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High-speed ADC front-end design challenges and trade-offs (a)

In Electronic Infomation Category: H | on April 12,2011

On ADC (ADC) front-end design, you must first declare that: it is an art. If not in the daily work of laboratory hands-on, pay no attention to the amplifier and LAN91C113-NU datasheet and transformer (balun) is the latest technology trends, then the front-end design, especially the high-frequency (> 100MHz IF) under the front-end design can be very difficult. Most designers will put the data sheet or application note design as a starting point, but compared with the designers really want to achieve, these designs provide the information may be incomplete. This article is not intended to give a high-speed ADC front-end design on a "formula", but to show, using optimal design of a transformer or amplifier are many factors to weigh. Converter topology and LAN91C113-NU price and there are many types, this paper is the sampling rate of 10MSPS or more buffered and LAN91C113-NU suppliers and unbuffered (switched capacitor) high-speed pipelined architecture. Receiver front-end is to determine the converter and the sampled signal or a key part of the quality of information. In the design, if not placed enough emphasis on this last level, the application will adversely affect the performance. By understanding the trade-off front-end design elements, designers can sample some or all of these methods to help develop the base band, band-pass (ie, super-Nyquist frequency), or broadband, high-performance front-end converter applications. This article is divided into two parts, the first introduces the basic theories and concepts; second part of the front-end design will be given specific design guidelines.

Understand the front-end to be achieved

First consider the converter front-end design needs to achieve what goals. No matter how this can not be overstated, because many lack the design considerations. The choice of most converter is based on sampling rate, full-power bandwidth, power, digital output topology, channel number and other relevant characteristics to suit specific applications. Most of the features which are considered the converter rated limits. For example, if the sampling rate exceeds the maximum sampling rate converter, it will adversely affect performance. Therefore, we assume that in all cases, the clock converter are the rated specifications and any other work within the specifications, the converter is not a limiting factor front-end design process. After

selected ADC, you must understand the conditions specified in the system design, design high-performance front-end basic elements need attention. It found that, for all converter front-end design, there are seven critical parameters are: input impedance, VSWR, passband flatness, bandwidth, SNR, SFDR and input drive level. When the designer to optimize the design trade-offs, these parameters can play a guiding role.

Input impedance is designed or rated load characteristic impedance. In most cases, it is 50. However, in some cases, may show a different design. Using a transformer, the input impedance of the load refers to the primary side of the transformer coupling network, including converters. Using the amplifier, the impedance of the load refers only to the amplifier input. Amplifier output match between the converter input is accomplished by other, often including anti-aliasing filter (AAF). In either case, you can use a different characteristic impedance load, and should match. The higher the bandwidth of the design, then the characteristics of the more important.

VSWR (Voltage Standing Wave Ratio) is a dimensionless parameter is reflected in the target bandwidth, how much power is reflected into the load. This parameter is also related to the input drive level. If the networks VSWR is high (> 1.5), the converter full-scale required to achieve the gain or drive capability is higher. Similarly, the design of the higher bandwidth (more losses), then the characteristics of the more important.

Passband flatness bandwidth is usually rated the content of the volatility of promise / ripple content. It can be a ripple effect or AAF filter roll-off characteristics. In any case, this parameter is usually expressed by dB (typically a few tenths of 1dB), which the target frequency range for the overall system gain setting is very important, see Figure 1.

Figure 1: Input drive level / pass-band flatness / bandwidth definitions

Bandwidth refers to the system used in the difference between starting and ending frequencies can narrow to wide. Bandwidth in the baseband (fsample / 2) or cover multiple Nyquist zones converter.

SNR (signal to noise ratio) required by the overall system noise level design decisions. In general, the higher the bandwidth of front-end design, the lower the SNR performance, because the design will be continuous sampling of unwanted broadband noise. Transformer or amplifier usually used between the converter to achieve the highest SNR AAF performance.

SFDR (spurious free dynamic range) required by the overall system dynamic range of decisions. Second and third harmonic distortion of the system is usually the biggest limiting factor. One must seriously look into is how to introduce, or both, if the converter itself out of the linearity, the dynamic range is severely restricted.

Input drive level and bandwidth, input impedance and VSWR characteristics. It is necessary to set specific application system gain, and highly dependent on the selected front-end devices, that is, transformers, amplifiers and AAF, which makes the drive level required to meet the requirements may be one of the most difficult.

