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Analog Devices Welcomes Hittite Microwave Corporation NO CONTENT ON THE ATTACHED DOCUMENT HAS CHANGED www.analog.com www.hittite.com THIS PAGE INTENTIONALLY LEFT BLANK v00.1203 HMC424LP3 PRODUCT NOTE HMC424LP3 Negative Biased Digital Attenuator TTL/CMOS Driver Circuit Design General Description The HMC424LP3 is a broadband 6-bit GaAs IC digital attenuator in a low cost leadless surface mount package. The attenuator covers a frequency range from DC to 13 GHz with a typical insertion loss of less than 4 dB. Major attenuation steps are 0.5(LSB), 1, 2, 4, 8, and 16 dB with total attenuation of 31.5 dB. Attenuation accuracy is excellent at ± 0.5 dB typical step error with an IIP3 of +32 dBm. A single Vee bias of -5V allows operation at frequencies down to DC while six control voltage inputs, toggled between 0 and -5V, are used to select each attenuation state. Introduction Negative biased digital attenuators have the advantage of operation over wide RF bandwidths and excellent attenuation accuracy down to DC. Conversely, positive biased digital attenuators have a low frequency limit imposed by the on-chip-capacitors used for RF grounding. Therefore a negative biased attenuator is preferred in applications requiring operation to very low frequency. In order to switch between attenuation states, the user must apply negative control voltages to the control input pins. Since control signals generated by common logic families are positive voltages, a driver circuit capable of translating these logic level signals to inputs compatible with the digital attenuator is required. In addition, the number of components needed to realize the driver circuit and the space required to implement the circuit on the PCB must be minimized to control manufacturing costs. The HMC424LP3, a negative bias digital attenuator, is realized as a cascade of switches and fixed attenuator pads as shown in the functional diagram of figure 1. The attenuation is set by adjusting each of the control pins (V1 – V6) to either a logic “high” or logic “low” per table 1. The input control voltages must be within the range specified in table 2 in order to operate the attenuator as specified. To operate this device from standard logic families, a driver circuit has been designed that interfaces TTL/CMOS logic to the HMC424LP3. 17 PRODUCT APPLICATION NOTES Attenuator Operation Figure 1 – HMC424LP3 functional diagram For price, delivery, and to place orders, please contact Hittite Microwave Corporation: 20 Alpha Road Chelmsford, MA 01824 Phone: 978-250-3343 Fax: 978-250-3373 Order Online at www.hittite.com 17 - 77 v00.1203 HMC424LP3 PRODUCT NOTE Control Voltage Input V1 16 dB V2 8 dB V3 4 dB V4 2 dB V5 1 dB V6 0.5 dB Attenuation State RF1 - RF2 Low Low Low Low Low Low Reference I.L. Low Low Low Low Low High 0.5 dB Low Low Low Low High Low 1 dB Low Low Low High Low Low 2 dB Low Low High Low Low Low 4 dB Low High Low Low Low Low 8 dB High Low Low Low Low Low 16 dB High High High High High High 31.5 dB Any Combination of the above states will provide an attenuation approximately equal to the sum of the bits selected. Table 1 – HMC424LP3 truth table PRODUCT APPLICATION NOTES 17 17 - 78 State Bias Condition Low 0 to -3V @ 70 µA Typ. High -5 to -4.2V @ 5 µA Typ. Table 2 – HMC424LP3 control voltages at Vee = -5V Driver Circuit Design The driver circuit to interface TTL/CMOS to the digital attenuator is shown in figure 2. This circuit is duplicated for each control line, resulting in a total of six circuits for the HMC424LP3 digital attenuator. Inverter Zener Diode TTL/CMOS 100 ohm 10K ohm To Attenuator Control Inputs -5V Figure 2 – Driver circuit schematic The driver circuit converts the output signals from the TTL/CMOS devices to 0V and -5V respectively for the inputs of the HMC424LP3. In figure 2, the zener diode is used to level-shift the TTL/CMOS outputs down by the zener voltage Vz. The standard zener voltage for commercially available zener diodes in this For price, delivery, and to place orders, please contact Hittite Microwave Corporation: 20 Alpha Road Chelmsford, MA 01824 Phone: 978-250-3343 Fax: 978-250-3373 Order Online at www.hittite.com v00.1203 HMC424LP3 PRODUCT NOTE range is -5.1V. The inverter logic gate is biased with 0V at the Vcc pin by grounding this pin and -5V at the ground terminal to operate the logic gate with negative voltage. Finally, a 100 ohm series resistance is added to minimize RF leakage between control pins. The input-output characteristic of the TTL/CMOS