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OP292GP Arkusz danych(PDF) 11 Page - Analog Devices |
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OP292GP Arkusz danych(HTML) 11 Page - Analog Devices |
11 / 20 page REV. B OP292/OP492 –11– voltage. However, the output can be offset by setting VREFERENCE from 0 V to 4 V, as long as the input common-mode voltage of the amplifier is not exceeded. VIN VREF 8 VOUT 5V 7 4 1 5 VOUT = 5 40k RG + VREF 20k 5k 20k 5k RG 1/2 OP292 1/2 OP292 Figure 4. A Single-Supply Instrumentation Amplifier In this configuration, while the output can swing to near zero volts, one needs to be careful because the input’s common-mode voltage range cannot operate to zero volts. This is because of the limitation of the circuit configuration where the first amplifier must be able to swing below ground in order to attain a 0 V common-mode voltage, which it cannot do. Depending on the gain of the instrumentation amplifier, the input common-mode extends to within about 0.3 V of zero. One can easily calculate the worst-case common-mode limit for a given gain. DAC Output Amplifier The OP292/OP492 are ideal for buffering the output of single- supply D/A converters. Figure 5 shows a typical amplifier used to buffer the output of a CMOS DAC that is connected for single- supply operation. To do that, the normally current output 12-bit CMOS DAC (R-2R ladder type) is connected backward to pro- duce a voltage output. This operating configuration necessitates a low voltage reference. In this case, a 1.235 V low-power reference is used. The relatively high output impedance (10 k ) is buffered by the OP292 and at the same time gained up to a much more usable level. The potentiometer provides an accurate gain trim for a 4.095 V full-scale, allowing 1mV increment per LSB of control resolution. The DAC8043 device comes in an 8-pin DIP package providing a cost-effective, compact solution to a 12-bit analog channel. VDD Clk Sri 1 2 3 4 8 7 6 5 DAC8043 5V 5V 5V 7.5k 1.235V Ad589 NC DIGITAL CONTROL LD SRI CLK 500k 8.45k VOUT 20k 1/2 OP292 1mV/LSB 0V 4.095V FS VREF VFB t0 VND LD SRI CLK VDD Figure 5. A 12-Bit Single-Supply DAC with Serial Bus Control A 50 Hz/60 Hz Single-Supply Notch Filter Figure 6 shows a notch filter that achieves nearly 30 dB of 60 Hz rejection while powered by only a single 12 V supply. The circuit also works well on 5 V systems. The filter utilizes a twin-T configu- ration whose frequency selectivity depends heavily on the relative matching of the capacitors and resistors in the twin-T section. Mylar is a good choice for the twin-T’s capacitors, and the relative matching of the capacitors and resistors determines the filter’s passband symmetry. Using 1% resistors and 5% capacitors produces satisfactory results. The amount of rejection and the Q of the filter is solely determined by one resistor, and is shown in the table. The bottom amplifier is used to split the supply to bias the amplifier to midlevel. The circuit can be modified to reject 50 Hz by simply changing the resistors in the twin-T section (Rl through R4) from 2.67 k to 3.16 k , and changing R5 to 1⁄2 of 3.16 k . For best results, the common value resistors can be from a resistor array for opti- mum matching characteristics. 1/4 OP492 C1 1 F C3 2 F (1 F 2) R5 1.335k (2.67k 2) R4 2.67k C2 1 F R6 100k RQ 8k 12V 12V R8 100k R9 100k C4 1 F 6V R7 1k R2 2.67k VOUT VIN NOTE FOR 50Hz APPLICATION CHANGE R12 R4 TO 3.16k AND R5 TO 1.58k (3.16k 2) FILTER Q 0.75 1.00 1.25 2.50 5.00 10.00 RQ (k ) 1.0 2.0 3.0 8.0 18 38 REJECTION (dB) 40 35 30 25 20 15 VOLTAGE GAIN 1.33 1.50 1.60 1.80 1.90 1.95 1/4 OP492 1/4 OP492 R3 2.67k R1 2.67k Figure 6. A Single-Supply 50 Hz/60 Hz Notch Filter VOUT 5V 5k 5k 1.78k 16.2k 100 F 2 3 1 8 4 6 5 7 5V VIN 1.1k 14.3k 0.01 F 0.022 F 3300pF 2200pF 1/2 OP292 1/2 OP292 Figure 7. A 4-Pole Bessel Low-Pass Filter Using Sallen-Key Topology A 4-Pole Bessel Low-Pass Filter The linear phase filter in Figure 7 is designed to roll off at a voiceband cutoff frequency of 3.6 kHz. The 4 poles are formed by two cascading stages of two-pole Sallen-Key filters. |
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