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AN314 Datasheet(PDF) 4 Page - Cirrus Logic |
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AN314 Datasheet(HTML) 4 Page - Cirrus Logic |
4 / 10 page ![]() 4 AN314REV1 AN314 selected input's voltage level and this charge will be dumped into the next input’s source when that switch closes. If an op-amp or high-impedance source is used for the input to the MUX, there could be an issue with settling when this charge is transferred through the switch. One of the functions of the anti-alias capacitor on the input side of the MUX in Figure 3 is to provide a reservoir of charge to the switch-channel capacitance. This is another reason why the on resistance of the MUX should be as low as possible – so the channel capacitance can be charged quickly. However, there is an inverse relationship between on resistance and channel capacitance because low on resis- tance requires larger transistors and larger transistors result in higher channel capacitance. The following table shows some examples of MUX devices along with their critical parameters. The devices grayed out are not recom- mended due to the parameters indicated. Table 1: Critical Parameters of Typical Multiplexers Notes: 1. Devices in gray are not recommended for this application due to the indicated parameters. 2. Parameters are typical values and are for comparative purposes only. 3. The manufacturer's data sheet should be referenced for min and max values The third parameter to be considered when selecting a MUX is the switching speed. What needs to be considered here is the total time from when the system selects another channel to when the conversion can begin. This includes MUX turn-on time and analog circuitry settling time. For example, if it is desired to convert with the CS556x at its maximum speed of 50 kSps (CONV held low), then from the time the RDY signal falls until the next conversion be- gins is 10 MCLKs (at 16 MHz) or 625 nanoseconds. This means that the MUX channel must change as RDY falls and its turn on time and all analog settling must occur in less than 625 nanoseconds. If a MUX requires 250 nanoseconds to turn on, there is only 325 nanoseconds for the analog circuitry to settle. If more settling time is required, the CONV pin timing can be delayed to slow down the conversion rate, allowing enough delay after the MUX channel is changed to ensure complete settling. The type of multiplexer selected will depend upon whether the converter is a single-ended-input or a differential- input ADC. The advantage of using a differential ADC rather than one with a single-ended input is an improvement Part Number # of Poles # of Inputs On/Off Speed ns On Res Ohms Off Cap pF On Cap pF Supply Voltage ADG706 1 16 50/14 2.5 13 200 1.8 to 5.5 ADG707 2 8 50/14 2.5 13 100 1.8 to 5.5 ADG787 2 2 22/6 2.5 16 40 1.8 to 5.5 ADG884 2 2 53/21 0.28 103 295 1.8 to 5.5 DG406 1 16 134/98 60 6 114 ±5 to ±20 DG407 2 8 134/98 60 6 57 ±5 to ±20 DG408 1 8 150/150 100 3 37 ±5 to ±20 DG409 2 4 150/150 100 3 25 ±5 to ±20 HI-506 1 16 250/250 180 10 62 ±15 HI-507 2 8 250/250 180 10 40 ±15 HI-508 1 8 250/250 180 10 27 ±15 HI-509 2 4 250/250 180 10 22 ±15 ISL43L220 2 2 12/5 0.23 115 224 1.1 to 4.5 MAX306 1 16 130/55 60 8 140 ±4.5 to ±20 MAX307 2 8 130/55 60 8 70 ±4.5 to ±20 MAX4617 1 8 7/4.5 8 5 32 2 to 5.5 MAX4618 2 4 7/4.5 8 5 21 2 to 5.5 MAX4635 2 2 12/5 2.5 9 32 1.8 to 5.5 |
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