/64 (38.4 kHz @ fCLK IN = 2.4576 MHz) 2 2 × fCLK IN/64 (76.8 kHz @ fCLK IN = 2.4576 MHz) 32 8 × fCLK IN/64 (307.2 kHz @ fCLK IN = 2.4576 MHz) 128 8 × fCLK IN/64 (307.2 kHz @ fCLK IN = 2.4576 MHz) Bipolar/Unipolar Inputs The analog input on the AD7715 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 input since the analog input cannot go more negative than −30 mV to ensure correct operation of the part. The input channel is fully differential. 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 AD7715 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 AD7715 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 (that is, 2.5 V ± 1.25 V). If AIN(−) is at AGND, the part cannot be configured for bipolar ranges in excess of ±30 mV. Bipolar or unipolar options are chosen by programming the B/U bit of the setup register. This programs the channel 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. |
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