AD8131

Rev. B | Page 18 of 20

APPLICATIONS

TWISTED-PAIR LINE DRIVER

The AD8131 has on-chip resistors that provide for a gain of 2

without any external parts. Several on-chip resistors are

trimmed to ensure that the gain is accurate, the common-mode

rejection is good, and the output is well balanced. This makes

the AD8131 very suitable as a single-ended-to-differential

twisted-pair line driver.

Figure 40 shows a circuit of an AD8131 driving a twisted-pair

line, like a Category 3 or Category 5 (Cat3 or Cat5), that is

already installed in many buildings for telephony and data

communications. The characteristic impedance of such a

transmission line is usually about 100 Ω. The outstanding

balance of the AD8131 output will minimize the common-

mode signal and therefore the amount of EMI generated by

driving the twisted pair.

The two resistors in series with each output terminate the line at

the transmit end. Since the impedances of the outputs of the

AD8131 are very low, they can be thought of as a short-circuit,

and the two terminating resistors form a 100 Ω termination at

the transmit end of the transmission line. The receive end is

directly terminated by a 100 Ω resistor across the line.

This back-termination of the transmission line divides the

output signal by two. The fixed gain of 2 of the AD8131 will

create a net unity gain for the system from end to end.

In this case, the input signal is provided by a signal generator

with an output impedance of 50 Ω. This is terminated with a

resistance of the source and termination is 25 Ω.The 24.9 Ω

impedance and minimizes any dc and gain errors.

distance, such as the output of an op amp, then no termination

tied to ground.

AD8131

8

2

3

6

5

4

+

+

1

24.9

Ω

49.9

Ω

100

Ω

10

μF

0.1

μF

49.9

Ω

49.9

Ω

10

μF

0.1

μF

+5V

–5V

RECEIVER

Figure 40. Single-Ended-to-Differential 100 Ω Line Driver

3 V SUPPLY DIFFERENTIAL A-TO-D DRIVER

Many newer ADCs can run from a single 3 V supply, which can

save significant system power. In order to increase the dynamic

range at the analog input, they have differential inputs, which

double the dynamic range with respect to a single-ended input.

An added benefit of using a differential input is that the

distortion can be improved.

The low distortion and ability to run from a single 3 V supply make

the AD8131 suited as an A-to-D driver for some 10-bit, single-

supply applications. Figure 41 shows a schematic for a circuit for an

AD8131 driving an AD9203, a 10-bit, 40 MSPS ADC.

The common mode of the AD8131 output is set at midsupply

a 0.1 μF capacitor. This provides for maximum dynamic range

between the supplies at the output of the AD8131. The 110 Ω

resistors at the AD8131 output, along with the shunt capacitors

form a one pole, low-pass filter for lowering noise and

antialiasing.

AD8131

8

2

3

6

+

3V

LPF

+3V

1

20pF

20pF

AD9203

26

25

28

AVDD

DRVDD

AINP

AINN

AVSS

DRVSS

3V

27

1

2

DIGITAL

OUTPUTS

0.1 F

10 F

0.1 F

0.1 F

110

Ω

110

Ω

24.9

Ω

49.9

Ω

10k

Ω

10k

Ω

Figure 41. Test Circuit for AD8131 Driving an AD9203, 10-Bit, 40 MSPS ADC

Figure 42 shows an FFT plot that was taken from the combined

devices at an analog input frequency of 2.5 MHz and a 40 MSPS

sampling rate. The performance of the AD8131 compares very

favorably with a center-tapped transformer drive, which has

typically been the best way to drive this ADC. The AD8131 has

the advantage of maintaining dc performance, which a

transformer solution cannot provide.