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ADC Guide, Part 14: Impedance Matching

Designing a matching circuit is fairly straightforward for a buffered input ADC because it offers almost constant impedance across all frequencies. As we have already seen, this is not the case with an unbuffered ADC. The aim of designing a matching circuit is to match the track mode impedance of the ADC so that the source sees only real (i.e., resistive) impedance at the signal frequency. It is worth noting that impedance matching is particularly important for high-frequency applications since an unmatched input would cause prominent reflections of the input signal and thus heavily distort it.

Figure 1

Block diagram of typical ADC input impedance matching

Block diagram of typical ADC input impedance matching

There can be many configurations of a matching circuit, and the exact implementation depends upon the end application. However, the general format remains the same as shown in Figure 1 . Here, the impedance matching block takes Zin into consideration and aims at making the effective impedance as seen by the source (Rin1 ) equal to Rs , which is the output impedance of the source.

Figure 2

Impedance matching for buffered input ADC

Impedance matching for buffered input ADC

For a buffered input ADC, the input impedance is very high and resistive. A simple resistive shunt can do the job of impedance matching in this case. Consider the example circuit shown in Figure 2 . Here, the input impedance of the buffer is Rin_buff . To match this impedance to the impedance of the source (Rsource ), a resistive shunt Rsh is added at the input terminal of ADC. The objective of this shunt is to match the effective input impedance (Rin ) of the ADC to the source impedance.

Equation 1 gives the solution for the design of a simple resistive shunt type matching circuit for buffered ADCs.

The design of an impedance matching circuit for an unbuffered ADC, however, is not as straightforward. It involves resistive as well as reactive elements of impedance. The key point to remember in this case is that the matching circuit is usually designed for a specific signal frequency. This is because the input impedance of the ADC itself is frequency dependent. Thus we have to first define a signal frequency, then design the impedance matching network considering the ADC’s input impedance at that signal frequency. There are various ways to achieve impedance matching once the signal frequency is known. Let us explore a few examples.

Figure 3

Resonant matching for unbuffered ADC

Resonant matching for unbuffered ADC

An impedance matching circuit for unbuffered ADCs can consist of a simple inductor. Since the ADC’s input impedance is capacitive, this inductor forms a resonant circuit with it (Figure 3 ). The inductor is designed such that the resonant frequency of the resultant circuit is equal to the signal frequency. Thus this circuit offers pure resistive impedance to the source at the signal frequency. Equation 2 can be used to find the value of inductance L for the matching circuit. In this equation, Cin is the input capacitance of the ADC, and fs is the signal frequency at which the matching network is to be designed:

This equation follows from the fact that L and Cin form a series resonant circuit. We can also design a matching circuit with parallel resonance. Once we have gotten rid of the reactive component of impedance, the resistive component can be matched with a resistive shunt similar to the one we have seen before.

For RF or IF applications, transformer-based matching circuits are more popular. The advantage of a transformer-based impedance matching circuit is that the transformer does not add any noise by itself. Besides this, a transformer-based circuit provides inherent single-ended to differential conversion, which, in turn, extends most of the advantages of differential input configuration to a single-ended signal. The actual matching circuit between the secondary winding of the transformer and the inputs of the ADC can be built in many ways per the specific needs of the application.

A resonant circuit similar to the one shown in Figure 3 can be used to nullify reactive the component of the ADC’s input impedance (Figure 4-A ). An additional consideration for a transformer-based circuits is that the impedance reflection seen by the secondary winding of the transformer towards its primary winding depends on the turn ratio of the transformer. Another commonly used transformer-based impedance matching circuit at baseband frequencies is shown in Figure 4-B . In this diagram, resistances R1 and R2 act to give sufficient isolation to the input signal from the ADC’s input capacitance.

Figure 4

Transformer based impedance matching: (A) RF/IF applications; (B) Baseband applications

Transformer based impedance matching: (A) RF/IF applications; (B) Baseband applications

The impedance matching block shown in Figure 1 is not always restricted strictly to the impedance matching circuit. This block may occasionally encompass an anti-aliasing low pass or band pass filter, depending on the system requirements. The effect of these filters on the impedance cannot be ignored while designing the matching circuit. Furthermore, when such a filter is present, it is recommended to place it at the transformer’s secondary winding. This helps to limit the noise that gets through to the ADC inputs.