The need to meet so many parameters, so the launch of a new design, all parameters will affect the design from different aspects. Weigh the various factors can sometimes be very difficult, overwhelming. One way is to use a spreadsheet or chart, RADAR chart is a good visualization tool, shown in Figure 2. In such maps, each parameter has its own axis. Designers have the flexibility to determine the proportion of each parameter, and to create a window in each axis. When all the design parameters are met, the closest to the center of the design will be the best choice.

Figure 2: RADAR chart example

Bandwidth priority

Start a new design, first need to decide is also the most important parameter is the bandwidth. Bandwidth for the design direction, guide the designer to open the path to success. There are essentially three types of front-end to choose from: base-band type, or super-Nyquist band-pass (sometimes referred to as narrow band) type and broadband type, shown in Figure 3. Choice of type depends on the specific application.

Figure 3: The base-band, band-pass and broadband

Bandwidth baseband design is from DC (or low-MHz area) to the converters Nyquist frequency. Expressed with the relative bandwidth, this means that about 100MHz or less. Such designs can be used amplifier or transformer (balun). Band-pass design means

converter bandwidth using only a small part of the band in high school, the only 20-60MHz bandwidth. For example, the center frequency can be as low as 100MHz. Today, in most cases the center frequency at 140MHz, 170MHz or 190MHz. However, the market shows the trend to a higher frequency. In essence, the designers simply use converter can be a small portion of bandwidth to complete the work. This design is usually a transformer or balun. However, if the SFDR performance of a high enough frequency, you can also use the amplifier.

Broadband design usually requires the full bandwidth of the design. Converter can provide much bandwidth, how much bandwidth is used. In the three designs, the widest bandwidth of this design, which is the most challenging front-end design. The bandwidth range of such applications DC (or low-MHz area) to + GHz region. Such design is often used broadband balun.

Before the next part of the discussion, I would like to talk about the bandwidth more points. Note that the full power converter bandwidth and available bandwidth converters are two concepts. Full power bandwidth refers to the rated based on the data sheet referred to in resolution and performance, the converter needed to accurately capture the signal bandwidth. It is usually far greater than the available bandwidth of the converter (probably the latter two times). Should be designed around the available bandwidth expansion. All design should avoid the use of rated full power bandwidth of a part or all of the highest frequency, or dynamic performance (SNR / SFDR) will decline, and become highly uncertain. To determine the bandwidth available converters, please refer to the manual or contact data application support. Typically, the data sheet will be provided to ensure the rated performance of the frequency, and even conducted a production test this.

Type of high-speed converter

Know the design of bandwidth, the next need to select the converter. There are essentially two types of high-speed converters are available: the buffer type and buffer type (switched capacitor). Although there are many different converters to choose from, but all applications are for this pipeline framework, because such a high sample rate converter with sufficient resolution and power consumption reasonable.

Common CMOS switched capacitor ADC input without built-in buffer, so the power consumption is much lower than the buffering. External front-end directly connected to the internal switched-capacitor ADC sample and hold (SHA) circuit. This has two problems. First, it will be in the sample and hold to switch between two modes, so the input impedance varies with frequency and mode. Second, from the internal sampling capacitor and the network will be a small amount of charge injection signal (mixed with the high frequency components, as shown in Figure 4) reflected back to the front-end design and input signals, which may cause the converter not connected to analog inputs source components to establish an error occurred. Generally speaking, when the frequency is low (

Figure 4: time-domain charge injection (single-ended) and frequency-domain charge injection

The differential input structure must, especially for frequency domain design. Differential front-end design to better conduct the charge into the common mode rejection, so that the design is not affected. For non-buffered input impedance of the converter, please refer to the converter data sheet or page. It may be in a separate table or data table below. If not, check with the manufacturer.

Buffered input converter is easier to design, the main downside is a higher power converter, because the buffer must be specially designed for high linearity and low noise characteristics. Input impedance is usually defined as a fixed differential R | | C resistance. It is buffered by a transistor-level, while the transistor level to low-impedance drive the conversion process, the charge injection spikes and switching transients decreased significantly. And switched-capacitor ADC, the input termination resistors in the analog input frequency range is almost unchanged, so the driver circuit design much easier. Figure 5 is buffered and unbuffered ADCs internal sample and hold circuit schematic diagram.

Figure 5: unbuffered and buffered ADC

Converter selection may be difficult, and now most of the designs have sought to lower power consumption, so designers often use unbuffered converter. High linearity performance is critical when power consumption is relatively unimportant, the commonly used buffer converter. It should be noted, the choice of what kind of converter, the design the higher the frequency, the front-end design more difficult. Select buffer alone converter can not solve all problems. However, in some cases, it may reduce the design complexity.

