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AD7242JN Arkusz danych(PDF) 6 Page - Analog Devices |
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AD7242JN Arkusz danych(HTML) 6 Page - Analog Devices |
6 / 12 page AD7242/AD7244 REV. A –6– CIRCUIT DESCRIPTION The AD7242/AD7244 contains two 12-bit/14-bit D/A convert- ers, each with an output buffer amplifier. The part also contains a reference input buffer amplifier for each DAC, and an on-chip 3 V reference. D/A Section The AD7242/AD7244 contains two 12-bit/14-bit voltage mode D/A converters, each consisting of highly stable thin-film resistors and high speed single-pole, double-throw switches. The simplified circuit diagram for the DAC section is shown in Figure 1. The three MSBs of the data word are decoded to drive the seven switches A-G. On the AD7242, the 9 LSBs switch a 9-bit R-2R ladder structure while on the AD7244, the 11 LSBs switch an 11-bit R-2R ladder structure. The output voltage from this converter has the same polarity as the reference voltage, REF IN. The REF IN voltage is internally buffered by a unity gain amplifier before being applied to the D/A converters and the bipolar bias circuitry. The D/A converter is configured and scaled for a 3 V reference, and the device is tested with 3 V applied to REF IN. Operating the AD7242/AD7244 at refer- ence voltages outside the ±5% tolerance range may result in degraded performance from the part. Figure 1. DAC Ladder Structure Internal Reference The on-chip reference is a temperature-compensated buried Zener reference that is factory trimmed for 3 V ± 10 mV. The reference can be used to provide both the reference voltage for the two D/A converters and the bipolar biasing circuitry. This is achieved by connecting REF OUT to REF INA and REF INB. The reference voltage can also be used for other components and is capable of providing up to 500 µA to an external load. The maximum recommended capacitance on the reference output pin for normal operation is 50 pF. If the reference output is required to drive a capacitive load greater than 50 pF, a 200 Ω resistor should be placed in series with the capacitive load. Decoupling the REF OUT pin with a series 200 Ω resistor and a parallel combination of a 10 µF tantalum capacitor and a 0.1 µF ceramic capacitor as in Figure 2 reduces the noise spectral density of the reference (see Figure 4). Using this decoupling scheme to generate the reference voltage for REF INA and REF INB gives a channel-to-channel isolation number of 110 dB (connecting REF OUT directly to REF INA and REF INB gives 80 dB). The channel-to-channel isolation is 110 dB using an external reference. External Reference In some applications, the user may require a system reference or some other external reference to drive the AD7242/AD7244 reference inputs. Figure 3 shows how the AD586 reference can be conditioned to provide the 3 V reference required by the AD7242/AD7244 reference inputs. Figure 2. Circuit Connection for REF OUT with an External Capacitive Load of Greater Than 50 pF Figure 3. AD586 Driving AD7242/AD7244 Reference Inputs |
Podobny numer części - AD7242JN |
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Podobny opis - AD7242JN |
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