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AD548BR датащи(PDF) 6 Page - Analog Devices |
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AD548BR датащи(HTML) 6 Page - Analog Devices |
6 / 8 page AD548 –6– REV. C Figure 22. Board Layout for Guarding Inputs INPUT PROTECTION The AD548 is guaranteed to withstand input voltages equal to the power supply potential. Exceeding the negative supply volt- age on either input will forward bias the substrate junction of the chip. The induced current may destroy the amplifier due to excess heat. Input protection is required in applications such as a flame detector in a gas chromatograph, where a very high potential may be applied to the input terminals during a sensor fault con- dition. Figure 23 shows a simple current limiting scheme that can be used. RPROTECT should be chosen such that the maxi- mum overload current is 1.0 mA (l00 k Ω for a 100 V overload, for example). Exceeding the negative common-mode range on either input terminal causes a phase reversal at the output, forcing the amplifier output to the corresponding high or low state. Exceed- ing the negative common-mode on both inputs simultaneously forces the output high. Exceeding the positive common-mode range on a single input doesn’t cause a phase reversal, but if both inputs exceed the limit the output will be forced high. In all cases, normal amplifier operation is resumed when input voltages are brought back within the common-mode range. Figure 23. Input Protection of IV Converter D/A CONVERTER OUTPUT BUFFER The circuit in Figure 24 shows the AD548 and AD7545 12-bit CMOS D/A converter in a unipolar binary configuration. VOUT will be equal to VREF attenuated by a factor depending on the digital word. VREF sets the full scale. Overall gain is trimmed by adjusting RIN. The AD548’s low input offset voltage, low drift and clean dynamics make it an attractive low power output buffer. The input offset voltage of the AD548 output amplifier results in an output error voltage. This error voltage equals the input offset voltage of the op amp times the noise gain of the amplifier. Figure 24. AD548 Used as DAC Output Amplifier That is: VOS Output =VOS Input 1+ RFB RO RFB is the feedback resistor for the op amp, which is internal to the DAC. RO is the DAC’s R-2R ladder output resistance. The value of RO is code dependent. This has the effect of changing the offset error voltage at the amplifier’s output. An output am- plifier with a sub millivolt input offset voltage is needed to preserve the linearity of the DAC’s transfer function. The AD548 in this configuration provides a 700 kHz small sig- nal bandwidth and 1.8 V/ µs typical slew rate. The 33 pF capaci- tor across the feedback resistor optimizes the circuit’s response. The oscilloscope photos in Figures 25 and 26 show small and large signal outputs of the circuit in Figure 24. Upper traces show the input signal VIN. Lower traces are the resulting output voltage with the DAC’s digital input set to all 1s. The AD548 settles to ±0.01% for a 20 V input step in 14 µs. 0% 10 5V 5µS 20V 100 90 Figure 25. Response to ±20 V p-p Reference Square Wave 0% 10 50mV 2µS 200mV 100 90 Figure 26. Response to ±100 mV p-p Reference Square Wave |
Аналогичный номер детали - AD548BR |
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Аналогичное описание - AD548BR |
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