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AD625JN датащи(PDF) 11 Page - Analog Devices |
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AD625JN датащи(HTML) 11 Page - Analog Devices |
11 / 15 page AD625 REV. D –11– Offset voltage and offset voltage drift each have two compo- nents: input and output. Input offset is that component of offset that is generated at the input stage. Measured at the output it is directly proportional to gain, i.e., input offset as measured at the output at G = 100 is 100 times greater than that measured at G = 1. Output offset is generated at the output and is constant for all gains. The input offset and drift are multiplied by the gain, while the output terms are independent of gain, therefore, input errors dominate at high gains and output errors dominate at low gains. The output offset voltage (and drift) is normally specified at G = 1 (where input effects are insignificant), while input offset (and drift) is given at a high gain (where output effects are negli- gible). All input-related parameters are specified referred to the input (RTI) which is to say that the effect on the output is “G” times larger. Offset voltage vs. power supply is also specified as an RTI error. By separating these errors, one can evaluate the total error inde- pendent of the gain. For a given gain, both errors can be com- bined to give a total error referred to the input (RTI) or output (RTO) by the following formula: Total Error RTI = input error + (output error/gain) Total Error RTO = (Gain × input error) + output error The AD625 provides for both input and output offset voltage adjustment. This simplifies nulling in very high precision appli- cations and minimizes offset voltage effects in switched gain applications. In such applications the input offset is adjusted first at the highest programmed gain, then the output offset is adjusted at G = 1. If only a single null is desired, the input offset null should be used. The most additional drift when using only the input offset null is 0.9 µV/°C, RTO. COMMON-MODE REJECTION Common-mode rejection is a measure of the change in output voltage when both inputs are changed by equal amounts. These specifications are usually given for a full-range input voltage change and a specified source imbalance. In an instrumentation amplifier, degradation of common-mode rejection is caused by a differential phase shift due to differences in distributed stray capacitances. In many applications shielded cables are used to minimize noise. This technique can create AD625 +VS –VS RF RG RF SENSE REFERENCE AD711 VOUT +INPUT –INPUT 100 Figure 32. Common-Mode Shield Driver common-mode rejection errors unless the shield is properly driven. Figures 32 and 33 show active data guards which are configured to improve ac common-mode rejection by “boot- strapping” the capacitances of the input cabling, thus minimiz- ing differential phase shift. AD625 +VS –VS RF RG RF AD712 100 100 VOUT SENSE REFERENCE –INPUT +INPUT –VS Figure 33. Differential Shield Driver GROUNDING In order to isolate low level analog signals from a noisy digital environment, many data-acquisition components have two or more ground pins. These grounds must eventually be tied to- gether at one point. It would be convenient to use a single ground line, however, current through ground wires and pc runs of the circuit card can cause hundreds of millivolts of error. Therefore, separate ground returns should be provided to mini- mize the current flow from the sensitive points to the system ground (see Figure 34). Since the AD625 output voltage is developed with respect to the potential on the reference termi- nal, it can solve many grounding problems. AD625 AD7502 –VS +VS –VS +VS AD583 SAMPLE AND HOLD HOLD CAP –VS +VS INPUT SIGNAL STATUS ANALOG OUT –VS +VS DIGITAL COMMON VLOGIC ANALOG POWER GROUND AD574A A/D CONVERTER Figure 34. Basic Grounding Practice for a Data Acquisition System |
Аналогичный номер детали - AD625JN |
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Аналогичное описание - AD625JN |
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