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AD7710 Datasheet(PDF) 14 Page - Analog Devices |
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AD7710 Datasheet(HTML) 14 Page - Analog Devices |
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14 / 28 page  REV. F –14– AD7710 Input Sample Rate The modulator sample frequency for the device remains at fCLK IN/512 (19.5 kHz @ fCLK IN = 10 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 III). The effective input impedance is 1/C × f S where C is the input sam- pling capacitance and fS is the input sample rate. Table III. Input Sampling Frequency vs. Gain Gain Input Sampling Frequency (fS) 1fCLK IN/256 (39 kHz @ fCLK IN = 10 MHz) 22 × f CLK IN/256 (78 kHz @ fCLK IN = 10 MHz) 44 × f CLK IN/256 (156 kHz @ fCLK IN = 10 MHz) 88 × f CLK IN/256 (312 kHz @ fCLK IN = 10 MHz) 16 8 × f CLK IN/256 (312 kHz @ fCLK IN = 10 MHz) 32 8 × f CLK IN/256 (312 kHz @ fCLK IN = 10 MHz) 64 8 × f CLK IN/256 (312 kHz @ fCLK IN = 10 MHz) 128 8 × f CLK IN/256 (312 kHz @ fCLK IN = 10 MHz) DIGITAL FILTERING The AD7710’s digital filter behaves like a similar analog filter, with a few minor differences. 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. On the other hand, analog filtering can remove noise super- imposed 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 AD7710 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%). Filter Characteristics The cutoff frequency of the digital filter is determined by the value loaded to bits FS0 to FS11 in the control register. At the maximum clock frequency of 10 MHz, the minimum cutoff frequency of the filter is 2.58 Hz while the maximum program- mable cutoff frequency is 269 Hz. Figure 6 shows the filter frequency response for a cutoff fre- quency of 2.62 Hz which corresponds to a first filter notch frequency of 10 Hz. This is a (sinx/x)3 response (also called sinc3) that provides >100 dB of 50 Hz and 60 Hz rejection. Programming a different cutoff frequency via FS0–FS11 does not alter the profile of the filter response, it changes the frequency of the notches as outlined in the Control Register section. FREQUENCY – Hz 0 –240 070 10 20 30 40 50 60 –40 –80 –120 –160 –200 –20 –60 –100 –140 –180 –220 Figure 6. Frequency Response of AD7710 Filter Since the AD7710 contains this on-chip, low-pass filtering, there is a settling time associated with step function inputs, and data on the output will be invalid after a step change until the settling time has elapsed. The settling time depends upon the notch frequency chosen for the filter. The output data rate equates to this filter notch frequency and the settling time of the filter to a full-scale step input is four times the output data period. In applications using both input channels, the settling time of the filter must be allowed to elapse before data from the second channel is accessed. Post Filtering The on-chip modulator provides samples at a 19.5 kHz output rate. The on-chip digital filter decimates these samples to pro- vide data at an output rate which corresponds to the pro- grammed first notch frequency of the filter. Since the output data rate exceeds the Nyquist criterion, the output rate for a given bandwidth will satisfy most application requirements. However, there may be some applications which require a higher data rate for a given bandwidth and noise performance. Applications which need this higher data rate will require some post filtering following the digital filter of the AD7710. For example, if the required bandwidth is 7.86 Hz but the required update rate is 100 Hz, the data can be taken from the AD7710 at the 100 Hz rate giving a –3 dB bandwidth of 26.2 Hz. Post filtering can be applied to this to reduce the bandwidth and output noise, to the 7.86 Hz bandwidth level, while maintaining an output rate of 100 Hz. Post filtering can also be used to reduce the output noise from the device for bandwidths below 2.62 Hz. At a gain of 128, the output rms noise is 250 nV. This is essentially device noise or white noise, and since the input is chopped, the noise has a flat frequency response. By reducing the bandwidth below 2.62 Hz, the noise in the resultant passband can be reduced. A reduction in bandwidth by a factor of two results in a √2 reduction in the output rms noise. This additional filtering will result in a longer settling time. |
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