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AD7937AR Datasheet(PDF) 7 Page - Analog Devices |
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AD7937AR Datasheet(HTML) 7 Page - Analog Devices |
7 / 8 page AD7937 –7– REV. 0 APPLICATION HINTS Output Offset: CMOS D/A converters in circuits such as Fig- ures 4 and 5 exhibit a code-dependent output resistance which in turn can cause a code-dependent error voltage at the output of the amplifier. The maximum amplitude of this error, which adds to the D/A converter nonlinearity, depends on VOS, where VOS is the amplifier input offset voltage. To maintain specified operation, it is recommended that VOS be no greater than (25 10 –6) (V REF) over the temperature range of operation. Suitable op amps are the AD711C and its dual version, the AD712C. These op amps have a wide bandwidth and high slew rate and are recommended for wide bandwidth ac applications. AD711/AD712 settling time to 0.01% is typically 3 µs. Temperature Coefficients: The gain temperature coefficient of the AD7937 has a maximum value of 5 ppm/ °C and typical value of 1 ppm/ °C. This corresponds to worst case gain shifts of 2 LSBs and 0.4 LSBs respectively over a 100 °C temperature range. When trim resistors R1 (R3) and R2 (R4) are used to adjust full scale range as in Figure 4, the temperature coefficient of R1 (R3) and R2 (R4) should also be taken into account. High Frequency Considerations: AD7937 output capacitance works in conjunction with the amplifier feedback resistance to add a pole to the open-loop response. This can cause ringing or oscillation. Stability can be restored by adding a phase compen- sation capacitor in parallel with the feedback resistor. This is shown as C1 and C2 in Figures 4 and 5. Feedthrough: The dynamic performance of the AD7937 depends upon the gain and phase stability of the output amplifier, together with the optimum choice of PC board layout and decoupling components. MICROPROCESSOR INTERFACING The byte loading structure of the AD7937 makes it very easy to interface the device to any 8-bit microprocessor system. Figure 8 shows an example 8-bit interface between the AD7937 and a generic 8-bit microcontroller/DSP processor. Pins D7 to D0 of the processor are connected to pins D7 to D0 of the AD7937. When writing to the DACs, the lower 8 bits must be written first, followed by the upper four bits. The upper four bits should be output on data lines D0 to D3. *ADDITIONAL PINS OMITTED FOR CLARITY ADDRESS DECODE AD7937* D7 D0 CS UPD WR D7 D0 R/ W DATA BUS CONTROLLER/ DSP PROCESSOR CLR FROM SYSTEM RESET A0 A0 A1 A1 UPPER BITS OF ADDRESS BUS * Figure 8. AD7937 8-Bit Interface |
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