Low Output Impedance MUX (HA5022) Application Note November 1996 AN9503.1 R3 sets the amplifier's frequency response, so it's best to check the manufacturer's data sheet before changing its value. Two common problems will surface when trying to multiplex multiple high-speed signals into a low-impedance load, such as an analog-to-digital converter. The first is the low load impedance, which tends to make amplifiers oscillate and thus causes gain errors. The second problem involves the multiplexer, which supplies no gain, introduces distortion, and limits the frequency response. R5, C1, and D1 make up an asymmetrical charge/discharge time circuit that configures U1 as a break-before-make switch to prevent both amplifiers from being active simultaneously. If this design is extended to more channels, the drive logic must be designed to be break-before-make. Also, the inhibit input is only functional when the channel switch input is high. R4 is enclosed in the feedback loop of the amplifier so that the large open-loop amplifier gain of U2 will present the load with a small closed-loop output impedance while keeping the amplifier stable for all values of load capacitance. Using op amps that have an enable/disable function, such as the HA5022, will eliminate the multiplexer problems. That's because the external multiplexer chip isn't needed, and the HA5022 can drive low-impedance (large capacitance) loads if a series isolation resistor is employed. Looking more closely at the circuit, both inputs are terminated in their characteristic impedance; 75 is typical for video applications, see Figure 1. Because the output cables usually are terminated in their characteristic impedance, the gain is 0.5. Consequently, amplifiers U2A and U2B are configured in a gain of +2 to set the circuit gain at 1. R2 and R3 determine the amplifier gain; if a different gain is desired, R2 should be changed according to the equation: G = (1 + R3/R2). The circuit shown was tested for the full range of capacitor values with no oscillations observed. Thus, the problem is solved. The circuit's frequency and gain characteristics are now those of the amplifier independent of any multiplexing action. This essentially solves the second problem. The multiplexer transition time is approximately 15s with the component values shown. R3A 681 INPUT B R2A 681 R1A 75 INPUT A D1A 1N4148 R1B 75 R4A 1 U2A 27 16 2 + 4 -5V 3 0.01F R5A U1C 2000 C1A 0.047F CHANNEL SWITCH R2B 681 R3B 681 7 U2B R4B - 10 6 + 13 27 5 +5V R5B 2000 INHIBIT U1A R6 100K U1B OUTPUT 0.01F NOTES: U1D C1B D1B 1N4148 0.047F 1. U2: HA5022 2. U1: CD4011 FIGURE 1. THIS LOW-OUTPUT IMPEDANCE MULTIPLEXER WILL SOLVE PROBLEMS OF OSCILLATION CAUSED BY LOW LOAD IMPEDANCE, AS WELL AS DISTORTION AND LIMITED FREQUENCY RESPONSE INTRODUCED BY THE MULTIPLEXER. THE SECOND PROBLEM IS SOLVED BECAUSE THE FREQUENCY AND GAIN CHARACTERISTICS BECOME THOSE OF THE AMPLIFIER, INDEPENDENT OF THE MULTIPLEXER. 1 1-888-INTERSIL or 321-724-7143 | Copyright (c) Intersil Corporation 1999 Application Note 9503 All Intersil semiconductor products are manufactured, assembled and tested under ISO9000 quality systems certification. Intersil semiconductor products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries. For information regarding Intersil Corporation and its products, see web site www.intersil.com Sales Office Headquarters NORTH AMERICA Intersil Corporation P. O. Box 883, Mail Stop 53-204 Melbourne, FL 32902 TEL: (321) 724-7000 FAX: (321) 724-7240 EUROPE Intersil SA Mercure Center 100, Rue de la Fusee 1130 Brussels, Belgium TEL: (32) 2.724.2111 FAX: (32) 2.724.22.05 2 ASIA Intersil (Taiwan) Ltd. 7F-6, No. 101 Fu Hsing North Road Taipei, Taiwan Republic of China TEL: (886) 2 2716 9310 FAX: (886) 2 2715 3029