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AD621A Datasheet(PDF) 11 Page - Analog Devices |
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AD621A Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page AD621 REV. B –11– – + AD705 5V 3k 3k 3k 3k AD621B ADC REF IN AGND DIGITAL DATA OUTPUT 20k 10k 20k – + 0.6mA MAX 0.10mA 1.3mA MAX 1.7mA Figure 5. A Pressure Monitor Circuit which Operates on a 5 V Power Supply Pressure Measurement Although useful in many bridge applications such as weigh-scales, the AD621 is especially suited for higher resistance pressure sensors powered at lower voltages where small size and low power become more even significant. Figure 5 shows a 3 k Ω pressure transducer bridge powered from 5 V. In such a circuit, the bridge consumes only 1.7 mA. Adding the AD621 and a buffered voltage divider allows the signal to be conditioned for only 3.8 mA of total supply current. Small size and low cost make the AD621 especially attractive for voltage output pressure transducers. Since it delivers low noise and drift, it will also serve applications such as diagnostic non- invasion blood pressure measurement. Wide Dynamic Range Gain Block Suppresses Large Common- Mode and Offset Signals The AD621 is especially useful in wide dynamic range applica- tions such as those requiring the amplification of signals in the presence of large, unwanted common-mode signals or offsets. Many monolithic in amps achieve low total input drift and noise errors only at relatively high gains (~100). In contrast the AD621’s low output errors allow such performance at a gain of 10, thus allowing larger input signals and therefore greater dynamic range. The circuit of Figure 6 ( ± 15 V supply, G = 10) has only 2.5 µV/°C max. VOS drift and 0.55 µ/V p-p typical 0.1 Hz to 10 Hz noise, yet will amplify a ±0.5 V differential signal while suppressing a ±10 V common-mode signal, or it will amplify a ±1.25 V differential signal while suppressing a 1 V offset by use of the DAC driving the reference pin of the AD621. An added benefit, the offsetting DAC connected to the reference pin allows removal of a dc signal without the associated time-constant of ac coupling. Note the representations of a differential and common-mode signal shown in Figure 6 such that a single-ended (or normal mode) signal of 1 V would be composed of a 0.5 V common-mode component and a 1 V differential component. Table I. Make vs. Buy Error Budget AD621 Circuit Discrete Circuit Error, ppm of Full Scale Error Source Calculation Calculation AD621 Discrete ABSOLUTE ACCURACY at TA = +25 °C Input Offset Voltage, µV 125 µV/20 mV (150 µV × 2/20 mV 16,250 15,000 Output Offset Voltage, µV N/A ((150 µV × 2)/100)/20 mV N/A 12,150 Input Offset Current, nA 2 nA × 350 Ω/20 mV (6 nA × 350 Ω)/20 mV 12,118 121,53 CMR, dB 110 dB →3.16 ppm, × 5 V/20 mV (0.02% Match × 5 V)/20 mV 12,791 14,988 Total Absolute Error 17,558 20,191 DRIFT TO +85 °C Gain Drift, ppm/ °C 5 ppm × 60°C 100 ppm/ °C Track × 60°C 13,300 12,600 Input Offset Voltage Drift, µV/°C1 µV/°C × 60°C/20 mV (2.5 µV/°C × 2 × 60°C)/20 mV 13,000 15,000 Output Offset Voltage Drift, µV/°C N/A (2.5 µV/°C × 2 × 60°C)/100/20 mV N/A 12,150 Total Drift Error 13,690 15,750 RESOLUTION Gain Nonlinearity, ppm of Full Scale 40 ppm 40 ppm 12,140 12,140 Typ 0.1 Hz–10 Hz Voltage Noise, µV p-p 0.28 µV p-p/20 mV (0.38 µV p-p × √2)120 mV 121,14 12,127 Total Resolution Error 121,54 121,67 Grand Total Error 11,472 36,008 G = 100, VS = ± 15 V. (All errors are min/max and referred to input.) |
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