Datasheet AD8042 (Analog Devices) - 16

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AD8042 Single-Ended-to-Differential Driver
The cable has a characteristic impedance of about 120 Ω. Each Using a cross-coupled, single-ended-to-differential converter driver output is back terminated with a pair of 60.4 Ω resistors (SEDC), the AD8042 makes a good general-purpose differential to make the source look like 120 Ω. The receive end is terminated line driver. This SEDC can be used for applications such as with 121 Ω, and the signal is measured differentially with a pair driving Category-5 (CAT-5) twisted pair wires. Figure 39 shows of scope probes. One channel on the oscilloscope is inverted a configuration for a circuit that performs this function that can and then the signals are added. be used for video transmission over a differential pair or various data communication purposes. Figure 40 shows the results of the circuit in Figure 39 driving 50 meters of CAT-5 cable.
+5V 0.1µF 10µF 1V 200mV 50ns RIN 100 R 1kΩ F 3 8 V V 90 IN 1kΩ IN 60.4Ω 1 49.9Ω AMP1 2 RA 1kΩ 50m R R B B V 1kΩ 1kΩ 121Ω OUT AD8042 VOUT RA 10 1kΩ 6 0% 7 60.4Ω
0
5 AMP2 200mV
-04
4
59 10 0
100Ω
39 0
0.1µF 10µF
Figure 40. Differential Driver Frequency Response 9- 05
–5V
01 Figure 39. Single-Ended-to-Differential Twisted Pair Line Driver
Single-Supply Differential A/D Driver
The single-ended-to-differential converter circuit is also useful Each of the op amps of the AD8042 is configured as a unity gain as a differential driver for video speed, single-ended, differential follower by the feedback resistors (RA). Each op amp output also input ADCs. Figure 41 is a schematic that shows such a circuit drives the other as a unity gain inverter via RB, c B reating a totally differentially driving an AD9220, a 12-bit, 10 MSPS ADC. symmetrical circuit.
+5V
If the noninverting input of AMP2 is grounded and a small
+5V 0.1µF
positive signal is applied to the noninverting input of AMP1, the output of AMP1 is driven to saturation in the positive
0.1µF 1kΩ 1kΩ
direction and the input of AMP2 is driven to saturation in the
3 8 VIN 1
negative direction. This is similar to the way a conventional op
2 +5V +5V +5V
amp behaves without any feedback.
1kΩ 0.1µF 0.1µF 0.1µF 28 15 26
If a resistor (R
1kΩ 1kΩ
F) is connected from the output of AMP2 to the
AD8042 DVDD AVDD AVDD
noninverting input of AMP1, negative feedback is provided, which
+5V 14 1kΩ OTR
closes the loop. An input resistor (R
VINA
IN) makes the circuit look
6 13 BIT 1 7
like a conventional inverting op amp configuration with
2.49kΩ VINB 12 BIT 2 5 11
differential outputs.
4 BIT 3 2.49kΩ 0.1µF CAPT 10 BIT 4 AD9220 9 0.1µF BIT 5
The gain of this circuit from input to either output is ±RF/RIN, or
10/16 0.1µF 8 CAPB BIT 6
the single-ended-to-differential gain is 2 × R
7
F/RIN. This gives the
0.1µF 18 BIT 7 VREF 6
circuit the advantage of being able to adjust its gain by changing
17 BIT 8 SENSE 5 BIT 9
a single resistor.
22 4 CML BIT 10 0.1µF 3 BIT 11 2 1 CLK BIT 12 CLOCK REFCOM DVSS AVSS AVSS
041
19 27 25 16
59- 10 0 Figure 41. AD8042 Differential Driver for the AD9220 12-Bit, 10 MSPS ADC Rev. E | Page 14 of 16 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION CONNECTION DIAGRAM TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS MAXIMUM POWER DISSIPATION ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS APPLICATIONS INFORMATION CIRCUIT DESCRIPTION DRIVING CAPACITIVE LOADS OVERDRIVE RECOVERY Single-Supply Composite Video Line Driver Single-Ended-to-Differential Driver Single-Supply Differential A/D Driver HDSL Line Driver LAYOUT CONSIDERATIONS OUTLINE DIMENSIONS ORDERING GUIDE