Window Comparator With Independent Adjustments (HA2841, HA4905) Application Note April 1998 AN9802 Introduction Window comparators, or limit comparators as they are sometimes called, determine when the input voltage is within a pre-selected range. The output voltage usually goes to a TTL high when the input voltage is within the pre-selected range. A typical window comparator circuit normally consists of two comparators with two reference adjustments: one for the low trip level, and one for the high trip level. If the reference voltages are derived from the same resistor divider string, hystersis can not be added without having the trip levels interact. If the reference voltages are independently derived they can drift in different directions causing the window to widen or possibly disappear. There might be a temptation to change the comparator to an op amp. This is poor practice for any circuit which requires high speed performance because the op amp output would saturate causing uncontrolled time delays. The comparator is subject to multiple switching caused by high frequency noise riding on slow rising input signals. The multiple switching is eliminated by the capacitor, C, which is placed across its input leads. Effectively, C couples the high frequency noise onto both comparator leads enabling it to be rejected by the comparator’s common-mode rejection capability. Notice in the waveform that there is no multiple switching taking place. The window comparator shown in Figure 1 circumvents these problems with independent trip and window adjustments. The HA2841 op amp (selected because it has high speed and low input currents) adds the input signal to the trip voltage developed by RT. The window width voltage RW is summed into the output of the op amp as an offset voltage. The op amp has two feedback loops each of which contains a steering diode, D1 or D2. The high open loop gain of the op amp insures that one diode will always be forward biased unless RW supplies the feedback current, thus when the sum of the input voltages (closed loop gain is one) and IRW passes through zero the current flow switches from one diode to the other. The output comparator senses the polarity switch across the diodes, and the comparator output voltage changes. This circuit yields a window comparator with independent adjustments, temperature stability, wide voltage range compatibility, and TTL compatibility. The HA4905 comparator has inputs that can work from the 15V supplies, thus the input voltage swing is matched to the op amp’s output voltage swing. Even though the input section of the HA4905 is connected to 15V, the output section can be connected from +5V to ground, thus offering a TTL compatible output voltage. RG1 10K +15V RF1 10K R 100K INPUT RW 10K +15 +5V +15V 4 3 + 7 D1 6 HA2841 4 - D2 2 +15V -15V + 1 47pF 1/4 3 HA4905 9 5 C 2 VO -15V -15 RT 5K 13 RG2 10K RF2 10K FIGURE 1. WINDOW COMPARATOR WITH INDEPENDENT ADJUSTMENTS RT sets the trip point anywhere between the power supplies, and RW sets the window width from zero, when the center tap voltage is zero, to about 90 percent of the input waveform. The trip point should be set first, then the window can be set to the desired width. Since the adjustments do not interact, the trip point stays constant as the window width is changed. FIGURE 2. 1 1-888-INTERSIL or 321-724-7143 | Copyright © Intersil Corporation 1999 Application Note 9802 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