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9 / 17 page REV. E –8– AD7703 DIGITAL FILTERING The AD7703’s digital filter behaves like an analog filter, with a few minor differences. First, since digital filtering occurs after the analog-to-digital conversion, it can remove noise injected during the conversion process. Analog filtering cannot do this. 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 AD7703 has overrange headroom built into the - modulator and digital filter that allows overrange excursions of 100 mV. If noise signals are larger than this, consideration should be given to analog input filtering, or to reducing the gain in the input channel so that a full-scale input (2.5 V) gives only a half-scale input to the AD7703 (1.25 V). This will provide an overrange capability greater than 100% at the expense of reducing the dynamic range by one bit (50%). FILTER CHARACTERISTICS The cutoff frequency of the digital filter is fCLK/409600. At the maximum clock frequency of 4.096 MHz, the cutoff frequency of the filter is 10 Hz and the data update rate is 4 kHz. Figure 9 shows the filter frequency response. This is a six-pole Gaussian response that provides 55 dB of 60 Hz rejection for a 10 Hz cutoff frequency. If the clock frequency is halved to give a 5 Hz cutoff, 60 Hz rejection is better than 90 dB. 1 10 100 FREQUENCY – Hz 20 0 –20 –40 –60 –80 –100 –120 –140 –160 f CLK = 1MHz f CLK = 2MHz f CLK = 4MHz Figure 9. Frequency Response of AD7703 Filter Since the AD7703 contains this low-pass filtering, there is a settling time associated with step function inputs, and data will be invalid after a step change until the settling time has elapsed. The AD7703 is, therefore, unsuitable for high speed multiplex- ing, where channels are switched and converted sequentially at high rates, as switching between channels can cause a step change in the input. However, slow multiplexing of the AD7703 is possible, provided that the settling time is allowed to elapse before data for the new channel is accessed. The output settling of the AD7703 in response to a step input change is shown in Figure 10. The Gaussian response has fast settling with no overshoot, and the worst-case settling time to ±0.0007% is 125 ms with a 4.096 MHz master clock frequency. 100 80 60 40 20 0 040 80 120 160 TIME – ms Figure 10. AD7703 Step Response USING THE AD7703 SYSTEM DESIGN CONSIDERATIONS The AD7703 operates differently from successive approximation ADCs or integrating ADCs. Since it samples the signal continu- ously, like a tracking ADC, there is no need for a start convert command. The 20-bit output register is updated at a 4 kHz rate, and the output can be read at any time, either synchronously or asynchronously. CLOCKING The AD7703 requires a master clock input, which may be an exter- nal TTL/CMOS compatible clock signal applied to the CLKIN pin (CLKOUT not used). Alternatively, a crystal of the correct frequency can be connected between CLKIN and CLKOUT, when the clock circuit will function as a crystal controlled oscillator. Figure 11 shows a simple model of the on-chip gate oscillator and Table II gives some typical capacitor values to be used with various resonators. AD7703 C2* C1* R1 5M X1 3 10pF 10pF gm = 1500 MHO *SEE TABLE II 2 Figure 11. On-Chip Gate Oscillator |
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