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AD8042ARZ-REEL Datasheet(PDF) 13 Page - Analog Devices |
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AD8042ARZ-REEL Datasheet(HTML) 13 Page - Analog Devices |
13 / 16 page REV. C AD8042 –13– Each of the AD8042’s op amps is configured as a unity gain follower by the feedback resistors (RA). Each op amp output also drives the other as a unity gain inverter via the two RBS, creating a totally symmetrical circuit. If the + input to Amp 2 is grounded and a small positive signal is applied to the + input of Amp 1, the output of Amp 1 will be driven to saturation in the positive direction and the input of Amp 2 driven to saturation in the negative direction. This is similar to the way a conventional op amp behaves without any feedback. If a resistor (RF) is connected from the output of Amp 2 to the + input of Amp 1, negative feedback is provided which closes the loop. An input resistor (RI) will make the circuit look like a conventional inverting op amp configuration with differential outputs. The gain of this circuit from input to either output will be ±RF/ RI. Or the single-ended-to-differential gain will be 2 ¥ RF/RI. This gives the circuit the advantage of being able to adjust its gain by changing a single resistor. The cable has a characteristic impedance of about 120 W. Each driver output is back terminated with a pair of 60.4 W resistors to make the source look like 120 W. The receive end is termi- nated with 121 W, and the signal is measured differentially with a pair of scope probes. One channel on the oscilloscope is inverted and then the signals are added. The scope photo in Figure 9 shows a 10 MHz, 2 V p-p input signal driving the circuit with 50 m of category 5 twisted pair wire. 10 0% 100 90 50ns 1V 200mV VIN VOUT 200mV Figure 9. Differential Driver Frequency Response Single-Supply Differential A/D Driver The single-ended-to-differential converter circuit is also useful as a differential driver for video speed, single-ended, differential input A/D converters. Figure 10 is a schematic that shows such a circuit differentially driving an AD9220, a 12-bit, 10-MSPS A/D converter. AD9220 VINA VINB CAPT CAPB VREF SENSE CML CLK 19 27 25 14 13 12 11 10 9 8 7 6 5 4 3 2 +5V 0.1 F DVDD AVDD AVDD REFCOM DVSS AVSS AVSS 16 28 15 26 OTR BIT1 BIT2 BIT3 BIT4 BIT5 BIT6 BIT7 BIT8 BIT9 BIT10 BIT11 BIT12 CLOCK 1 0.1 F 0.1 F 18 17 22 10/16 8 3 2 6 5 7 4 +5V 1 AD8042 2.49k 0.1 F 1k VIN 1k 1k 1k 1k 1k 0.1 F +5V +5V 0.1 F 2.49k 0.1 F 0.1 F +5V 0.1 F +5V 0.1 F Figure 10. AD8042 Differential Driver for the AD9220 12-Bit, 10-MSPS A/D Converter The circuit was tested with a 1 MHz input signal and clocked at 10 MHz. An FFT response of the digital output is shown in Figure 11. Pin 5 is biased at 2.5 V by the voltage divider and bypassed. This biases each output at 2.5 V. VIN is ac coupled such that VIN going positive makes VINA go positive and VINB go in the negative direction. The opposite happens for a negative going VIN. 1 4 9 7 2 3 6 8 5 FUND FRQ 1000977 SMPL FRQ 10000000 THD –82.00 SNR 71.13 SINAD 70.79 SFDR –86.74 2nd –88.34 3rd –86.74 4th –99.26 5th –90.67 6th –99.47 7th –91.16 8th –97.25 9th –91.61 HARMONICS (dBc) Figure 11. FFT of AD9220 Output When Driven by AD8042 |
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