Differentially Driven ADC with Transformer - Page 1
Forum section: electronics / rf-microwave

author: yammy_96
author_rank: Newbie
timestamp: August 20, 2025, 09:12:33 pm
content: I am designing an L-Band Receiver.The MAX2120 is being used as the tuner IC. The chip provides differential IQ outputs. The datasheet specifies an IQ single-ended output impedance of 30 ohm. And so my first question for analysis purposes: can the IQ differential output be modeled as a single-ended output with 30ohm source impedance?https://www.analog.com/media/en/technical-documentation/data-sheets/max2120.pdfThe AD9629 is being used as the ADC. The chip takes differential inputs, and for my application I am only using the in-phase channel coming from the MAX2120. The datasheet for the AD9629 suggests using an rf transformer input frequencies below 10 MHz.https://www.analog.com/media/en/technical-documentation/data-sheets/AD9629.pdfThe MAX2120 includes a programmable low-pass filter at IF. My IF frequency will be set to 5 MHz.Will it be a good idea to take the MAX2120 output and send it through a MABA-007159-000000 BALUN as shown in the attached schematic? Would I need some sort of impedance matching between the IQ output and the transformer? How is this calculated using datasheet values of the MAX2120 and BALUN? Can I analyze the circuit as a 30-ohm single-ended source feeding the BALUN with pin 1 grounded? Do you have any recommended resources to learn about this sort of circuit design?Then from the BALUN to take that into an ADC driver like AD8352, then to some sort of differential matching to the AD9629 ADC. Do you have any suggestions for a simpler design?Here is my uncertainty in the design of the ADC front end.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: August 21, 2025, 01:29:15 am
content: I wouldn't struggle too much trying to model this as they were seemingly quite sloppy with it. The evaluation board uses opamps, nothing is matched.If I understand it correctly the outputs are meant to drive a high impedance (2k) load and the output levels are provided accordingly. Keep everything reasonably close on the board and you'll be fine.The balun you linked is not going to work, because you need one with a center tap on the secondary side to connect the ADC common mode voltage to.You could use something like this:https://www.minicircuits.com/pdfs/ADTT4-1+.pdfThe 1:4 impedance ratio gives you a 2x voltage boost so the 1Vpp nominal output swing of the mixer fits to the 2Vpp full scale range of the ADC without an amplifier.If you need more gain then you can look at differential amp, like the recommended ADA4938, but if you don't explicitly want it just hook it up directly through the balun.HTH

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 21, 2025, 02:51:36 pm
content: Quote from: Marsupilami on August 21, 2025, 01:29:15 amA center tap isn't necessary. The AD9629 datasheet shows several possible implementations, including some where the VCM bias is applied through a RC bias tee. Using a simple 1:1 voltage-mode balun with no CT should work fine, and the VCM can be fed to each differential output via a resistor.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: August 21, 2025, 04:56:38 pm
content: Quote from: mtwieg on August 21, 2025, 02:51:36 pmProbably yes. My concern was that since the mixer assumes a HiZ output these VCM resistors will have to be fairly large. I didn't look into it enough to gather whether that's a problem due to a bias current or alike. It is likely fine, but then also a $7.32 center taped transformer can solve this with 1 component.If the high value VCM resistors are fine and no additional gain is needed, then it is also enough to AC couple the IF, there is no need for a transformer at all.

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 22, 2025, 02:21:05 pm
content: Quote from: Marsupilami on August 21, 2025, 04:56:38 pmYeah, the MAX2120 datasheet gives almost no guidance on terminating the baseband ports. It specifies output swing with a 2K load, so I'm guessing all other specs are valid for that load. Should also be capable of driving Vcm biasing resistors.But it's unclear if the MAX2120 outputs are suitable for driving an ADC without a proper buffer amp. Could be that a passive network degrades performance, regardless of whether you use a transformer at all.

author: jwet
author_rank: Frequent Contributor
timestamp: September 06, 2025, 04:09:25 pm
content: There were several app notes for the 2112/2120 that give better design guidance. As far as your question, the outputs are like single ended outputs in complement with an output Z of 30 ohms.One link to an app note is:https://www.analog.com/en/resources/design-notes/extended-characterization-for-the-max2112max2120-satellite-tuners.htmlThere are likely others notes- these were heavily supported parts by Maxim for design ins. I left Maxim before ADI acquisition (actually as part of it...) so I don't know the present situation.For most of these parts, the volumes were high and the real customers were well known to Maxim; they were basically derivatives of customs done for some very big guys. Maxim had several nice RF apps labs and a half dozen engineers in the US and Asia that did the detailed design for these things for our customers. The lead engineer was Dave DeVries, I think he's moved on. I worked at Maxim when these parts were hot but wasn't deeply involved in RF design. You could contact ADI and get past the gatekeepers on the main apps line to get to an RF apps guy. I think using an appropriate minicircuits TX or balun makes some sense depenging on your ADC input S11, etc.Good luck.

