AD8042

Rev. E | Page 14 of 16

Single-Ended-to-Differential Driver

Using a cross-coupled, single-ended-to-differential converter

(SEDC), the AD8042 makes a good general-purpose differential

line driver. This SEDC can be used for applications such as

driving Category-5 (CAT-5) twisted pair wires. Figure 39 shows

a configuration for a circuit that performs this function that can

be used for video transmission over a differential pair or various

data communication purposes.

10µF

60.4Ω

60.4Ω

100Ω

1kΩ

1kΩ

49.9Ω

1kΩ

1kΩ

1kΩ

1kΩ

0.1µF

50m

121Ω

AMP1

8

3

2

6

5

7

4

1

AMP2

+5V

AD8042

10µF

0.1µF

–5V

Figure 39. Single-Ended-to-Differential Twisted Pair Line Driver

Each of the op amps of the AD8042 is configured as a unity gain

symmetrical circuit.

B

If the noninverting input of AMP2 is grounded and a small

positive signal is applied to the noninverting input of AMP1,

the output of AMP1 is driven to saturation in the positive

direction and the input of AMP2 is driven to saturation in the

negative direction. This is similar to the way a conventional op

amp behaves without any feedback.

noninverting input of AMP1, negative feedback is provided, which

like a conventional inverting op amp configuration with

differential outputs.

circuit the advantage of being able to adjust its gain by changing

a single resistor.

The cable has a characteristic impedance of about 120 Ω. Each

driver output is back terminated with a pair of 60.4 Ω resistors

to make the source look like 120 Ω. The receive end is terminated

with 121 Ω, 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.

Figure 40 shows the results of the circuit in Figure 39 driving

50 meters of CAT-5 cable.

50ns

1V

100

10

0%

90

200mV

200mV

Figure 40. 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 ADCs. Figure 41 is a schematic that shows such a circuit

differentially driving an AD9220, a 12-bit, 10 MSPS ADC.

VINA

VINB

CAPT

CAPB

VREF

SENSE

CML

CLK

AD9220

19

27

25

14

13

12

11

10

9

8

7

6

5

4

3

2

+5V

0.1µF

0.1µF

0.1µF

0.1µF

0.1µF

0.1µF

0.1µF

2.49kΩ

1kΩ

1kΩ

1kΩ

1kΩ

1kΩ

1kΩ

2.49kΩ

0.1µF

DVDD

AVDD

AVDD

REFCOM DVSS AVSS AVSS

16

28

15

26

OTR

BIT 1

BIT 2

BIT 3

BIT 4

BIT 5

BIT 6

BIT 7

BIT 8

BIT 9

BIT 10

BIT 11

BIT 12

CLOCK

1

18

17

22

10/16

8

3

2

6

5

7

4

+5V

1

+5V

+5V

+5V

+5V

AD8042

0.1µF

0.1µF

Figure 41. AD8042 Differential Driver for the AD9220 12-Bit, 10 MSPS ADC