Introduction to the programmable differential amplifier

As high-speed systems become more complex and design cycles become shorter, engineers need to find high-performance yet flexible components to facilitate their system designs. One method is to use components that offer very high performance, yet are easy to reconfigure to produce new products, or can be quickly reconfigured to correct issues discovered late in the design cycle. Finding parts that offer both high performance and flexibility is a challenge. Now there is a new class of differential amplifiers to meet the needs of engineers. 

This article explains the architectural features of the industry’s first programmable differential amplifier (PDA), including its easy-to-change voltage gain, consistent bandwidth over gain settings, constant input impedance over gain settings, excellent linearity, and low-noise figure. The PDA is compared to the fully differential amplifier (FDA) and digital variable gain amplifier (DVGA). Configuring the PDA’s gain and input configurations are also discussed. 

The industry’s first PDA, the LMH6881, is a high-speed, high-performance, fully differential amplifier. With a bandwidth of 2.4 GHz and an OIP3 of 44 dBm, it addresses a wide variety of signal conditioning applications from radio equipment to high-speed test and measurement systems. The PDA combines the best of FDAs and DVGAs. It offers superior noise and distortion performance over the entire gain range without external resistors, enabling the use of one device for many applications requiring different gain settings.

Figure 1. Typical connection of the programmable differential amplifier

PDA vs. FDA

The PDA was designed to make differential amplifier circuits easy to implement. It can be used in place of a traditional fully differential, fixed gain amplifier (Figure 1). Because the gain is easily changed, the PDA supports last minute design changes while accommodating variability in other parts of the signal chain.

Traditional FDAs require external components to set the closed-loop gain (Figures 3 and 4). The PDA’s gain can be easily changed by either the external gain set pins or with the internal serial peripheral interface (SPI) register. For fixed gain applications, the external gain set pins can be connected directly to ground or the 5V supply. 

Figure 2. PDA frequency response with 4 dB voltage gain steps

In contrast to a typical FDA, the PDA offers consistent bandwidth performance over a wide voltage gain range. With an FDA, the bandwidth, noise figure and distortion performance all change significantly over the usable gain range. The FDA has large changes in loop gain when the closed-loop gain is altered. As shown in Figure 2, the PDA’s frequency response is insensitive to voltage gain. The frequency response graph of a traditional FDA (Figure 3) shows a frequency response with the well-documented gain bandwidth product.

The input is terminated at 100 Ohm for differential signals, and the input termination is independent of gain. In contrast, a FDA has to balance input impedance with realistic gain set resistor values. Two FDA circuits are shown in Figures 4 and 5, with one set at 8 dB of gain and the other at 26 dB. From this example, it’s obvious that the input impedance changes drastically between the two gain settings. In addition to the input impedance, the noise contributed by external resistors is different.

 Figure 4. Traditional FDA with 8 dB gain

 Figure 5. FDA with 26 dB gain