Use of ISL55016 as a Single-Ended to Differential Converter ® Application Note September 16, 2008 AN1418.0 Introduction Impact of Balun on Layout The ISL55016 Silicon Bipolar amplifier can match a 75Ω single-ended source to a 100Ω differential load. This feature makes the ISL55016 ideal for a radios where the tuner chip has a differential input, such as Satellite TV. Here we will introduce the application circuit and discuss the advantages of this part. Specifically, the differential nature of the output removes the need for a balun, which improves both board area and frequency response. In contrast with the ISL55016 is its predecessor, the ISL55012. The amplifier will be followed with a Balun structure to transfer the single-ended signal to a differential tuner. The ISL55016 can be used to substitute the Balun Structure and LNA to save PCB space and components (Figure 3). The differential capabilities of the ISL55016 allow a reduction in excess of 50%. The ISL55012 is a predecessor to the ISL55016. It has similar functions with the exception of the output, which is limited to single-ended operation. We will discuss using the ISL55016 to substitute for the ISL55012 with a balun structure. GND GND ISL55012 ISL55012 RF RF IN IN C C A balun is a passive structure which converts between balanced and unbalanced electrical signals (Figure 1). Some normal balun structures include classical transformers, transmission line baluns and 180° power divider/combiners. RF IN RF IN BALUN DIFFERENTIAL SIGNAL FIGURE 1. BLOCK DIAGRAM OF A BALUN Typical Application Circuit C C C C C BALUN Balun CC VDD DIFFERENTIAL Differential OUT Out Vdd C C C C CC L R R L ISL55016 ISL55016 VDD Vdd Figure 4 shows gain measured for the ISL55016 and ISL55012 with a transmission line balun on a customer board as shown in Figure 3. One of the design goals of the ISL55016 is to provide gain no less than the ISL55012 at 0.5GHz to 2GHz. Also the ISL55016 can be used at low frequency down to 50MHz, where the ISL55012 can not work well. 19 The ISL55016 is a true differential amplifier and doesn’t need a balun. Figure 2 shows the typical application circuit of ISL55016. The ISL55016 can be used as differential-in and differential-out as well since Pin 2 and Pin 3 are equivalent, balanced inputs. +5V C6 C3 1 6 R1 ISL55016 2 5 4 3 C2 L1 R2 L2 C5 RF OUT 2 15 13 ISL55016 11 9 7 5 500M ISL55012+TRANSMISSION LINE BALUN 1.0G 1.5G 2.0G FREQUENCY (Hz) 2.5G 3.0G FIGURE 4. GAIN OF ISL55012 WITH A TRANSMISSION LINE BALUN vs ISL55016 The ISL55016 shows a little higher gain than the ISL55012 with balun structure. In this way, Figure 4 shows the realization of the ISL55016 as a single-ended to differential converter. Trade-off Between Power and OIP2 RF OUT 1 C4 FIGURE 2. APPLICATION CIRCUIT 1 17 dB The ISL55016 can be used as a single-ended input. Pin 2 and Pin 3 are equivalent. Either one can be used as the input; the other one needs to be connected to an AC Ground. The input is internally matched to 75Ω single-ended and the output is matched to 50Ω single-ended or 100Ω differentially. C1 RF IN 2 C C C DIFFERENTIAL Differential OUT Out C C C FIGURE 3. COMPARISON OF THE LAYOUTS OF HE ISL55012 WITH A BALUN AND THE ISL55016 (RELATIVE SIZE) SINGLE-ENDED SIGNAL RF IN 1 C C R R C LL Vdd VDD VDD Vdd C C L L R R GND GND C C Balun L L L C Once the signal is truly balanced, even order distortion drops to zero. The ISL55016 converts the signal to differential early in its signal path thus reducing second order distortion and boosting OIP2. Nevertheless, the ISL55016 does generate CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright © Intersil Americas Inc. 2008. All Rights Reserved. All other trademarks mentioned are the property of their respective owners. Application Note 1418 some even order distortion because of the asymmetric loading of its input and improving the device linearity will improve OIP2. The values of R1 and R2 (Figure 1) have been characterized for two options, 27Ω and 0Ω. Decreasing the R1 and R2 value will increase the voltage across the output transistor leading to an increase in the dissipation power. At the same time, it will increase the amplitude of the compression and OIP2, OIP3. This allows a trade-off between the power dissipation and the distortion. Thermal Management Matching at the Input and Output Choices of Components In PCB layout design, a matching network is needed, especially at the input. The Figure 5 shows the matching network used for the ISL55016 Evaluation Board. 12mm of 100Ω trace and 8mm of 50Ω trace are used to form the input matching network while 4mm of 100Ωr trace forms the output matching network on the FR4 material. ISL55016 is designed for wide bandwidth Applications, 50MHz-3GHz. The decoupling and RF choke components should be chosen carefully for different frequency applications. Tables 1 and 2 list the component information for the Evaluation Board. Vdd VDD FREQUENCY BAND 1 0100Ω 0ohm L L22 C C22 L1 L1 1 0100Ω 0ohm C1 C1 5 050Ω ohm RRF F IIN N TABLE 1. LIST OF COMPONENTS (50MHz~300MHz) R FOUT OUT 5 050Ω o h m RF CC3 3 10 0ohm 100Ω The power dissipation of ISL55016 is about 500mW. The PCB layout shown in Figure 5 provides the good thermal connectivity between the device back plate and the main ground plane, hence the copper and vias holes are needed on either side of the chip. So via holes have two functions to perform, providing a good RF ground for C2, C3 and back plate and providing a low thermal conductivity to the ground plane which dispenses the heat. RRF F OUT OUT 7 5 o75Ω hm 5 050Ω ohm 2200pF 0603 C4, C5 2200pF 0603 L1, L2 2.2µH Multilayer Ferrite/0603 C3 1nF 0603 C6 0.1µF 1206 R1, R2 27Ω/0Ω 0402 TABLE 2. LIST OF COMPONENTS (300MHz~3GHz) Figure 6 shows the improvement in input matching (S11) provided by this network of traces. At 2GHz, the input match is improved 3dB and remains below a -10dB level. 0 -5 -10 dB DESCRIPTION/DIMENSIONS C1, C2 FIGURE 5. PCB LAYOUT OF MATCHING NETWORK EVAL BOARD WITH MATCHING NETWORK -15 VALUE FREQUENCY BAND VALUE DESCRIPTION/DIMENSIONS C1, C2 100pF 0603 C4, C5 100pF 0603 L1, L2 100nH Surface Mount/0402 C3 100pF 0603 C6 0.1µF 1206 R1, R2 27Ω/0Ω 0402 -20 PROBE STATION -25 -30 0 0.5G 1.0G 1.5G 2.0G FREQUENCY (Hz) 2.5G 3.0G FIGURE 6. |S11| vs FREQUENCY Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that the Application Note or Technical Brief is current before proceeding. For information regarding Intersil Corporation and its products, see www.intersil.com 2 AN1418.0 September 16, 2008