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AD8264 Datasheet(PDF) 29 Page - Analog Devices |
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AD8264 Datasheet(HTML) 29 Page - Analog Devices |
29 / 40 page Data Sheet AD8264 Rev. B | Page 29 of 40 IPPx VPOS IPNx VOLx VOHx INTERPOLATOR PREAMP 6dB (2×) OFSx OPPx GAIN INTERFACE 100Ω 747Ω 107Ω 100Ω + – VOCM COMM GNHx GNLO BIAS VNEG NONINVERTING AMPLIFIER INPUT INVERTING AMPLIFIER INPUT (NOT USED) POWER SUPPLIES VGAx SINGLE-ENDED HS VGA OUTPUT DIFFERENTIAL VGA OUTPUT OFFSET ADJUST OUTPUT COMMON-MODE VOLTAGE ADJUSTMENT DIFFERENTIAL GAIN CONTROL INPUTS 1.2V p-p MAX @ ±2.5V 2V p-p MAX @ ±3.5V TO ±3.3V 3V p-p MAX@ ±5V (PREAMP DRIVE LIMITED) 2.3nV/√Hz DIFFERENTIAL OUTPUT NEVER LIMITS BECAUSE VGA LIMITS FIRST. DIFFERENTIAL OUTPUT SWING = 2x VGA OUT 5.2V p-p MAX @ ±2.5V 8V p-p MAX @ ±3.5V TO ±3.3V 15V p-p MAX @ ±5V 73nV/√Hz PREAMP OUTPUT (NOT USED) 2.6V p-p MAX @ ±2.5V 4V p-p MAX @ ±3.5V TO ±3.3V 7.5V p-p MAX @ ±5V 34nV/√Hz COMPOSITE GAIN IS +6dB TO +30dB ATTENUATOR –24dB TO 0dB FIXED GAIN VGA AMPLIFIER 18dB (8×) DIFFERENTIAL OUTPUT AMPLIFIER 6dB (2×) 1kΩ 2kΩ 1kΩ 2kΩ 1 1 2 2 3 3 Figure 111. Single-Channel Block Diagram POST AMPLIFIER From the preamp input to the VGA output (VGAx), the gain is noninverting. As can be seen in Figure 111, the VGAx pins drive the positive input of the differential amplifier. The gain is inverting from the input of the preamp to the output pin at VOLx, and the gain is noninverting to the output VOHx. Other than the input from VGAx, each differential amplifier has two additional inputs: VOCM and OFSx. A common VOCM pin is shared among all four postamplifiers, while separate OFSx pins are provided for each channel. VOCM Pin The VOCM pin sets the common-mode voltage of the differential output and must be biased by an external voltage. When driving a dc-coupled ADC, the voltage typically comes from the ADC reference, as shown in the Applications Information section. If dc level shift is not necessary, the VOCM pin is connected to ground. OFSx Pins The OFSx pins are the inverting inputs of the differential post amplifiers and can be used to prebias a differential dc offset at the output. This is very useful when the input is a unipolar pulse because the user can set up the gain and the offset in such a way as to optimally map a unipolar pulse into the full-scale input of an ADC, while dc coupling throughout. If dc offset is not desired, then connect the OFSx pins to ground. However, the OFSx pins can also be used as separate inputs if the user wants this function. NOISE At maximum gain, the preamplifier is the primary contributor of noise and results in a differential output referred noise of roughly 73 nV/Hz. The noise at the VGAx outputs is 34 nV/Hz, and because of the gain-of-2, the VGA output noise is amplified by 6 dB to 68 nV/Hz. The differential amplifier, including the gain setting resistors, contributes another 26 nV/Hz, and the rms sum results in a total noise of 73 nV/Hz. At the lowest gain, the noise at the VGA output is approximately 19 nV/Hz, and when multiplied by two, it results in 38 nV/Hz at the differential output; again, rms summing this with the 26 nV/Hz of the differential amplifier causes the total output referred noise to be approximately 46 nV/Hz. The input referred noise to the preamplifier at maximum gain is 2.3 nV/Hz and increases with decreasing gain. Note that all noise numbers include the necessary gain setting resistors. |
Número de pieza similar - AD8264_16 |
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Descripción similar - AD8264_16 |
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