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AD7886BD Datasheet(PDF) 7 Page - Analog Devices |
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AD7886BD Datasheet(HTML) 7 Page - Analog Devices |
7 / 16 page AD7886 –7– REV. B OFFSET AND GAIN ADJUSTMENT In most digital signal processing (DSP) applications, offset and full-scale errors have little or no effect on system performance. Offset error can usually be eliminated in the analog domain by ac coupling. Full-scale errors do not cause problems as long as the input signal is within the full dynamic range of the ADC. For applications requiring that the input signal range match the full analog input dynamic range of the ADC, offset and full- scale errors must be adjusted to zero. UNIPOLAR OFFSET AND GAIN ERROR ADJUSTMENT If absolute accuracy is an application requirement, offset and gain can be adjusted to zero. Offset error must be adjusted be- fore gain error. Zero offset is achieved by adjusting the offset of the op amp driving the analog input (i.e., A1 in Figure 6). For zero offset error, apply a voltage of 1 LSB to AIN and adjust the op amp offset until the ADC output code flickers between 0000 0000 0000 and 0000 0000 0001. 0 V to 5 V Range: 1 LSB = 1.22 mV 0 V to 10 V Range: 1 LSB = 2.44 mV For zero gain, error apply an analog input voltage equal to FS–1 LSB (last code transition) at AIN and adjust R3 until the ADC output code flickers between 1111 1111 1110 and 1111 1111 1111. 0 V to 5 V Range: FS–1 LSB = 4.99878 V 0 V to 10 V Range: FS–1 LSB = 9.99756 V AD7886* SUM + – AD586 VOUT +V IN GND +V C1 10 µF C2 0.1 µF AD707 VIN1 VIN2** AGND V SS V DD AIN R1 82k R2 56k R3 5k 0 TO 5V OR 0 TO 10V – + AD845 A1 5V + 5REF + 5V + 3.5V – V REF 5V – *ADDITIONAL PINS OMITTED FOR CLARITY **0 TO 5V RANGE: CONNECT VIN2 TO VIN1 0 TO 10V RANGE: CONNECT VIN2 TO AGND Figure 6. Unipolar Operation with Gain Error Adjust BIPOLAR OPERATION Bipolar operation is achieved by providing a +10 V span on the VIN1 input while offsetting the VIN2 input by +5 V. A typical circuit is shown in Figure 7. The output code is off- set binary. The ideal input/output transfer characteristic is shown in Figure 8. The LSB size is (10/4096) V = 2.44 mV. AD7886* SUM + – AD586 V OUT +V IN GND +V C1 10 µF C2 0.1 µF AD707 VIN1 VIN2 AGND V SS V DD AIN *ADDITIONAL PINS OMITTED FOR CLARITY 5V + 5REF + V REF 5V ± 5V + 5V – 3.5V – Figure 7. Bipolar Operation 00...000 00...001 01...101 01...110 11...111 11...110 11...101 10...000 10...001 10...010 01...111 FS = 10V 1LSB = FS 4096 VIN, INPUT VOLTAGE – LSBs OUTPUT CODE 1LSB – +1LSB – 1LSB 2 FS + 2 FS – +1LSB Figure 8. Ideal Input/Output Characteristics for Bipolar Operation |
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