author: radiolistener
author_rank: Super Contributor
timestamp: September 07, 2025, 03:34:35 am
content: Quote from: yammy_96 on August 20, 2025, 09:12:33 pmYou are on the right track in thinking about a transformer interface between the MAX2120 and the AD9629. A wideband RF transformer is generally the best solution when driving a differential ADC input: it provides isolation, impedance transformation, good common-mode rejection, and what’s often called a "noise-free" gain.That said, a transformer also comes with trade-offs. The main one is its low-frequency response: below a few hundred kHz (depending on the part) the insertion loss rises, and it completely blocks DC. If your application does not require preserving the DC component or very low frequencies, then a transformer front end is usually the highest-performance and simplest option. At 5 MHz IF, you are well inside the comfortable operating range of most broadband RF transformers.Regarding impedance matching: the transformer itself is the impedance matching element. If the MAX2120 presents ~30 Ω single-ended source impedance and the ADC wants to see ~100–200 Ω differential (depending on how you bias/terminate the input), then you simply select the appropriate impedance ratio. For example:30 Ω → 30 Ω: use a 1:1 impedance ratio transformer (turns ratio 1:1).30 Ω → 120 Ω: use a 1:4 impedance ratio transformer (turns ratio 1:2).In practice, impedance ratios beyond about 1:16 (turns ratio 1:4) are rarely practical, because transformer bandwidth and linearity degrade.Unfortunately, the MAX2120 datasheet only specifies output impedance in simplified terms, and the AD9629 datasheet doesn’t list a simple "input impedance" - instead it describes an equivalent switched-capacitor network. That’s why it is hard to do a purely paper design. In reality, the final choice of transformer and termination network usually requires prototyping and measurement to confirm performance.To summarize:1) Yes, a transformer front end is a very good idea at 5 MHz IF if DC is not required. This approach is generally the most effective if your goal is to achieve the best state-of-the-art performance.2) The transformer also performs your impedance matching, you just pick the ratio according to source and load impedances.3) Expect to do some bench testing to refine the match, since neither datasheet gives a perfectly clear picture of the real impedances.PS: I cannot state the exact input impedance of the AD9629, but high-speed ADC with wide dynamic range often exhibit an effective input impedance on the order of 1–2 kΩ. These devices are usually well matched to a 50 Ω source when driven through RF transformer with a turns ratio of 1:4, corresponding to an impedance ratio of 1:16 (50 Ω × 16 ≈ 800 Ω, reasonably close to 1 kΩ). Higher turns ratios are generally not practical, as they introduce significant bandwidth limitations and are constrained by the physical properties of the transformer and distortions.If wider bandwidth is a priority, you can trade off perfect matching by using a transformer with a lower ratio, this will degrade the noise figure but allow you to extend the usable frequency range. In practice, the difference in noise figure between using transformers with a turns ratio of 1:2 versus 1:4, or between 1:1 versus 1:2, is approximately 6 dB. Ultimately, it comes down to a design choice: whether you prioritize wider bandwidth or a 6 dB improvement in noise performance.After all, the optimal configuration is best determined by prototyping and measurement, since datasheet values alone rarely capture all real-world effects.

author: gf
author_rank: Super Contributor
timestamp: September 07, 2025, 12:54:03 pm
content: Quote from: yammy_96 on August 20, 2025, 09:12:33 pmThe IF frequency of the MAX2120 is actuallyzero Hz; it's adirect conversion receiver. Its lowpass filter determines the IF bandwidth, not the IF frequency. [ There seems to be an additional high-pass corner, though, at 400 Hz to ensure DC stability. ]If the lowpass is set to 5 Mhz, then the -3dB IF passband is -5MHz...+5Mhz (i.e. 10 MHz double-sided bandwidth, centered at 0Hz), with a -400Hz...+400Hz notch in the center. 5Mhz:400Hz is a 12500:1 ratio. I'm not sure if a transformer can easily cover such a large frequency ratio with flat response?You are still free, of course, to tune the MAX2120 to an offset frequency below the carrier frequency rather than the carrier frequency itself, such that the carrier frequency maps to say 5 MHz in the MAX2120 output. In this case, the MAX2120's lowpass filter would only act as a pre-filter and as an anti-aliasing filter for the ADCs. The actual IF filtering (and any necessary down-conversion from non-zero IF to DC), would need to be done after analog-to-digital conversion in the DSP. Advantage is that you can avoid the notch in the center of the IF passband. But your IF bandwidth can only be <= 50% of the MAX2120's zero-IF bandwidth.So I wonder what's your intended usage, and what maximum IF bandwidth you need to cover? Do you want to use the MAX2120 as direct conversion receiver (what it basically is), or do you want to 'abuse' it just as image rejection mixer with non-zero IF? [ where 'abuse' is not meant in the strict sense; it's still a legitimate use case ]

author: yammy_96
author_rank: Newbie
timestamp: September 14, 2025, 03:38:03 pm
content: Thanks for breaking this down clearly.From antenna I receive -160dBm, LNA 40dB GAIN 0.4dB NF. The band of interest is 3MHz wide at around 1.42GHz. This is the front end before the tuner sectioon.The MAX2120 has its own gain and filtering. Downconverting to 5Mhz IF. Sampling at 20MSPS with the AD9629 and doing FFT averages on FPGA to pick out galactic noise on the hydrogen line.Would you suggest using an ADC driver IC or would the BALUN with appropriate turns ratio suffice.

author: gf
author_rank: Super Contributor
timestamp: September 15, 2025, 07:24:36 pm
content: Is the goal simply to detect the 1.42 GHz CW signal with a power level of -160 dBm so that it stands out slightly from the noise floor?EDIT: misinterpreted the datasheetDo you really need a 3 MHz bandwidth? And do you have a preselection filter in front of the MAX2120 to prevent input noise from overdriving its output? Although the LNA improves the NF, it even worsens the potential for overdrive.

author: mawyatt
author_rank: Super Contributor
timestamp: September 15, 2025, 07:58:18 pm
content: Quote from: yammy_96 on September 14, 2025, 03:38:03 pmThe SNR at the ADC input will be ~ -51dB!!!Best

author: gf
author_rank: Super Contributor
timestamp: September 16, 2025, 11:57:35 am
content: Quote from: mawyatt on September 15, 2025, 07:58:18 pmTheoretically, a sufficient number of DFT points (narrow detection bandwidth) can do wondersI wonder, however: What is theminimum usefuldetection bandwidth imposed by the phase noise of the MAX2120's LO and ADC jitter?