driver circuit is shown in figure 3 while a summary of the voltages at different nodes of driver circuit in shown figure 4. -5 Input HIGH for HMC424LP3 DRIVER OUTPUT (V) -4 -3 Input LOW for HMC424LP3 -2 -1 0 0 1 2 3 4 5 DRIVER INPUT (V) A B C D 100 ohm 10K ohm -5V A TTL/CMOS Output Signal B Input of the Inverter C Output of the Inverter D Control Input of HMC424LP3 LOW (0 to +0.5V) -5V 0V LOW HIGH (+2.5V to +5V) 0V -5V HIGH Figure 4 – Summary of voltage levels Since one driver circuit is required for each control input of the attenuator, figure 2 is duplicated six times to create the complete schematic of the driver and attenuator shown in figure 5. In order to realize the circuit of figure 5 in a minimum PCB design area, integration of the circuit elements was a major design criterion. Circuit elements were obtained in an array format including the hex inverter array, the recently-developed zener diode array1 and the 10kΩ resistor array. For price, delivery, and to place orders, please contact Hittite Microwave Corporation: 20 Alpha Road Chelmsford, MA 01824 Phone: 978-250-3343 Fax: 978-250-3373 Order Online at www.hittite.com PRODUCT APPLICATION NOTES 17 Figure 3 – Input-Output Characteristic of Driver Circuit 17 - 79 v00.1203 HMC424LP3 PRODUCT NOTE PRODUCT APPLICATION NOTES 17 17 - 80 Figure 5 – Complete driver schematic Driver Circuit Layout Figure 6 shows the physical layout of the application circuit board for HMC424LP3 with the driver circuit. The board material is Rogers 4350 with a total thickness of 10 mils, selected to achieve good RF performance through 15 GHz. Coplanar lines are utilized to minimize radiation from the RF traces and to minimize their width. C1 is a bypass capacitor added to minimize bias voltage noise and ripple from the external supplies. U2 is the hex inverter and U3 is the zener diode array. For price, delivery, and to place orders, please contact Hittite Microwave Corporation: 20 Alpha Road Chelmsford, MA 01824 Phone: 978-250-3343 Fax: 978-250-3373 Order Online at www.hittite.com v00.1203 HMC424LP3 PRODUCT NOTE Figure 6 – HMC424LP3 evaluation board with driver circuit Circuit Performance Since the digital attenuator is being supplied with its specified control voltage levels and there is no change to the RF circuit, there is no degradation in RF performance of the HMC424LP3. Full attenuation range and accuracy is maintained over the entire operating frequency range as shown in figure 7. Moreover, with the use of high speed CMOS inverter technology with very low current consumption, the driver circuit adds only one gate delay to the settling time and switching speed. For price, delivery, and to place orders, please contact Hittite Microwave Corporation: 20 Alpha Road Chelmsford, MA 01824 Phone: 978-250-3343 Fax: 978-250-3373 Order Online at www.hittite.com PRODUCT APPLICATION NOTES 17 17 - 81 v00.1203 HMC424LP3 PRODUCT NOTE NORMALIZED ATTENUATION (dB) 0 -5 -10 -15 -20 0.5 dB 1 dB 2 dB 4 dB 8 dB 16 dB 31.5 dB -25 -30 -35 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 FREQUENCY (GHz) Figure 7 – HMC424LP3 normalized attenuation Conclusion PRODUCT APPLICATION NOTES 17 This product note has detailed the design, layout and performance of a driver circuit interfacing the HMC424LP3 digital attenuator to TTL/CMOS outputs in a minimum board space. The driver circuit does not limit the RF performance of the attenuator or appreciably degrade switching speed. This circuit topology can be utilized to drive a wide variety of negative bias switch and attenuator products available from Hittite Microwave Corporation including the HMC307QS16G and HMC335G16 5-bit digital attenuators, the HMC344LP3 SP4T and the HMC322LP4 SP8T. (Endnotes) 1 17 - 82 For a complete description of this product please contact Microsemi Corporation or visit www.microsemi.com For price, delivery, and to place orders, please contact Hittite Microwave Corporation: 20 Alpha Road Chelmsford, MA 01824 Phone: 978-250-3343 Fax: 978-250-3373 Order Online at www.hittite.com v00.1203 HMC424LP3 PRODUCT NOTE Notes: PRODUCT APPLICATION NOTES 17 For price, delivery, and to place orders, please contact Hittite Microwave Corporation: 20 Alpha Road Chelmsford, MA 01824 Phone: 978-250-3343 Fax: 978-250-3373 Order Online at www.hittite.com 17 - 83 v00.1203 HMC424LP3 PRODUCT NOTE Notes: PRODUCT APPLICATION NOTES 17 17 - 84 For price, delivery, and to place orders, please contact Hittite Microwave Corporation: 20 Alpha Road Chelmsford, MA 01824 Phone: 978-250-3343 Fax: 978-250-3373 Order Online at www.hittite.com