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AD7228ABP Scheda tecnica(PDF) 5 Page - Analog Devices |
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AD7228ABP Scheda tecnica(HTML) 5 Page - Analog Devices |
5 / 8 page AD7228A REV. A –5– OP AMP SECTION Each voltage-mode D/A converter output is buffered by a unity gain noninverting CMOS amplifier. This buffer amplifier is tested with a 2 k Ω and 100 pF load but will typically drive a 2 k Ω and 500 pF load. The AD7228A can be operated single or dual supply. Operating the part from single or dual supplies has no effect on the positive- going settling time. However, the negative-going settling time to voltages near 0 V in single supply will be slightly longer than the settling time for dual supply operation. Additionally, to ensure that the output voltage can go to 0 V in single supply, a transis- tor on the output acts as a passive pull-down as the output volt- age nears 0 V. As a result, the sink capability of the amplifier is reduced as the output voltage nears 0 V in single supply. In dual supply operation, the full sink capability of 400 µA at 25°C is maintained over the entire output voltage range. The single sup- ply output sink capability is shown in Figure 4. The negative VSS also gives improved output amplifier performance allowing an extended input reference voltage range and giving improved slew rate at the output. Figure 4. Single Supply Sink Current The output broadband noise from the amplifier is 300 µV peak-to-peak. Figure 5 shows a plot of noise spectral density versus frequency. Figure 5. Noise Spectral Density vs. Frequency DIGITAL INPUTS The AD7228A digital inputs are compatible with either TTL or 5 V CMOS levels. All logic inputs are static-protected MOS gates with typical input currents of less than 1 nA. Internal in- put protection is achieved by on-chip distributed diodes. SUPPLY CURRENT The AD7228A has a maximum IDD specification of 22 mA and a maximum ISS of 20 mA over the –55 °C to +125°C tempera- ture range. This maximum current specification is actually de- termined by the current at –55 °C. Figure 6 shows a typical plot of power supply current versus temperature. Figure 6. Power Supply Current vs. Temperature APPLYING THIS AD7228A UNIPOLAR OUTPUT OPERATION This is the basic mode of operation for each channel of the AD7228A, with the output voltage having the same positive po- larity as VREF. Connections for unipolar output operation are shown in Figure 7. The AD7228A can be operated from single or dual supplies as outlined earlier. The voltage at the reference input must never be negative with respect to GND. Failure to observe this precaution may cause parasitic transistor action and possible device destruction. The code table for unipolar output operation is shown in Table II. Figure 7. Unipolar Output Circuit |
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