Now that we have had a look at the commonly used impedance matching networks, we can complete the discussion of ADCs in general and their properties. We have seen how these properties affect the measurement systems either positively or negatively. We have also discussed how to address the negative effects of some of the properties of ADCs. In our next installment, we’ll discuss various types of ADCs, their architectures, and their peculiarities.

About the authors:

Sachin Gupta is working as a Senior Applications Engineer in the PSoC 1 Applications group with Cypress Semiconductor. He holds a Bachelor’s degree in electronics and communications from Guru Gobind Singh Indraprastha University, Delhi. He has several years of experience in mixed signal application development. He can be reached at sgup@cypress.com.

Akshay Phatak is an Applications Engineer with Cypress Semiconductor. He holds a Bachelor's degree in electronics and telecommunications from the College of Engineering, Pune, India. He likes to work on mixed-signal embedded systems. He can be reached at akay@cypress.com.

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14 comments on “ADC Guide, Part 14: Impedance Matching

  1. amrutah
    April 2, 2013

    Is using the inductance the only method to match the impedence? The input parasitic capacitance to ground can form a parallel resonant circuit. If the series cap and the parasitic cap are of the same order than the 2 resonant can cause problems…

  2. amrutah
    April 2, 2013

    “For RF or IF applications, transformer-based matching circuits are more popular.”

    Usage of the transformer wil  help by isolating the input of the ADC from source but it will make the system bulky.  Only from Impedence matching perspective is transformer used or are there any other advantages?

  3. Vishal Prajapati
    April 3, 2013

    Sachin/Akshay, Can you suggest some good books or literature for impedence matching? Can we try to simulate this reflection of signal or impedance matching in the PSPICE or similar software?

  4. Hughston
    April 3, 2013

    Transformer coupling is very convenient to use for an ac differential signal in a mixed signal system with multiple grounds. It gives great CMRR between noisy grounds. I think you will be flat enough over your bandwidth with a transformer because because the IF frequency is going to stay within a limited enough range. I am making an assumption with that last sentance.

  5. Brad Albing
    April 4, 2013

    And even tho' it's been mentioned before, it bears repeating: Even with a good quality, properly wound transformer, you will lose that good CMRR if you don't pay careful attention to your PC board layout  – parts placement, trace length, matching and co-locating traces, etc.

  6. Brad Albing
    April 4, 2013

    Pick your inductance value carefully with respect to the frequency (or bandwidth of the frequencies) that matter in your system. Also, adding resistance to lower the Q of the resonant circuit can help some.

  7. amrutah
    April 4, 2013

    Akshay,

       This is one of the topic very difficult to understand and get a intuitive feel for the subject.  Though there are tons of data available on the subject, but it would be helpful if you could point to some good material for impedence matching, understanding Schmitt charts….

  8. Brad Albing
    April 30, 2013

    We'll get some info from the authors. I'll let them know.

  9. Brad Albing
    April 30, 2013

    We'll need to get a blog posted regarding PC layout techniques that speaks to this specific issue.

  10. jacksonman
    February 1, 2018

    Is it possible to put the anti-aliasing filter before the transformer so as to design the filter as single ended only, giving the advantage of having less inductors that might introduce lots of tolerances?

    My frequency of operation is 50 MHz and I need to design a 7th order filter.

    What would be the benefit of having the filter after the transformer instead of before it?. From noise filtering point of view I donot see any difference!

    Thanks,

    Karim

     

  11. Steve Taranovich
    February 1, 2018

    @jacksonman—If you move the anti-alias filter before the transformer—be sure to calculate the reflected transformer impedance that will be seen by the ADC internal front-end. You will need to take care that you have the proper impedance that the internal ADC front end will need to see for optimum bandwidth, plus if the front end is not properly designed to match the interna; ADC input—this can lead to less bandwidth, degraded amplitude, phase imbalance, and sometimes degraded pass-band ripple.

    I suggest reviewing the following app notes:

    http://www.analog.com/media/en/training-seminars/tutorials/MT-228.pdf

    http://www.ti.com/lit/an/slaa416a/slaa416a.pdf 

    Check the ADC to see if it has an input buffer as well.

  12. jacksonman
    February 2, 2018

    Steve….Thank you for your reply and useful refereneces.

  13. raff5184
    May 14, 2019

    Hi, I'm not sure if this thread is stll active. Anyways, how do you choose the inductance L  for the matching if the input signal is not monotone but has a more or less wide spectrum? What frequency should you consider?

  14. Steve Taranovich
    May 14, 2019

    @raff5184—I am asking the authors to respond

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