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ADUC841 Datasheet(PDF) 28 Page - Analog Devices |
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ADUC841 Datasheet(HTML) 28 Page - Analog Devices |
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28 / 88 page  ADuC841/ADuC842/ADuC843 Rev. 0 | Page 28 of 88 Table 11. Some Single-Supply Op Amps Op Amp Model Characteristics OP281/OP481 Micropower OP191/OP291/OP491 I/O Good up to VDD, Low Cost OP196/OP296/OP496 I/O to VDD, Micropower, Low Cost OP183/OP283 High Gain-Bandwidth Product OP162/OP262/OP462 High GBP, Micro Package AD820/AD822/AD824 FET Input, Low Cost AD823 FET Input, High GBP Keep in mind that the ADC’s transfer function is 0 V to VREF, and that any signal range lost to amplifier saturation near ground will impact dynamic range. Though the op amps in Table 11 are capable of delivering output signals that very closely approach ground, no amplifier can deliver signals all the way to ground when powered by a single supply. Therefore, if a negative supply is available, you might consider using it to power the front end amplifiers. If you do, however, be sure to include the Schottky diodes shown in Figure 31 (or at least the lower of the two diodes) to protect the analog input from undervoltage conditions. To summarize this section, use the circuit in Figure 31 to drive the analog input pins of the parts. Voltage Reference Connections The on-chip 2.5 V band gap voltage reference can be used as the reference source for the ADC and DACs. To ensure the accuracy of the voltage reference, you must decouple the CREF pin to ground with a 0.47 µF capacitor, as shown in Figure 32. Note that this is different from the ADuC812/ADuC831/ADuC832. BUFFER BUFFER 0.47 µF 51 Ω VREF = NC CREF 2.5V BAND GAP REFERENCE ADuC841/ADuC842/ADuC843 Figure 32. Decoupling VREF and CREF If the internal voltage reference is to be used as a reference for external circuitry, the CREF output should be used. However, a buffer must be used in this case to ensure that no current is drawn from the CREF pin itself. The voltage on the CREF pin is that of an internal node within the buffer block, and its voltage is critical for ADC and DAC accuracy. The parts power up with their internal voltage reference in the off state. If an external voltage reference is preferred, it should be connected to the CREF pin as shown in Figure 33. Bit 6 of the ADCCON1 SFR must be set to 1 to switch in the external reference voltage. To ensure accurate ADC operation, the voltage applied to CREF must be between 1 V and AVDD. In situations where analog input signals are proportional to the power supply (such as in some strain gage applications), it may be desirable to connect the CREF pin directly to AVDD. Operation of the ADC or DACs with a reference voltage below 1 V, however, may incur loss of accuracy, eventually resulting in missing codes or non- monotonicity. For that reason, do not use a reference voltage lower than 1 V. BUFFER 51 Ω CREF EXTERNAL VOLTAGE REFERENCE 1 = EXTERNAL 0 = INTERNAL 0.1 µF 2.5V BAND GAP REFERENCE ADuC841/ADuC842/ADuC843 ADCCON1.6 VDD VREF = NC Figure 33. Using an External Voltage Reference Configuring the ADC The parts’ successive approximation ADC is driven by a divided down version of the master clock. To ensure adequate ADC operation, this ADC clock must be between 400 kHz and 8.38 MHz. Frequencies within this range can be achieved easily with master clock frequencies from 400 kHz to well above 16 MHz, with the four ADC clock divide ratios to choose from. For example, set the ADC clock divide ratio to 8 (i.e., ADCCLK = 16.777216 MHz/8 = 2 MHz) by setting the appropriate bits in ADCCON1 (ADCCON1.5 = 1, ADCCON1.4 = 0). The total ADC conversion time is 15 ADC clocks, plus 1 ADC clock for synchronization, plus the selected acquisition time (1, 2, 3, or 4 ADC clocks). For the preceding example, with a 3-clock acquisition time, total conversion time is 19 ADC clocks (or 9.05 µs for a 2 MHz ADC clock). In continuous conversion mode, a new conversion begins each time the previous one finishes. The sample rate is then simply the inverse of the total conversion time described previously. In the preceding example, the continuous conversion mode sample rate is 110.3 kHz. |
Similar Part No. - ADUC841_15 |
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Similar Description - ADUC841_15 |
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