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AD7869AQ Datasheet(PDF) 7 Page - Analog Devices |
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AD7869AQ Datasheet(HTML) 7 Page - Analog Devices |
7 / 16 page AD7869 –7– REV. A 500 200 100 50 20 10 50 100 200 1k 2k 10k 20k 100k FREQUENCY – Hz REF OUT OUTPUT WITH ALL 0s LOADED REF OUT DECOUPLED AS SHOWN IN FIGURE 2 TA = +25°C VDD = +5V VSS = –5V Figure 3. Noise Spectral Density vs. Frequency INPUT/OUTPUT TRANSFER FUNCTIONS A bipolar circuit for the AD7869 is shown in Figure 4. The analog input/output voltage range of the AD7869 is ±3 V. The designed code transitions for the ADC occur midway be- tween successive integer LSB values (i.e., 1/2 LSB, 3/2 LSB, 5/2 LSB . . . FS –3/2 LSBs). The input/output code is 2s Complement Binary with 1 LSB = FS/16384 = 366 µV. The ideal transfer function is shown in Figure 5. AD7869* RO ADC RI DAC AGND *ADDITIONAL PINS OMITTED FOR CLARITY VIN VOUT ANALOG OUTPUT RANGE = ±3V ANALOG INPUT RANGE = ±3V R1 200 C2 0.1µF C1 10µF Figure 4. Basic Bipolar Operation -FS 2 FS = 6V 1LSB = FS 16384 0V 011...111 011...110 000...010 000...001 000...000 111...111 111...110 100...001 100...000 INPUT VOLTAGE OUTPUT CODE 2 -1LSB FS + Figure 5. Input/Output Transfer Function OFFSET AND FULL SCALE ADJUSTMENT In most digital signal processing (DSP) applications, offset and full-scale errors have little or no effect on system performance. Offset error can always 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 have to be adjusted to zero. ADC ADJUSTMENT Figure 6 has signal conditioning at the input and output of the AD7869 for trimming the endpoints of the transfer functions of both the ADC and the DAC. Offset error must be adjusted be- fore full-scale error. For the ADC, this is achieved by trimming the offset of A1 while the input voltage, V1, is 1/2 LSB below ground. The trim procedure is as follows: apply a voltage of –183 µV (–1/2 LSB) at V1 in Figure 6 and adjust the offset volt- age of A1 until the ADC output code flickers between 11 1111 1111 1111 (3FFF HEX) and 00 0000 0000 0000 (0000 HEX). AD7869* *ADDITIONAL PINS OMITTED FOR CLARITY AGND A1 V1 INPUT VOLTAGE RANGE = ±3V R1 10k R2 500 R3 10k R5 10k R4 10k VIN VOUT A2 R6 10k R7 500 R8 10k R10 10k R9 10k V0 OUTPUT VOLTAGE RANGE = ± 3V Figure 6. AD7869 with Input/Output Adjustment ADC gain error can be adjusted at either the first code transi- tion (ADC negative full scale) or the last code transition (ADC positive full scale). The trim procedures for both cases are as follows (see Figure 6). ADC Positive Full-Scale Adjustment Apply a voltage of 2.99945 V (FS/2 – 3/2 LSBs) at V1. Adjust R2 until the ADC output code flickers between 01 1111 1111 1110 (1FFE HEX) and 01 1111 1111 1111 (1FFF HEX). ADC Negative Full-Scale Adjustment Apply a voltage of –2.99982 V (–FS/2 + 1/2 LSB) at V1 and ad- just R2 until the ADC output code flickers between 10 0000 0000 0000 (2000 HEX) and 10 0000 0000 0001 (2001 HEX). DAC ADJUSTMENT Op amp A2 is included in Figure 6 for the DAC transfer func- tion adjustment. Again, offset must be adjusted before full scale. To adjust offset, load the DAC with 00 0000 0000 0000 (0000 HEX) and trim the offset of A2 to 0 V. As with the ADC adjust- ment, gain error can be adjusted at either the first code transi- tion (DAC negative full scale) or the last code transition (DAC positive full scale). The trim procedures for both cases are as follows: DAC Positive Full-Scale Adjustment Load the DAC with 01 1111 1111 1111 (1FFF HEX) and ad- just R7 until the op amp output voltage is equal to 2.99963 V (FS/2 – 1 LSB). DAC Negative Full-Scale Adjustment Load the DAC with 10 0000 0000 0000 (2000 HEX) and adjust R7 until the op amp output voltage is equal to –3 V (–FS/2). |
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