Video Difference Amplifier Brings Versatility to Low Voltage Applications – Design Note 343 Jon Munson Introduction The LT®6552 is a specialized dual-differencing 75MHz operational amplifier ideal for rejecting common mode noise as a video line receiver. The input pairs are designed to operate with equal but opposite large-signal differences and provide exceptional high frequency common mode rejection (CMRR of 65dB at 10MHz), thereby forming an extremely versatile gain block structure that minimizes component count in most situations. The dual input pairs are free to take on independent common mode levels, while the two voltage differentials are summed internally to form a net input signal. The LT6552 is optimized for low supply voltages, ranging from 3V to 12V between the rails. In single supply operation at 3.3V, the input range includes ground and an output that can swing within 400mV of either supply rail when driving a 150Ω load. Additionally, a shutdown feature is provided to allow easy power management in supply current-sensitive applications. Dual Input Pair Zaps Common Mode Noise Pickup The basic application of the LT6552 is that of a high CMRR difference amplifier as shown in Figure 1. Only two resistors are required to set the difference gain (Gain = 2 in this example). For unity gain, no additional components are required. Single supply operation usually requires a means of output offset control so that signals aren’t clipped. The dual input structure of the LT6552 makes this easy (see VDC(ADJ) in Figure 1). The circuit shown in Figure 1 is especially effective in removing ground noise from video signals in vehicular and industrial applications, as shown in Figure 2, killing common mode noise by over a factor of 1000 and providing a lower solution cost than other op amp based topologies because of the fewer required components. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. VIN 5V 3 2 CABLE VDC(ADJ) 1 8 + – REF FB 7 LT6552 6 75Ω VOUT 4 75Ω RF 500Ω RG 500Ω RECOVERED OUTPUT DN343 F01 CF 8pF Figure 1. Noise Rejecting Sense Amplifier with DC Adjust 08/04/343_conv NOISY INPUT DN343 F02 Figure 2. Common Mode Noise Eliminated from Video Signal Perform Video Rate Analog Arithmetic Because of its dual differencing input structure, the LT6552 is able to readily process both additive (noninverting) and subtractive (inverting) variables without the need for complicated resistor networks. Figure 3 shows that the output is a function of three input variables with SHDN V3 V2 V1 3 2 1 8 5 + – V+ 7 REF FB 6 LT6552 VO Conclusions By virtue of its dual differencing input structure and ability to operate from low supply voltages, the LT6552 provides a versatile and high performing gain block for modern analog applications. Of particular value is the part’s ability to recover difference signals in the presence of strong common mode interference, such as in vehicular and closed-circuit video applications. 4 RF RG VO = (V1 – V2 + V3) DN343 F03 RF + RG RG Figure 3. General Purpose Arithmetic Block 3V 499Ω 499Ω the overall scaling set by the feedback resistors. This property provides a useful means of differential to single ended conversion with offset control or performing multivariate functions such as the YPBPR to RGB video adapter application shown in Figure 4. The circuit of Figure 4 uses the fewest possible amplification stages to accomplish the needed matrix conversion functions while operating on the lowest supply voltages (±3V). Input black levels should be near 0V for best results and sync on Y is mapped to all three outputs, though typically only needed on G. 3V 499Ω 8 8 7 8.2pF FB 1 REF 6 LT6552 2 – SHDN 3 + 5 4 1 2 3 7 FB REF – + LT6552 SHDN 4 499Ω 5.6pF 6 75Ω G 75Ω 5 –3V –3V 3V 499Ω 909Ω Y 8 1 1R 21.5Ω 21.5Ω 53.6Ω 11.3Ω 2 3 FB REF 7 LT6552 2.2pF 6 75Ω R – SHDN + 5 75Ω 4 –3V 42.2Ω 3V 499Ω 8 1 1B 2 49.9Ω 25.5Ω 3 3:t1R (:ot1Bot1R #:t1B FB REF – + 7 LT6552 SHDN 4 –3V 1.30k 1pF 6 75Ω B 75Ω 5 BW (±0.5dB) > 25MHz BW (–3dB) > 36MHz IS ≈ 70mA DN343 F04 Figure 4. YPBPR to RGB Component Video Converter Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn343f_conv LT/TP 0804 305K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2004