Amplifier or a transformer?

Know the bandwidth and converters, the next step should be selected front-end topology: Amplifiers (active) or transformers (passive). Despite their respective advantages and disadvantages of both, but also on the specific application, but hope the following points that help to get to the root. Amplifier will increase the noise front-end design and the need for power (power consumption). The advantage is that unlike the amplifier gain bandwidth associated with the same transformer, and generally have a fixed input and output impedance. In general, the available voltage gain bandwidth of transformer is much lower than 1:1. As for the amplifier, when using or in need of greater gain, bandwidth is only slightly reduced. In the passband region, the amplifier generally has better gain flatness. Transformer is not. Transformers are passive devices, without increasing the noise, do not consume power. However, the symmetry may challenge the transformer, causing spurious problems. Note that the transformer is far from ideal device. If used improperly, may reduce the parasitic effect of the performance of any design, especially at higher frequencies (+100 MHz) and voltage gain of the time when used.

Transformer using an amplifier, not the main reason is that the former can get a better pass-band flatness. If this feature is essential for the design, the amplifier produces a smaller change in the whole frequency range usually? 0.1 dB. Fluctuating response to the transformer, and if you must use the flatness is important, you may need to "fine-tuning." Another advantage of the amplifier is a good drive capability. Transformer is not suitable for driving long traces on the PCB, which is mainly used for directly connected with the converter. If asked to "drive / coupler" in a remote location or on a different board, it is strongly recommended to use amplifiers.

DC-coupled amplifiers may be used as a reason, because the transformer itself is AC coupled. Although Barron can be coupled DC, but not recommended Barron, as provided in the core may change the characteristics of the bias, resulting in front-end performance. If the DC is the application of an important part of the spectrum, then the current can be considered, including a number of AD8138 and ADA4937 amplifier and so on. Amplifier can provide dynamic isolation, about 30dB to 40dB reverse isolation in order to suppress non-converter input buffer in the current transient spikes caused by recoil. If the design requires broadband gain, then the amplifier and ADC analog input matching better than transformers. Another feature is the need of balancing the bandwidth and noise. Frequencies higher than 150MHz for the design of the transformer can better maintain the SNR and SFDR performance. However, in the first or second Nyquist zone, transformers and amplifiers are available.

Major consideration when choosing an amplifier are summarized as follows

Bandwidth: As mentioned above, if the bandwidth of the new design is very important, you should ensure that the amplifier has sufficient bandwidth, and it should be higher than the actual needs of the design bandwidth. Thus, the amplifier will be able to properly set up, to resolve the signal converter to sample information. If the front-end design of insufficient bandwidth, the amplifier will not be created correctly, this will cause signal errors. Minimum error signal amplifier to allow the amount of resolution of the converter should be selected decisions. Front-end design of the output signal swing is also an important factor. It determines whether the amplifier to meet the full-scale converter input range. Input range of high-speed converter is typically 2V peak to peak differential. Most amplifiers can meet this range, but there are other factors that may limit the choice of amplifier, such as linearity and margin and so on. Be sure to check the data sheet typical operating characteristics graph. For unbuffered converter, common mode range is very important. Converter common-mode range of the required level by the half-supply voltage (AVDD / 2) setting. Over the years, the converter supply voltage range has been reduced, it is now difficult to find common mode voltage specifications

Compared to amplifiers, transformers (balun) has many different characteristics. When designing choose this device should consider these characteristics. Voltage gain, impedance ratio, bandwidth and insertion loss, amplitude and phase imbalance, the return loss are some of the features. Other requirements may include power rating, configuration type (Barron or transformer, etc.) and the center tap option. Transformer design is not always straightforward. For example, transformer characteristics change with frequency, which will is expected to cast a shadow. Some of the transformer on the ground, and the center tap coupled layout sensitive. Do not complete the data sheet of transformer selected as the sole basis for the transformer. Experience can play a big role here.

