Eight Questions and Answers on ADC Driver Circuit Design

balance - Unbalanced transformer is always only used AC Decoupling. If your application needs DC Decoupling, the best choice is to use support DC Decoupled FDA 。

FDA yes Integrated circuit 。 Therefore, balance with - Compared with unbalanced transformer, FDA Its design, manufacturing and packaging take up much less board area and height. As part of the floor area analysis, you It should also be taken into account that in some cases a single FDA Two much larger back-to-back configurations can be completed (to reduce even times Harmonic distortion ）Balance of - The task of unbalanced transformer.

Balanced unbalanced transformers do not provide power gain. Moreover, when providing voltage gain, they are subject to the impedance of the system in the boost voltage configuration (using turn ratio to generate voltage gain). This makes it difficult to obtain the desired turn to voltage gain while using the required filter impedance. In fact, balanced unbalanced transformers often have insertion losses. FDA provides true signal gain and flexibility in impedance transformation.

balance - The unbalanced transformer cannot buffer the impedance from the power supply. In existence from ADC Input sampling / It is particularly meaningful to maintain the kick back interference energy of the circuit, or where the selected filter may cause load problems. FDA Provides such advantages that you It can flexibly configure the circuit for the exact voltage gain required, drive the selected impedance and buffer the load from the power supply.

Although balanced - Unbalanced transformers usually do not impose great restrictions on the conversion rate, but differential amplifiers also do not need to limit the conversion rate. LMH5401FDA Capable of supporting more than 17,500V/μs Conversion rate and 80 ps Rise time of.

Although balanced - The unbalance transformer is a passive component and does not consume power, but because it is located at ADC Signal loss and even balance in previous downconversion receivers and filters - The loss in the unbalanced transformer itself leads to the need to deploy a IF Gain stage. FDA Will provide you Required IF Gain and possibly eliminate balance - Unbalanced transformer.

Compared with balanced unbalanced transformer, the use of FDA will increase the noise of signal path, but in many cases you All require an IF amplifier. Typically, the RF LNA will set the noise performance.

Many designers prefer to use balanced unbalanced transformers because they think their distortion performance is better than that of amplifiers. This may be true at very high frequencies (>>1 GHz), but amplifier manufacturers such as TI provide FDA with amplitude balance and distortion indicators superior to balanced unbalanced transformers at many common frequencies (especially in the frequency range from DC to 1GHz).

For example, our LMH5401 FDA can provide a balance error of>60dBc at 100MHz, while it is close to 50dBc at 1GHz. This provides harmonic distortion of approximately 100 dBc (at 100 MHz) and approximately 60 dBc (at 1 GHz). See Figure 1 for more details on amplitude balance errors.

chart one ： LMH5401 Balance error when performing single ended to differential conversion

Roughly speaking, LM5401 and LM3401 have the distortion performance necessary to drive 16 bit ADC (up to about 300 MHz), 14 bit ADC (up to about 500 MHz), 12 bit ADC (up to about 1 GHz), and 8 bit and 10 bit ADC (up to 2GHz).

Therefore, in general, FDA can integrate two or three functions in a single chip - an IF amplifier and one or two balanced unbalanced transformers - while improving IF amplification performance and achieving low power consumption, small size, high bandwidth and excellent distortion indicators. This is no exception even in single ended to differential configurations.

Figure 2 shows a typical application circuit with an "active balanced unbalanced transformer" configuration, which is provided with gain, buffer and single terminal to differential conversion by LMH5401 FDA before a GSPS ADC (such as 12 bit, 4 GSPS RF sampling ADC12J4000).

chart two : Display active balance - Unbalanced transformer configuration LMH5401 Application circuit

The table below gives a quick reference of some advantages and disadvantages of balanced unbalanced transformers and FDA.

So, next time you When it is necessary to drive a high-speed, differential input ADC, FDA such as LMH3401 or LMH3401 must be considered. For many applications below 1 GHz, FDA will not only shorten you It can also provide wider bandwidth support, smaller size, DC coupling and simplified design.

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