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AD7225KP Arkusz danych(PDF) 9 Page - Analog Devices |
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AD7225KP Arkusz danych(HTML) 9 Page - Analog Devices |
9 / 12 page AD7225 REV. B –9– Figure 14. AD7225 Bipolar Output Circuit Table IV. Bipolar (Offset Binary) Code Table DAC Latch Contents MSB LSB Analog Output 1 1 1 1 1 1 1 1 +V REF 127 128 1 0 0 0 0 0 0 1 +V REF 1 128 1 0 0 0 0 0 0 0 0 V 0 1 1 1 1 1 1 1 –V REF 1 128 0 0 0 0 0 0 0 1 –V REF 127 128 0 0 0 0 0 0 0 0 –V REF 128 128 = –V REF AGND BIAS The AD7225 AGND pin can be biased above system GND (AD7225 DGND) to provide an offset “zero” analog output voltage level. Figure 15 shows a circuit configuration to achieve this for channel A of the AD7225. The output voltage, VOUT A, can be expressed as: VOUT A = VBIAS + DA (VIN) where DA is a fractional representation of the digital word in DAC latch A. (0 ≤ D A ≤ 255/256). Figure 15. AGND Bias Circuit For a given VIN, increasing AGND above system GND will re- duce the effective VDD–VREF which must be at least 4 V to en- sure specified operation. Note that because the AGND pin is common to all four DACs, this method biases up the output voltages of all the DACs in the AD7225. Note that VDD and VSS of the AD7225 should be referenced to DGND. AC REFERENCE SIGNAL In some applications it may be desirable to have ac reference signals. The AD7225 has multiplying capability within the up- per (VDD – 4 V) and lower (2 V) limits of reference voltage when operated with dual supplies. Therefore ac signals need to be ac coupled and biased up before being applied to the reference in- puts. Figure 16 shows a sine wave signal applied to VREF A. For input signal frequencies up to 50 kHz the output distortion typi- cally remains less than 0.1%. The typical 3 dB bandwidth figure for small signal inputs is 800 kHz. Figure 16. Applying an AC Signal to the AD7225 APPLICATIONS PROGRAMMABLE TRANSVERSAL FILTER A discrete-time filter may be described by either multiplication in the frequency domain or convolution in the time domain i.e. Y ω () = H ω ()X ω () or y n =∑ k =1 N h kXn –k +1 The convolution sum may be implemented using the special structure known as the transversal filter (Figure 17). Basically, it consists of an N-stage delay line with N taps weighted by N co- efficients, the resulting products being accumulated to form the output. The tap weights or coefficients hk are actually the non- zero elements of the impulse response and therefore determine the filter transfer function. A particular filter frequency response is realized by setting the coefficients to the appropriate values. This property leads to the implementation of transversal filters whose frequency response is programmable. Figure 17. Transversal Filter |
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