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AD571JD Datasheet(PDF) 5 Page - Analog Devices |
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AD571JD Datasheet(HTML) 5 Page - Analog Devices |
5 / 8 page AD571 REV. A –5– NOTE: During a conversion transient currents from the analog common terminal will disturb the offset voltage. Capacitive de- coupling should not be used around the offset network. These transients will settle as appropriate during a conversion. Capaci- tive decoupling will “pump up” and fail to settle resulting in conversion errors. Power supply decoupling which returns to analog signal common should go to the signal input side of the resistive offset network. 0000000100 0000000011 0000000010 0000000001 0000000000 0V 10mV 30mV 50mV INPUT VOLTAGE OUTPUT CODE NORMAL CHARACTERISTICS REFERRED TO ANALOG COMMON 0000000100 0000000011 0000000010 0000000001 0000000000 0V 10mV 30mV 50mV INPUT VOLTAGE OUTPUT CODE OFFSET CHARACTERISTICS WITH 2.7 Ω IN SERIES WITH ANALOG COMMON Figure 8. AD571 Transfer Curve—Unipolar Operation (Approximate Bit Weights Shown for Illustration, Nominal Bit Weights 9.766 mV) BIPOLAR CONNECTION To obtain the bipolar –5 V to +5 V range with an offset binary output code the bipolar offset control pin is left open. A –5.0 volt signal will give a 10-bit code of 0000000000; an in- put of 0.00 volts results in an output code of 1000000000; +4.99 volts at the input yields 1111111111. The nominal trans- fer curve is shown in Figure 9. INPUT VOLTAGE – mV 0 –30 –20 –10 0 +10 +20 +30 10000 00000 01111 11111 01111 11110 10000 00010 10000 00001 OUTPUT CODE Figure 9. AD571 Transfer Curve—Bipolar Operation ZERO OFFSET The apparent zero point of the AD571 can be adjusted by inserting an offset voltage between the analog common of the device and the actual signal return or signal common. Figure 7 illustrates two methods of providing this offset. Figure 7a shows how the converter zero may be offset by up to ±3 bits to correct the device initial offset and/or input signal offsets. As shown, the circuit gives approximately symmetrical adjustment in unipolar mode. In bipolar mode R2 should be omitted to obtain a sym- metrical range. AD571 AIN ACOM INPUT SIGNAL R1 10 Ω R2 7.5k Ω R3 4.7k Ω R4 10k Ω SIGNAL COMMON +15V ZERO OFFSET ADJ ±3 BIT RANGE –15V Figure 7a. AD571 AIN ACOM INPUT SIGNAL SIGNAL COMMON R1 2.7 Ω OR 5 Ω POT 1/2 BIT ZERO OFFSET Figure 7b. Figure 8 shows the nominal transfer curve near zero for an AD571 in unipolar mode. The code transitions are at the edges of the nominal bit weights. In some applications it will be pref- erable to offset the code transitions so that they fall between the nominal bit weights, as shown in the offset characteristics. This offset can easily be accomplished as shown in Figure 7b. At bal- ance (after a conversion) approximately 2 mA flows into the analog common terminal. A 2.7 Ω resistor in series with this terminal will result in approximately the desired 1/2 bit offset of the transfer characteristics. The nominal 2 mA analog common current is not closely controlled in production. If high accuracy is required, a 5 Ω potentiometer (connected as a rheostat) can be used as R1. Additional negative offset range may be obtained by using larger values of R1. Of course, if the zero transition point is changed, the full-scale transition point will also move. Thus, if an offset of 1/2 LSB is introduced, full-scale trimming as described on previous page should be done with an analog in- put of 9.985 volts. |
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