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AD624 Datasheet(PDF) 8 Page - Analog Devices |
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AD624 Datasheet(HTML) 8 Page - Analog Devices |
8 / 15 page REV. C AD624 –8– directly proportional to gain i.e., input offset as measured at the output at G = 100 is 100 times greater than at G = 1. Output offset is independent of gain. At low gains, output offset drift is dominant, while at high gains input offset drift domi- nates. Therefore, the output offset voltage drift is normally specified as drift at G = 1 (where input effects are insignificant), while input offset voltage drift is given by drift specification at a high gain (where output offset effects are negligible). All input- related numbers are referred to the input (RTI) which is to say that the effect on the output is “G” times larger. Voltage offset vs. power supply is also specified at one or more gain settings and is also RTI. By separating these errors, one can evaluate the total error inde- pendent of the gain setting used. In a given gain configura- tion both errors can be combined to give a total error referred to the input (R.T.I.) or output (R.T.O.) by the following formula: Total Error R.T.I. = input error + (output error/gain) Total Error R.T.O. = (Gain × input error) + output error As an illustration, a typical AD624 might have a +250 µV out- put offset and a –50 µV input offset. In a unity gain configura- tion, the total output offset would be 200 µV or the sum of the two. At a gain of 100, the output offset would be –4.75 mV or: +250 µV + 100 (–50 µV) = –4.75 mV. The AD624 provides for both input and output offset adjust- ment. This optimizes nulling in very high precision applications and minimizes offset voltage effects in switched gain applica- tions. In such applications the input offset is adjusted first at the highest programmed gain, then the output offset is adjusted at G = 1. GAIN The AD624 includes high accuracy pretrimmed internal gain resistors. These allow for single connection program- ming of gains of 1, 100, 200 and 500. Additionally, a variety of gains including a pretrimmed gain of 1000 can be achieved through series and parallel combinations of the internal resis- tors. Table I shows the available gains and the appropriate pin connections and gain temperature coefficients. The gain values achieved via the combination of internal resistors are extremely useful. The temperature coefficient of the gain is dependent primarily on the mismatch of the temperature coefficients of the various internal resistors. Tracking of these resistors is extremely tight resulting in the low gain TCs shown in Table I. If the desired value of gain is not attainable using the inter- nal resistors, a single external resistor can be used to achieve any gain between 1 and 10,000. This resistor connected between AD624 G = 100 RG2 –VS OUTPUT SIGNAL COMMON VOUT 10k –INPUT RG1 G = 200 G = 500 +INPUT INPUT OFFSET NULL +VS Figure 28. Operating Connections for G = 200 Table I. Temperature Gain Coefficient Pin 3 (Nominal) (Nominal) to Pin Connect Pins 1 –0 ppm/ °C– – 100 –1.5 ppm/ °C13 – 125 –5 ppm/ °C 13 11 to 16 137 –5.5 ppm/ °C 13 11 to 12 186.5 –6.5 ppm/ °C 13 11 to 12 to 16 200 –3.5 ppm/ °C12 – 250 –5.5 ppm/ °C 12 11 to 13 333 –15 ppm/ °C 12 11 to 16 375 –0.5 ppm/ °C 12 13 to 16 500 –10 ppm/ °C11– 624 –5 ppm/ °C 11 13 to 16 688 –1.5 ppm/ °C 11 11 to 12; 13 to 16 831 +4 ppm/ °C 11 16 to 12 1000 0 ppm/ °C 11 16 to 12; 13 to 11 Pins 3 and 16 programs the gain according to the formula RG = 40 k G −1 (see Figure 29). For best results RG should be a precision resis- tor with a low temperature coefficient. An external RG affects both gain accuracy and gain drift due to the mismatch between it and the internal thin-film resistors R56 and R57. Gain accuracy is determined by the tolerance of the external RG and the absolute accuracy of the internal resistors ( ±20%). Gain drift is determined by the mismatch of the temperature coefficient of RG and the tem- perature coefficient of the internal resistors (–15 ppm/ °C typ), and the temperature coefficient of the internal interconnections. AD624 RG2 –VS REFERENCE VOUT –INPUT RG1 2.105k +INPUT +VS OR 1.5k 1k G = + 1 = 20 20% 40.000 2.105 Figure 29. Operating Connections for G = 20 The AD624 may also be configured to provide gain in the out- put stage. Figure 30 shows an H pad attenuator connected to the reference and sense lines of the AD624. The values of R1, R2 and R3 should be selected to be as low as possible to mini- mize the gain variation and reduction of CMRR. Varying R2 will precisely set the gain without affecting CMRR. CMRR is determined by the match of R1 and R3. AD624 G = 100 RG2 –VS VOUT –INPUT RG1 G = 200 G = 500 +INPUT +VS RL R3 6k R2 5k R1 6k (R2||20k ) + R1 + R3) (R2||20k ) G = (R1 + R2 + R3) || RL 2k Figure 30. Gain of 2500 |
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