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AD7669JR Datenblatt(PDF) 9 Page - Analog Devices |
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AD7669JR Datenblatt(HTML) 9 Page - Analog Devices |
9 / 20 page AD7569/AD7669 –9– REV. B CIRCUIT DESCRIPTION D/A SECTION The AD7569 contains an 8-bit, voltage-mode, D/A converter that uses eight equally weighted current sources switched into an R-2R ladder network to give a direct but unbuffered 0 V to +1.25 V output range. The AD7669 is similar, but contains two D/A converters. The current sources are fabricated using PNP transistors. These transistors allow current sources that are driven from positive voltage logic and give a zero-based output range. The output voltage from the voltage switching R-2R lad- der network has the same positive polarity as the reference; therefore, the D/A converter can be operated from a single power supply rail. The PNP current sources are generated using the on-chip bandgap reference and a control amplifier. The current sources are switched to either the ladder or AGNDDAC by high speed p-channel switches. These high-speed switches ensure a fast set- tling time for the output voltage of the DAC. The R-2R ladder network of the DAC consists of highly stable, thin-film resistors. A simplified circuit diagram for the D/A converter section is shown in Figure 3. An identical D/A converter is used as part of the A/D converter, which is discussed later. Figure 3. DAC Simplified Circuit Diagram OP AMP SECTION The output from the D/A converter is buffered by a high speed, noninverting op amp. This op amp is capable of developing ±2.5 V across a 2 kΩ and 100 pF load to AGND DAC. The am- plifier can be operated from a single +5 V supply to give two unipolar output ranges, or from dual supplies ( ±5 V) to allow two bipolar output ranges. The feedback path of the amplifier contains a gain/offset net- work that provides four voltage ranges at the output of the op amp. The output voltage range is determined by the RANGE and VSS inputs. (See Table I in the Pin Function Description section.) The four possible output ranges are: 0 V to +1.25 V, 0 V to +2.5 V, ±1.25 V and ±2.5 V. It should be noted that whichever range is selected for the output amplifier also applies to the input voltage range of the A/D converter. The output amplifier settles to within 1/2 LSB of its final value in typically less than 500 ns. Operating the part from single or dual supplies has no effect on the positive-going settling time. However, the negative-going output settling time to voltages near 0 V in single supply will be slightly longer than the settling time to negative full scale for dual supply operation. Addition- ally, to ensure that the output voltage can go to 0 V in single supply, a transistor on the output acts as a passive pull-down with output voltages near 0 V with VSS = 0 V. This means that the sink capability of the amplifier is reduced as the output volt- age nears 0 V in single supply. In dual supply operation the full sink capability of 1.25 mA is maintained over the entire output voltage range. For all other parameters, the single and dual supply perfor- mances of the amplifier are essentially identical. The output noise from the amplifier, with full scale on the DAC, is 200 µV peak-to-peak. The spot noise at 1 kHz is 35 nV/ √Hz with all 0s on the DAC. A noise spectral density versus frequency plot for the amplifier is shown in the typical performance graphs. VOLTAGE REFERENCE The AD7569/AD7669 contains an on-chip bandgap reference that provides a low noise, temperature compensated reference voltage for both the DAC and the ADC. The reference is trimmed for absolute accuracy and temperature coefficient. The bandgap reference is generated with respect to VDD. It is buff- ered by a separate control amplifier for both the DAC and the ADC reference. This can be seen in the DAC ladder network configuration in Figure 3. DIGITAL SECTION The data pins on the AD7569/AD7669 provide a connection between the external bus and DAC data inputs and ADC data outputs. The threshold levels of all digital inputs and outputs are compatible with either TTL or 5 V CMOS levels. Internal input protection of all digital pins is achieved by on-chip distrib- uted diodes. The data format is straight binary when the part is used in single supply (VSS = 0 V). However, when a VSS of –5 V is applied, the data format becomes twos complement. This data format ap- plies to the digital inputs of the DAC and the digital outputs of the ADC. ADC SECTION The analog-to-digital converter on the AD7569/AD7669 uses the successive approximation technique to achieve a fast conver- sion time of 2 µs and provides an 8-bit parallel digital output. The reference for the ADC is provided by the on-chip bandgap reference. Conversion start is controlled by ST or by CS and RD. Once a conversion has been started, another conversion start should not be attempted until the conversion in progress is completed. Exercising the RESET input does not affect conversion; the RESET input resets the INT line high, which is useful in inter- rupt driven systems where a READ has not been performed at the end of the previous conversion. The INT line does not have to be cleared at the end of conversion. The ADC will continue to convert correctly, but the function of the INT line will be affected. Figure 4 shows the operating waveforms for a conversion cycle. The analog input voltage, VIN, is held 50 ns typical after the fall- ing edge of ST or (CS & RD). The MSB decision is made ap- proximately 50 ns after the second falling edge of the input CLK following a conversion start. If t1 in Figure 4 is greater than 50 ns, then the falling edge of the input CLK will be seen as the first falling clock edge. If t1 is less than 50 ns, the first fall- ing clock edge of the conversion will not occur until one clock cycle later. The succeeding bit decisions are made approxi- mately 50 ns after a CLK edge until conversion is complete. |
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