Wideband A/D Converter Front-End Design Considerations: Amplifier or Transformer Drive for the ADC?
By Rob Reeder and Jim Caserta, Analog Devices

Design of the input configuration, or “front end,” ahead of a high-performance analog-to-digital converter (ADC) is critical to achieving desired system performance. Optimizing the overall design depends on many factors, including the nature of the application, system partition, and ADC architecture. The following questions and answers highlight the important practical considerations affecting the design of an ADC front end using amplifier and transformer circuitry.

Q. What is the fundamental difference between amplifiers and transformers?
A. An amplifier is an active element, while a transformer is passive. Amplifiers, like all active elements, consume power and add noise; transformers consume no power and add negligible noise. Both have dynamic effects to be dealt with.

Q. Why would you use an amplifier?
A. Amplifier performance has fewer limitations than those of transformers. If dc levels must be preserved, an amplifier must be used, because transformers are inherently ac-coupled devices. On the other hand, transformers provide galvanic isolation if needed. Amplifiers provide gain more easily because their output impedance is essentially independent of gain. On the other hand, a transformer’s output impedance increases with the square of the voltage gain– which depends on the turns ratio. Amplifiers provide flatter response in the pass band, free of the ripple due to the parasitic interactions in transformers.

Q. How much noise does an amplifier typically add, and what can I do to reduce this?
A. A typical amplifier that might be considered, the ADA4937, for example, when configured for G = 1, has an output noise spectral density of 5.7 nV/v Hz at high frequencies, compared to the 11.6-nV/vHz input noise spectral density of the 80-MSPS AD9446-80 ADC. The problem here is that the amplifier has a noise bandwidth equivalent to the full bandwidth of the ADC, around 500 MHz, while the ADC noise is folded to one Nyquist zone (40 MHz). Without a filter, the integrated noise then becomes 128 µV rms for the amplifier and 92 µV rms for the ADC. Mathematically, this degrades the overall system SNR (signal-to-noise ratio) by 4.7 dB. To confirm this experimentally, the measured SNR, with the ADA4937 driving the AD9446-80, is 75.6 dBFS, and the noise floor is -117.7 dB. With a transformer drive, the SNR is 80.7 dBFS. The driver amplifier has thus degraded the SNR by 5 dB.

To make better use of the ADC’s SNR, a filter is inserted between the amplifier and ADC. With a 100-MHz two-pole filter, the amplifier’s integrated noise becomes 71 µV rms, degrading the ADC’s SNR by only 1.7 dB. Use of the 2-pole filter improves the SNR performance of the circuit to 79 dBFS, with a noise floor of -121.2 dB. The 2-pole filter is built with 24-O resistors and 30-nH inductors in series with each of the amplifier’s outputs, and a 47-pF differentially connected capacitor.

Q. How do high-speed amplifiers and ADCs compare in power consumption?
A. This depends on the amplifier and ADC used. Two typical amplifiers, with similar power consumption, are the AD8352, which draws 37 mA @ 5 V (185 mW), and the ADA4937, which draws 40 mA @ 5 V (200 mW). Overall power consumption can be reduced by about one-third, with slightly degraded performance, by using a 3.3-V supply. ADCs feature more diversity in power consumption, depending on resolution and speed. The 16-bit, 80-MSPS AD9446-80 draws 2.4 W, the 14-bit, 125-MSPS AD9246-125 draws 415 mW, and the 12-bit, 20-MSPS AD9235-20 draws only 95 mW.

Q. When do you need to use a transformer?
A. Transformers offer the biggest performance advantage compared to amplifiers at very high signal frequencies and when significant additional noise cannot be tolerated at the ADC input.

Q. How do transformers and amplifiers differ when providing gain?
A. The main difference is in the impedance they present to the ADC input, which directly affects system bandwidth. A transformer’s input and output impedance are related by the square of the turns ratio, while an amplifier’s input and output impedance are essentially independent of gain.