Important consideration in choosing a transformer are summarized as follows

Ideally, the signal gain is equal to the transformer turns ratio. Although the voltage transformer or balun in the noise-free gain in itself, but use a transformer with a voltage gain would amplify the signal noise. Also may seriously affect the bandwidth. Simply as a transformer with a nominal gain of the wideband passband filter. Therefore, the greater the gain of the transformer, the lower the bandwidth, gain flatness characteristics of the design is also more difficult. Transformer voltage gain may vary greatly, as no gain, ripple and roll-off will be more pronounced. Today, it is difficult to find good GHz performance transformer impedance ratio of 1:4. In short, users should be vigilant, if you intend to use the 1:4,1:8, and 1:16 impedance ratio of the transformer to improve or optimize the signal chain, the last level of the noise figure should be well considered and verified in the laboratory . Selection and performance due to bandwidth restrictions, so its obvious drawbacks, the performance will not exceed 1:1 or 1:4 impedance ratio of the design. Transformer insertion loss means the loss within the specified frequency, the transformer data sheet is the most common measurement specification. Return loss for the primary, means the termination of the transformer secondary side does not match the effective impedance. For example, if the secondary and primary turns turns the square of the ratio of 4:1, when the secondary side impedance 200 termination, it should have 50 impedance will be reflected to the primary termination. However, this relationship is not accurate reflection of the original edge of the impedance change with frequency, the following example.

First, find the front-end design of the center frequency return loss. In this case, we use the 110MHz. For ideal transformer, the Zo is 50, but it is not. Can be seen from Equation 3, Zo is lower than the ideal value.

return loss (RL) = -18.9 dB @ 110MHz = 20 * log (50-Zo/50 + Zo) Formula 1

10 ^ (-18.9/20) = (50-Zo/50 + Zo) formula 2

Zo = 39.8 Formula 3 Solving Equation 3

then get the primary impedance Zo and the ideal ratio of the secondary. Impedance on the primary side and then the ideal and the actual impedance of the secondary ask the same ratio.

Z (the primary reflection impedance) / Z (secondary ideal impedance) = Z (primary ideal impedance) / Z (impedance of the secondary reflection) Formula 4

39.8/200 = 50 / X formula 5

Solving X,

X = 251 formula 6

General, with the impedance ratio increases, changes in return loss increases. Matching transformer or balun design using front-end should pay attention to this point.

On the transformer or balun, the amplitude and phase imbalance is the most important performance characteristics. They measure and the ideal of the single-ended signal the deviation, amplitude equal to phase difference of 180 degrees. When the design requires high-frequency (100MHz or more), the design may be based on these two technical specifications, understanding the signal provided to the converter linearity. In general, the deviation is larger, the greater the decline in performance. Start, be sure to select those in the data sheet published this information in the transformer or balun. If the data sheet does not exist in this information, it is likely that it is not suitable for high frequency applications. Remember, as the frequency increases, the nonlinear transformer also growth phase imbalance is usually the main converter into the even distortion (mainly second harmonic distortion). If the expected spurious features even close, do not rush to blame the converter, you should check the front-end design.

If the design uses a 1:4 or higher impedance than the transformer, should pay attention to this parameter at low frequencies will become more worse. This is because the transformer impedance ratio of 1:1 compared to the transformer primary will double the number of turns and secondary reference difference between the parasite becomes higher. For in-depth understanding of high-frequency phase and amplitude imbalance can affect the linearity of the transformer or balun performance, see reference 6.

Use in high frequency transformer or balun, in order to respond to the second harmonic distortion, you can try to use more than one transformer or balun cascade. You can use the two transformers (shown in Figure 6, in some cases you can use three) to help in the high frequencies will be more fully single-ended signals into differential signals. The disadvantage is that space, cost and insertion loss will increase. Another suggestion is to use other transformers. Transformer on the market better, for example, Anaren Inc (Anaren) has a patented design, which uses non-core topology, allowing only a single device to achieve bandwidth of Gigabit regional expansion to provide greater balance, and its size is less than standard iron core transformer.

Figure 6: Two-transformer topology.

Remember, not all manufacturers use the same method to specify the performance of the transformer, even if the size was similar to the operation of the transformer under the same situation may be different. Front-end design choices transformer is to collect and understand the best way to consideration all the specifications of the transformer, and obtain a copy of the manufacturer data sheet did not specify the other major items. In addition can also use the network analyzer to measure the performance of the transformer.

Use of multiple transformers, the last thing to note is that play an important role in the layout shown in Figure 7. To maintain optimal performance at high frequencies, the additional symmetry as far as possible the layout of the transformer. Otherwise, the use of multiple front-end design of the transformer may be useless.

Figure 7: Two-Barrons symmetrical layout (above) and non-symmetrical layout (below), the configuration of the schematic diagram shown in Figure 6.

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