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AD1672 датащи(PDF) 11 Page - Analog Devices |
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AD1672 датащи(HTML) 11 Page - Analog Devices |
11 / 20 page AD1672 REV. 0 –11– REFERENCE INPUT The nominal reference input is 2.5 V, taken with respect to REFCOM. The reference input pin (REFIN) can be connected to the reference output pin (REFOUT) or a standard external +2.5 Volt reference can be selected to meet specific system re- quirements. Figure 17 shows the AD1672’s REFIN being driven from an external precision voltage reference such as the REF192. If an external reference is used, REFOUT can be ef- fectively disabled by tying it directly to the positive supply volt- age VCC. Note that if the reference is changed during a conversion, all three conversions in the pipeline will be invalidated. The nominal 2.5 V reference input is rescaled to 1.25 V via a resistor divider network as shown in Figure 18. The nominal resistance of this resistor network is 8 k Ω. Table IV summa- rizes various 2.5 V references for use with the AD1672 (see REFERENCE OUTPUT section). Table IV. Suitable 2.5 Volt References Drift Initial Accuracy (ppm/ C) % REF43B 10 (max) 0.1 REF192E 5 (max) 0.08 AD780B 3 (max) 0.04 0.1µF 10µF +VCC +VCC AD780 OUTPUT GND VS 1µF AD1672 REFIN REFOUT Figure 17. Circuit using AD780 Optional +2.5 V Reference Input Circuit REFERENCE NOISE REDUCTION Figure 18 shows the proper connection of two external 1 µF capacitors, CNR1 and CNR2, associated with the two noise re- duction nodes NCOMP1 and NCOMP2 (Pins 25 and 26) of the AD1672. Although these noise reduction capacitors, CNR, are not required for stability, the most optimum noise and distortion per- formance will be attained with a 1 µF ceramic capacitor connected from each Noise Reduction node to ACOM. CNR1 is associated with the output of the DAC amp and is required for optimum dis- tortion performance and SHA settling time. It provides a low source impedance for signal-dependent, transient currents asso- ciated with this node. CNR2 reduces the noise contribution from the reference buffer amplifier. CNR2 1µF 1µF CNR1 1µF NCOMP2 REFOUT REFIN AD1672 REFCOM NCOMP1 DAC AMP REF AMP 4k Ω 4k Ω 20 23 24 25 26 Figure 18. Reference Noise Reduction REFERENCE OUTPUT The AD1672 includes an onboard +2.5 V curvature compen- sated bandgap reference that has been laser trimmed for both absolute value and temperature drift. The reference must be externally compensated with a capacitor of 1 µF or greater from REFOUT to REFCOM. The reference is specified to provide an additional load current up to 500 µA. For large dynamic loads and/or external loads greater than 500 µA, an external buffer amplifier or pull-up re- sistor is required. If a pull-up resistor is used, its value should be selected such that REFOUT will not be required to sink any current under all operating conditions. Figure 19 illustrates the load regulation of the reference. The power supply rejection of the reference is better than 0.2 % of FSR at dc. LOAD CURRENT – µA 2.500 2.493 2.490 0 100 200 300 400 500 600 700 800 900 1000 2.499 2.494 2.492 2.491 2.498 2.496 2.497 2.495 Figure 19. Reference Output Voltage vs. Load Current DIGITAL OUTPUTS The AD1672 output data is presented in positive true straight binary for both the 0 V to 2.5 V and 0 V to 5 V unipolar input ranges and positive true offset binary for the ±2.5 V bipolar range. Table V indicates the output data formats for the three analog input ranges. Users requiring twos complement encoding for the ±2.5 V range may simply invert the MSB (Pin 13). Table V. Output Data Format Input Voltage Range +FS (V) –FS (V) LSB (mV) 0 to +2.5 +2.5 0.0 0.610 0 to +5.0 +5.0 0.0 1.22 –2.5 to +2.5 +2.5 –2.5 1.22 –FS+1/2 LSB –FS –FS –1/2 LSB +FS +FS –1/2 LSB OTR +FS –1 1/2 LSB OTR DATA OUTPUTS 1 1111 111 1111 0 1111 1111 1111 0 1111 1111 1110 0 0000 0000 0001 0 0000 0000 0000 1 0000 0000 0000 Figure 20. Output Data Format OBSOLETE |
Аналогичный номер детали - AD1672_15 |
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Аналогичное описание - AD1672_15 |
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