For example, when a G = 2 transformer is used from a 50-O source impedance, the impedance seen at the secondary side of the transformer is 200 O. The AD9246 ADC has a differential input capacitance of 4 pF which, coupled with the 200-O transformer impedance, reduces the ADC’s -3-dB bandwidth from 650 MHz to 200 MHz. Extra series resistance and differential capacitance are often needed to improve performance and reduce kickback from the converter, which can limit -3-dB bandwidth further, possibly to 100 MHz.

When a low-output-impedance amplifier–such as the ADA4937–is used, the result is a very low source impedance, usually less than 5 O. 25-O transient-limiting resistors can be used in series with each ADC input; in the case of the AD9246, the ADC’s full 650-MHz analog input bandwidth would be usable.

So far the discussion has been about -3-dB bandwidths. When tighter flatness is needed, say 0.5 dB in a 1-pole system, the -3-dB bandwidth needs to be about 3X wider. For 0.1-dB flatness with one pole, the ratio increases to 6.5X. If 0.5-dB flatness is required at up to 150 MHz, a -3-dB bandwidth greater than 450 MHz is required, which is difficult to attain with a G = 2 transformer but is straightforward with a low-output-impedance amplifier.

Q. What are the factors to consider in choosing a transformer or an amplifier to drive an ADC?
A. They can be boiled down to a half-dozen parameters, outlined in this table:

Applications in which key parameters are in conflict require additional analysis and trade-offs.

Q. Can you show me some examples of transformer and amplifier drive circuits?
A. Figure 1 shows four examples of ADC input configurations using a transformer.
In baseband applications (a), the input impedance is much higher, so the match is more straightforward than, and not as critical as, the match at higher frequencies. Usually, small-value series resistors will suffice to damp out the charge injection with a differentially connected capacitor. This simple filter attenuates the broadband noise, achieving optimal performance.

In order to get a well-matched input in broadband applications (b), try to make the input’s real (resistive) component predominate. Minimize the capacitive terms with inductors or ferrite beads in shunt or series with the analog front end. This can yield good bandwidth, improve gain flatness, and provide better performance (SFDR), as seen using the AD92xx switched-capacitor ADC family.

For buffered high-IF applications (c), a double-balun configuration is shown, with a filter similar to the baseband configuration. This allows inputs of up to 300 MHz and provides good balancing to minimize even-order distortions.

For narrow-band (resonant) applications (d), the topology is similar to broadband. However, the match is in shunt instead of series, to narrow the bandwidth to the frequency specified.

When using an amplifier with a buffered or unbuffered ADC in baseband applications, the design is fairly straightforward (Figure 2). Just make sure that the common-mode voltage of the amplifier is shared with the ADC, and use a simple low-pass filter to get rid of the unwanted broadband noise (a). For IF applications (b and c), the matching network is essentially similar to that in baseband, but usually has shallower roll-off. Inductors or ferrite beads can be used on the outputs of the amplifier to help extend the bandwidth if needed. This is not always necessary, however, because the amplifier’s characteristics are less prone to changing over the band of interest than those of transformers. For narrow-band or resonant applications (d), the filter is matched to the output impedance of the amplifier to cancel the input capacitance of the ADC. Usually a multipole filter is used to get rid of broadband noise outside the frequency region of interest.

Q. Would you summarize the important points?
A. When facing a new design, remember to:
• Understand the level of design difficulty
• Rank the important parameters in your design
• Include the ADC input impedance and the external components in the input circuit
  when determining the total load on the transformer or amplifier

When choosing a transformer, always remember:
• Not all transformers are created equal
• Understand transformer specifications
• Ask the manufacturer for parameters that are not given, and/or do modeling
• High-IF designs are sensitive to transformer phase imbalance
• Two transformers or baluns may be needed for very high-IF designs to suppress
  the even-order distortions

