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AD623AN Arkusz danych(PDF) 13 Page - Analog Devices |
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AD623AN Arkusz danych(HTML) 13 Page - Analog Devices |
13 / 16 page AD623 –13– REV. C are common mode (the same on both in amp inputs) and are not applied differentially. This second low pass network, R1+R2 and C3, has a –3 dB frequency equal to: 1/(2 π (R1+R2) (C3)). Using a C3 value of 0.047 µF as shown, the –3 dB signal BW of this circuit is approximately 400 Hz. The typical dc offset shift over frequency will be less than 1.5 µV and the circuit’s RF signal rejection will be better than 71 dB. The 3 dB signal band- width of this circuit may be increased to 900 Hz by reducing resistors R1 and R2 to 2.2 k Ω. The performance is similar to that using 4 k Ω resistors, except that the circuitry preceding the in amp must drive a lower impedance load. The circuit of Figure 43 should be built using a PC board with a ground plane on both sides. All component leads should be as short as possible. Resistors R1 and R2 can be common 1% metal film units but capacitors C1 and C2 need to be ±5% tolerance devices to avoid degrading the circuit’s common-mode rejection. Either the traditional 5% silver mica units or Panasonic ±2% PPS film capacitors are recommended. RG +VS –IN VOUT LOCATE C1–C3 AS CLOSE TO THE INPUT PINS AS POSSIBLE –VS 0.01 F REFERENCE AD623 0.33 F 0.01 F 0.33 F C3 0.047 F C2 1000pF 5% C1 1000pF 5% R2 4.02k 1% R1 4.02k 1% +IN Figure 43. Circuit to Attenuate RF Interference In many applications shielded cables are used to minimize noise; for best CMR over frequency the shield should be properly driven. Figure 44 shows an active guard drive that is configured to improve ac common-mode rejection by “bootstrapping” the capacitances of input cable shields, thus minimizing the capaci- tance mismatch between the inputs. RG 2 –INPUT +INPUT 100 AD623 VOUT AD8031 +VS REFERENCE –VS RG 2 Figure 44. Common-Mode Shield Driver GROUNDING Since the AD623 output voltage is developed with respect to the potential on the reference terminal, many grounding problems can be solved by simply by tying the REF pin to the appropri- ate “local ground.” The REF pin should, however, be tied to a low impedance point for optimal CMR. The use of ground planes is recommended to minimize the impedance of ground returns (and hence the size of dc errors). In order to isolate low level analog signals from a noisy digital environment, many data-acquisition components have separate analog and digital ground returns (Figure 45). All ground pins from mixed signal components such as analog-to-digital converters should be returned through the “high quality” analog ground DIGITAL POWER SUPPLY 0.1 F VIN1 VIN2 VDD AGND DGND AD7892-2 ADC 12 AGND VDD PROCESSOR 0.1 F 0.1 F 0.1 F AD623 ANALOG POWER SUPPLY +5V –5V GND GND +5V Figure 45. Optimal Grounding Practice for a Bipolar Supply Environment with Separate Analog and Digital Supplies VIN VDD AGND DGND AD7892-2 ADC 12 DGND VDD PROCESSOR 0.1 F AD623 0.1 F 0.1 F POWER SUPPLY +5V GND Figure 46. Optimal Ground Practice in a Single Supply Environment |
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