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AD670AD датащи(PDF) 7 Page - Analog Devices |
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AD670AD датащи(HTML) 7 Page - Analog Devices |
7 / 13 page AD670 REV. A –6– Table I. AD670 Input Selection/Output Format Truth Table INPUT RANGE/ BPO/UPO FORMAT OUTPUT FORMAT 0 0 Unipolar/Straight Binary 1 0 Bipolar/Offset Binary 0 1 Unipolar/2s Complement 1 1 Bipolar/2s Complement DIFF STRAIGHT BINARY +VIN –VIN VIN (FORMAT = 0, BPO/UPO = 0) 0 0 0 0000 0000 128 mV 0 128 mV 1000 0000 255 mV 0 255 mV 1111 1111 255 mV 255 mV 0 0000 0000 128 mV 127 mV 1 mV 0000 0001 128 mV –127 mV 255 mV 1111 1111 Figure 5a. Unipolar Output Codes (Low Range) OFFSET BINARY 2s COMPLEMENT DIFF (FORMAT = 0, (FORMAT = 1, +VIN –VIN VIN BPO/UPO = 1) BPO/UPO = 1) 0 0 0 1000 0000 0000 0000 127 mV 0 127 mV 1111 1111 0111 1111 1.127 V 1.000 V 127 mV 1111 1111 0111 1111 255 mV 255 mV 0 1000 0000 0000 0000 128 mV 127 mV 1 mV 1000 0001 0000 0001 127 mV 128 mV –1 mV 0111 1111 1111 1111 127 mV 255 mV –128 mV 0000 0000 1000 0000 –128 mV 0 –128 mV 0000 0000 1000 0000 Figure 5b. Bipolar Output Codes (Low Range) Calibration Because of its precise factory calibration, the AD670 is intended to be operated without user trims for gun and offset; therefore, no provisions have been made for such user trims. Figures 6a, 6b, and 6c show the transfer curves at zero and full scale for the unipolar and bipolar modes. The code transitions are positioned so that the desired value is centered at that code. The first LSB transition for the unipolar mode occurs for an input of +1/2 LSB (5 mV or 0.5 mV). Similarly, the MSB transition for the bipolar mode is set at –1/2 LSB (–5 mV or –0.5 mV). The full scale transition is located at the full scale value –1 1/2 LSB. These values are 2.545 V and 254.5 mV. 6a. Unipolar Transfer Curve Figure 4a. CMRR Over Frequency Figure 4b. AD670 Input Rejects Common-Mode Ground Noise Good common-mode performance is useful in a number of situ- ations. In bridge-type transducer applications, such performance facilitates the recovery of differential analog signals in the pres- ence of a dc common-mode or a noisy electrical environment. High frequency CMRR also becomes important when the ana- log signal is referred to a noisy, remote digital ground. In each case, the CMRR specification of the AD670 allows the integrity of the input signal to be preserved. The AD670’s common-mode voltage tolerance allows great flexibility in circuit layout. Most other A/D converters require the establishment of one point as the analog reference point. This is necessary in order to minimize the effects of parasitic voltages. The AD670, however, eliminates the need to make the analog ground reference point and A/D analog ground one and the same. Instead, a system such as that shown in Figure 4b is possible as a result of the AD670’s common-mode performance. The resistors and inductors in the ground return represent un- avoidable system parasitic impedances. Input/Output Options Data output coding (2s complement vs. straight binary) is selected using Pin 12, the FORMAT pin. The selection of input format (bipolar vs. unipolar) is controlled using Pin 11, BPO/UPO. Prior to a write/convert, the state of FORMAT and BPO/UPO should be available to the converter. These lines may be tied to the data bus and may be changed with each conver- sion if desired. The configurations are shown in Table I. Output coding for representative signals in each of these configurations is shown in Figure 5. An output signal, STATUS, indicates the status of the conver- sion. STATUS goes high at the beginning of the conversion and returns low when the conversion cycle has been completed. |
Аналогичный номер детали - AD670AD |
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Аналогичное описание - AD670AD |
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