Driving ADCs: Amplifier or balun?

Baluns are often used to convert a single-ended signal to differential, without adding noise and while maintaining good distortion. A common example of that is in the driver circuit for a high-speed, differential-input analog-to-digital converter (ADC). 

Have you ever considered using a differential amplifier to replace a balun in the RF/IF signal chain? If you haven’t, you should. While they don’t make sense for all applications, fully differential amplifiers (FDAs) offer some advantages over baluns.

Here are some questions that’ll help you determine if a balun or FDA is best suited for your design.

Does your application require dc-coupling?

Baluns are always AC-coupled only. If your application requires dc coupling, your best option is a FDA that supports DC coupling. 

Is your application space-constrained?

FDAs are integrated circuits. As a result, FDA design, manufacturing and packaging yields a much smaller footprint and height than baluns. As part of the analysis of footprint, you should also consider that a single FDA can in some cases do the job of two much larger baluns configured back-to-back to reduce even-order harmonic distortion.

Does your signal path need power gain at this stage in the receiver?

Baluns cannot provide power gain. And when providing voltage gain, they are slave to the system’s impedances in the step-up voltage configuration (using the turns ratio to create voltage gain). This makes it difficult to use the filter impedances you want while getting the turns-ratio voltage gain desired. In fact, baluns often have insertion loss. FDAs provide real signal gain and flexibility in impedance transformations.

Does the ADC and/or filter need buffering?

Baluns cannot buffer the load from the source. This is particularly of interest in cases where there is glitch energy kicking back from an ADC input sample/hold circuit, or perhaps the filter of choice is causing loading challenges. FDAs provide these benefits, which gives you flexibility in how you configure your circuit for the exact voltage gain needed while driving the impedance of choice and buffering the load from the source.

Are you concerned about slew rate in the amplifier?

While a balun won’t usually limit slew rate much, the differential amplifier does not have to limit slew rate either. The LMH5401 FDA can support >17,500 V/us of slew rate and 80-ps rise time

Do you think a balun is best suited for your system because it’s passive?

While a balun is passive and burns no power, many times an IF gain stage is needed due to signal losses in the down-converting receiver and filters prior to the ADC, or even due to loses in the balun itself. An FDA will provide the IF gain you need while potentially eliminating the balun.

Won’t the FDA add noise?

Using an FDA will add noise to the signal path compared to a balun, but in many cases you’ll need an IF amplifier anyway. And typically, the RF LNA will set the noise performance. 

Will the FDA limit my distortion performance?

Many designers prefer baluns because they think they provide superior distortion over amplifiers.  That may be true at very high frequencies (>>1GHz), but amplifier manufacturers like TI provide FDAs that are better specified for amplitude balance and distortion than baluns at many common-use frequencies, especially in the DC-1GHz range.

For instance, our LMH5401 FDA provides >60dB balance error at 100MHz and nearly 50dBc at 1GHz. This provides harmonic distortion of ~100dBc at 100MHz and ~60dBc at 1GHz. See Figure 1 for more details on amplitude balance error.

Figure 1: LMH5401 balance error converting single to differential

Roughly speaking, the LMH5401 and LMH3401 have the distortion performance necessary to drive 16-bit ADCs up to ~300MHz, 14-bit ADCs up to ~500MHz, 12-bit ADCs up to ~1GHz, and 8- and 10-bit ADCs as high as 2GHz.

So in summary, FDAs can integrate two or three functions into one chip - the IF amplifier and one or two baluns - while improving the IF amplification performance with low power, smaller size, higher bandwidth and excellent distortion. This is true even in single-to-differential configurations.

Figure 2 shows a typical application circuit in an ‘active balun’ configuration, with the LMH5401 FDA providing gain, buffering and single-to-differential conversion prior to a GSPS ADC, such as the 12-bit, 4-GSPS RF-sampling ADC12J4000.

Figure 2: LMH5401 application circuit showing active balun configuration

The table below provides a quick reference for some of the advantages and disadvantages of baluns and FDAs.

So next time you need to drive a high-speed, differential-input ADC, be sure to consider an FDA like the LMH3401 or LMH5401. For many applications below 1GHz, the FDA will reduce your bill of materials while providing wider bandwidth support, smaller size, dc-coupling and easier design.

Learn more about how to use an FDA as an ‘active balun’ to drive an ADC in a traning presentation prepared by my colleague Michael Steffes, who's a high-speed Systems Engineer and Senior Member of Technical Staff.  

Additional resources:

• View the LMH3401 or LMH5401 datasheets.
• Learn how to reduce the impact of common mode noise on differential signal paths.
• Learn how to use an LDO to reduce high-speed power supply issues.