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AD629A 데이터시트(PDF) 9 Page - Analog Devices |
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AD629A 데이터시트(HTML) 9 Page - Analog Devices |
9 / 12 page REV. A AD629 –9– PROCESSOR 0.1 F AD7892-2 ANALOG POWER SUPPLY +5V GND –5V +5V DIGITAL POWER SUPPLY VDD AGND DGND VIN1 VIN2 VOUT 12 VDD GND 0.1 F GND REF(–) –IN +IN –VS +VS REF(+) AD629 0.1 F 0.1 F Figure 31. Optimal Grounding Practice for a Bipolar Supply Environment with Separate Analog and Digital Supplies PROCESSOR 0.1 F POWER SUPPLY +5V GND VDD AGND DGND VIN VREF VOUT VDD GND 0.1 F REF(–) –IN +IN –VS +VS REF(+) AD629 0.1 F ADC Figure 32. Optimal Ground Practice in a Single Supply Environment If there is only a single power supply available, it must be shared by both digital and analog circuitry. Figure 32 shows how to minimize interference between the digital and analog circuitry. In this example, the ADC’s reference is used to drive the AD629’s REF(+) and REF(–) pins. This means that the reference must be capable of sourcing and sinking a current equal to VCM/ 200 k Ω. As in the previous case, separate analog and digital ground planes should be used (reasonably thick traces can be used as an alternative to a digital ground plane). These ground planes should be connected at the power supply’s ground pin. Separate traces (or power planes) should be run from the power supply to the supply pins of the digital and analog circuits. Ideally, each device should have its own power supply trace, but these can be shared by a number of devices as long as a single trace is not used to route current to both digital and analog circuitry. Using a Large Sense Resistor Insertion of a large shunt resistance across the input Pins 2 and 3 will imbalance the input resistor network, introducing a common- mode error. The magnitude of the error will depend on the common-mode voltage and the magnitude of RSHUNT. Table I Table II. Recommended Values for 2-Pole Butterworth Filter Corner Frequency R1 R2 C1 C2 Output Noise (p-p) No Filter 3.2 mV 50 kHz 2.94 k Ω ± 1% 1.58 k Ω ± 1% 2.2 nF ± 10% 1 nF ± 10% 1 mV 5 kHz 2.94 k Ω ± 1% 1.58 k Ω ± 1% 22 nF ± 10% 10 nF ± 10% 0.32 mV 500 Hz 2.94 k Ω ± 1% 1.58 k Ω ± 1% 220 nF ± 10% 0.1 µF ± 10% 100 µV 50 Hz 2.7 k Ω ± 10% 1.5 k Ω ± 10% 2.2 µF ± 20% 1 µF ± 20% 32 µV shows some sample error voltages generated by a common-mode voltage of 200 V dc with shunt resistors from 20 Ω to 2000 Ω. Assuming that the shunt resistor has been selected to utilize the full ±10 V output swing of the AD629, the error voltage becomes quite significant as RSHUNT increases. Table I. Error Resulting from Large Values of RSHUNT (Uncompensated Circuit) RS ( ) Error VOUT (V) Error Indicated (mA) 20 0.01 0.5 1000 0.498 0.498 2000 1 0.5 If it is desired to measure low current or current near zero in a high common-mode environment, an external resistor equal to the shunt resistor value may be added to the low impedance side of the shunt resistor as shown in Figure 33. VOUT REF(–) –IN +IN –VS NC +VS REF(+) AD629 380k 380k 380k 20k NC = NO CONNECT 0.1 F +VS RSHUNT 8 7 6 5 1 2 3 4 RCOMP –VS 0.1 F ISHUNT 21.1k Figure 33. Compensating for Large Sense Resistors Output Filtering A simple 2-pole low-pass Butterworth filter can be implemented using the OP177 at the output of the AD629 to limit noise at the output, as shown in Figure 34. Table II gives recommended component values for various corner frequencies, along with the peak-to-peak output noise for each case. VOUT REF(–) –IN +IN –VS NC +VS REF(+) AD629 380k 380k 380k 20k NC = NO CONNECT 0.1 F +VS 8 7 6 5 1 2 3 4 –VS 0.1 F 0.1 F 0.1 F +VS –VS C2 R2 C1 R1 OP177 21.1k Figure 34. Filtering of Output Noise Using a 2-Pole Butterworth Filter |
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