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Amplifier to ADC Interface

Amplifier to ADC Interface

System designs using amplifiers and analog-to-digital converters (ADC) should focus on using the best features of each device. Amplifiers provide power gain, isolation, voltage gain, and impedance transformation. Data converters would seem more simple, offering only the digitization of a voltage; however, the ADC sample rate is a degree of flexibility that can be quite powerful. This application note focuses on combining the best performance characteristics of amplifiers and ADCs for high speed, high linearity data capture. This document concentrates mainly on the LMH5401 family of amplifiers and the ADC32RF4x series of ADCs.

The LMH5401 family of amplifiers includes the LMH3401 fixed-gain amplifier, the LMH5401 fullydifferential amplifier (FDA) and the LMH3404, dual-fixed-gain amplifier. Because the ADC32RF45 ADC is a dual-channel device, this document focuses on the LMH3404 amplifiers; however, the other amplifiers in the family can be used with similar design guidelines.

This application note covers a very specific system design. The design focuses on an RF signal with 750 MHz of bandwidth and a single-ended signal source, such as an antenna or mixer. The system consists of the amplifier, a simple, anti-alias filter, and the ADC.

The LMH3404 amplifier offers excellent performance up to 1-GHz signal bandwidth. This amplifier also offers 20 dB of gain and up to a 5-V differential signal swing. It is an excellent choice for our 750-MHz signal bandwidth system.

The LMH3404 amplifier has 7 GHz of –3-dB bandwidth, and the ADC34RF45 has an input bandwidth of 4 GHz. Without a noise filter between the amplifier and the ADC, the ADC samples all noise in the 4-GHz bandwidth of the ADC. In order to reduce the sampled noise, and also to reduce the harmonic distortion of the sampled signal a low-pass filter is used between the amplifier and the ADC.

The ADC32RF45 is a high performance 14-bit, 3.0-GSPS ADC. With a 1.5-GHz first Nyquist zone, this ADC affords a very large amount of flexibility in filter design. With a 750-MHz desired signal bandwidth, there is an additional 1.5 GHz of frequency guard band to ensure that undesired noise and harmonic distortion products are rejected. Half of the frequency guardband is between the 750-MHz desired signal bandwidth and the 1.5-GHz first Nyquist band of the ADC. The other half of the frequency guardband is in the ADC second Nyquist band. This illustrates the benefit of oversampling, which is basically using a faster than necessary ADC to improve signal fidelity.

Designing a filter to pass the desired frequencies is fairly easy. However, one of the largest drawbacks to real filter implementation is the loss of signal through the filter, or insertion loss. This signal loss contributes dB-for-dB to the ADC noise figure. What may be even worse is that the amplifier driving the ADC will generate distortion at multiples of the filter loss. For example, if a filter has 7 dB of loss, the amplifier needs to drive a signal 7 dB stronger. This results in second-order products with 14 dB higher levels and third-order products will be 21 dB worse. Some of these distortion products (intermodulation in particular) cannot be filtered out, so keeping filter loss to a minimum is critical to system performance. For this reason this application report focuses on using a low loss, low component count filter.

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