When choosing an amplifier, always remember:
• Note the noise specification
• Understand amplifier specifications
• For low-IF or baseband frequencies, use the AD8138/AD8139
• For mid-IF, use the ADA4937
• For high-IF designs, use the AD8352
• Amplifiers are less sensitive to imbalance and automatically suppress even-order distortions
• Some amplifiers can dc-couple to the ADC’s input, e.g., the AD8138/AD8139 and
  ADA4937/ADA4938
• Amplifiers inherently isolate the input source from output loading effects and can
  therefore be more useful than a transformer for dealing with sensitive input sources
• Amplifiers can drive long distances and are especially useful when system partition
  dictates two or more boards in a design
• Amplifiers may require another supply domain and will always add to system power
  requirements

When choosing an ADC, always remember:
• Is the ADC internally buffered?
• Switched-capacitor ADCs have a time-varying input impedance and are more difficult to
  design with at high-IFs
• If using an unbuffered ADC, always input-match in the track mode
• Buffered ADCs are easier to design with, even at high IFs
• Buffered ADCs tend to burn more power

Finally:
• Baseband designs are the easiest with either ADC type
• Use ferrite beads or low-Q inductors to tune out the input capacitance on
  switched-capacitor ADCs. This maximizes input bandwidth, creates a better input
  match, and maintains SFDR
• Two transformers may be needed to deal with high IFs

Q. How about some references for further reading?
A. Application Notes
AN-742, Frequency-Domain Response of Switched-Capacitor ADCs.
AN-827, A Resonant Approach to Interfacing Amplifiers to Switched-Capacitor ADCs.
ADC Switched-Capacitor Input Impedance Data (S-parameters) for AD9215, AD9226, AD9235, AD9236, AD9237, AD9244, AD9245. Go to their web pages, click on Evaluation Boards, upload Microsoft Excel spreadsheet.

B. Papers
Reeder, Rob. “Transformer-Coupled Front-End for Wideband A/D Converters.”. Analog Dialogue 39-2. 2005. pp. 3-6.
Reeder, Rob. Mark Looney, and Jim Hand. “Pushing the State of the Art with Multichannel A/D Converters.” Analog Dialogue 39-2. 2005. pp. 7-10.
Kester, Walt. “Which ADC Architecture is Right for Your Application?” Analog Dialogue 39-2. 2005. pp. 11-18.
Reeder, Rob and Ramya Ramachandran. “Wideband A/D Converter Front-End Design Considerations– When to Use a Double Transformer Configuration.” Analog Dialogue 40-3. 2006. pp. 19-22.

C. Informative Data Sheets
AD9246, 80-/105-/125-MSPS 14-Bit, 1.8-V, Switched-Capacitor ADC
AD9445 105-/125-MSPS 14-Bit, 5-/3.3-V, Buffered ADC
AD9446 16-Bit 80-/100-MSPS Buffered ADC
AD8138 Low-Distortion Differential ADC Driver
AD8139 Ultralow Noise Fully Differential ADC Driver
AD8350 1.0-GHz Differential Amplifier
AD8351 Low-Distortion Fully Differential RF/IF Amplifier
AD8352 2-GHz Ultralow Distortion Differential RF/IF Amplifier
ADA4937
ADA4938

About the Authors
Jim Caserta is a design engineer in the Analog Semiconductor Components Division Technology group. He received a BS and MS degrees in Electrical Engineering from the University of Florida. From 1998-2000 he was employed at Motorola Semiconductor Product Sector (now Freescale) as a mixed-signal designer, and from 2002-2003 was employed by the University of Florida Department of Radiology as an RF engineer. He holds one patent and has authored or co-authored about 10 journals and conference publications.

Rob Reeder is a Senior Application Engineer for the High-Speed Converter Group at Analog Devices. He received an Associates Degree in Applied Sciences from Southern Illinois University and Bachelors and Masters Degrees in Electrical Engineering Northern Illinois University.

Analog Devices
www.analog.com
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