AN314 CIRRUS | Alldatasheet
Document overview
- Manufacturer or author: harrison bates
- PDF pages: 10
Technical content
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- REQUIREMENTS FOR MULTIP LEXING THE CS556X/7X/8X
For higher-throughput in multiplexed applications, it is desirable for the converter to fully settle on each conversion. In other words, the digital filter's output must accurately reflect the analog value on the input during the conversion. plexer and the ADC must fully settle to the new value before the sampling begins. Figure 2. Amplifier and Filter between Multiplexer and ADC ever, many SAR ADC data sheets warn that a certain "quiet" period should be observed to prevent coupling of noise. samples of the input signal during the conversion (see Figure 3). Figure 3. Multiplexer Timing Requirements another input. Figure 4 illustrates this arrangement. Analog Input must be settled here . settled within 10 MCLKs after RDY falls.
Figure 4. Amplifier(s) and Filter(s) Placed Before Multiplexer ple capacitor to a value very close to the input voltage us ing current from the supply pins rather than the input pin. changed, an amplifier with gain or attenuation can be used. inputs increase. For example, a 16- to-1 MUX will usually have a higher drai n capacitance than an 8-to-1 MUX.
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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 possib le – so the channel capacitance can be charged quickly. However, there is an inverse relationship between on resistance a nd channel capacitance because low on resis- tance requires larger transistors and la rger 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 exam ple, 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. Th is means that the MUX channel must change as RDY falls and its turn on time and all analog settling must oc cur 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 multiplexe r selected will depend upon whether the converter is a sing le-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
cuit would need to be designed using a low-noise op-amp such as illustrated in Figure 5. Figure 5. Single-ended-to-differential Conversion Circuit be used with a single-pole MUX.
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- SELECTING AN AMPLIFIER If an amplifier is needed to change the full-scale range or to buffer the inputs and it is placed between the MUX and the ADC, one of the most critical parameters will be settling time. However, as mentioned above, if the amplifier is placed before the MUX (one on each input of the MUX wher e needed) this requirement is greatly relaxed. In this- case, signal-to-noise, distortion, DC offset, and stability over temperature become the primary selection criteria for most applications. Each application will determine which parameters are most critical so it becomes difficult to spec- ify an op-amp and circuit configuration that will work in all situations. The CS3003 and CS3004 operational ampli- fiers will be suitable for many applic ations due to their excellent DC perf ormance, moderate bandwidth, and low noise. 6. OPTIMIZED CIRCUITS FOR MULT IPLEXING CS556X/7X/8X ADCs The following are examples of circuits that optimize the performance of the CS556x/7x/8x family of high-throughput ADCs for a number of general-purpose applications. Each circuit is followed by a brief circuit description with the key selection criteria noted.
Figure 6. 2-input, Single-ended-to-differential, Multiple Input Range, Multiplexed System ic input current when the converter samples.
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Figure 7. 8-input, Single-ended, Multiple Input Range, Multiplexed System a buffer. In order to maintain DC accuracy, the series resistance in the unbuffered inputs needs to be kept very low.
- CONCLUSION The CS556x/7x/8x family of devices offers an exceptiona l alternative to SAR devices in multiplexed applications. Due to their digital filter with single-conversion settling, the input to the ADC can change from one end of full scale to the other between conversions and st ill maintain full accu racy. Since delta-sigma co nverters have far superior differential non-linearity (DNL) specifications and excellent noise performance, they provide a performance upgrade path from conventional SAR ADCs. They also offer 24-bit resolution which is unattainable with SAR devices today.
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REVISION HISTORY
REV1 AUG 2007 Initial Release Contacting Cirrus Logic Support For all product questions and inquiries, contact a Cirrus Logic Sales Representative. To find the one nearest to you, go to http://www.cirrus.com IMPORTANT NOTICE Cirrus Logic, Inc. and its subsidiaries ("Cirrus") believe that the information contained in this document is accurate and reliable. However, the information is subject to change without notice and is provided "AS IS" without warranty of any kind (express or implied). Customers are advised to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgment, including those pertaining to warranty, indemnification, and limitation of liability. No responsibility is assumed by Cirrus for the use of this information, including use of this information as the basis for manufacture or sale of any items, or for infringement of patents or other rights of third parties. This document is the property of Cirrus and by furnishing this information, Cirrus grants no license, express or implied under any patents, mask work rights, copyrights, trademarks, trade secrets or other intellectual property rights. Cirrus owns the copyrights associated with the information contained herein and gives con- sent for copies to be made of the information only for use within your organization with respect to Cirrus integrated circuits or other products of Cirrus. This consent does not extend to other copying such as copying for general distribution, advertising or promotional purposes, or for creating any work for resale. CERTAIN APPLICATIONS USING SEMICONDUCTOR PRODUCTS MAY INVOLVE POTENTIAL RISKS OF DEATH, PERSONAL INJURY, OR SEVERE PROP- ERTY OR ENVIRONMENTAL DAMAGE (“CRITICAL APPLICATIONS”). CIRRUS PRODUCTS ARE NOT DESIGNED, AUTHORIZED OR WARRANTED FOR USE IN AIRCRAFT SYSTEMS, MILITARY APPLICATIONS, PRODUCTS SURGICALLY IMPLANTED INTO THE BODY, AUTOMOTIVE SAFETY OR SECURITY DE- VICES, LIFE SUPPORT PRODUCTS OR OTHER CRITICAL APPLICATIONS. INCLUSION OF CIRRUS PRODUCTS IN SUCH APPLICATIONS IS UNDERSTOOD TO BE FULLY AT THE CUSTOMER’S RISK AND CIRRUS DISCLAIMS AND MAKES NO WARRANTY, EXPRESS, STATUTORY OR IMPLIED, INCLUDING THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR PARTICULAR PURPOSE, WITH REGARD TO ANY CIRRUS PRODUCT THAT IS USED IN SUCH A MANNER. IF THE CUSTOMER OR CUSTOMER’S CUSTOMER USES OR PERMITS THE USE OF CIRRUS PRODUCTS IN CRITICAL APPLICA- TIONS, CUSTOMER AGREES, BY SUCH USE, TO FULLY INDEMNIFY CIRRUS, ITS OFFICERS, DIRECTORS, EMPLOYEES, DISTRIBUTORS AND OTHER AGENTS FROM ANY AND ALL LIABILITY, INCLUDING ATTORNEYS’ FEES AND COSTS, THAT MAY RESULT FROM OR ARISE IN CONNECTION WITH THESE USES. Cirrus Logic, Cirrus, and the Cirrus Logic logo designs are trademarks of Cirrus Logic, Inc. All other brand and product names in this document may be trademarks or service marks of their respective owners.