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AD815
(Rev.:RevC)

High Output Current Differential Driver

Analog Devices 

AD815

1k⍀

+15V

0.1␮F

3

8

1/2

AD826

1

2

1k⍀

1k⍀

1k⍀

6

1/2

AD826 7

5

4 0.1␮F

+15V

100⍀

RG

100⍀

100⍀

0.1␮F

4

8

1/2

AD815

6

5

RF

499⍀

RF

10

499⍀

1/2

AD815 9

11

7

10␮F

RL

–15V

0.1␮F

–15V

10␮F

Figure 52. Differential Driver with Single-Ended

Differential Converter

Another means for creating a differential signal from a single-

ended signal is to use a transformer with a center-tapped

secondary. The center tap of the transformer is grounded and

the two secondary windings are connected to obtain opposite

polarity signals to the two inputs of the AD815 amplifiers. The

bias currents for the AD815 inputs are provided by the center

tap ground connection through the transformer windings.

One advantage of using a transformer is its ability to provide

isolation between circuit sections and to provide good common-

mode rejection. The disadvantages are that transformers have

no dc response and can sometimes be large, heavy, and expensive.

This circuit is shown in Figure 53.

+15V

100⍀

0.1␮F 10␮F

4

8

1/2

AD815 6

50⍀

5

50⍀

1k⍀

200⍀

1k⍀

RL

10

1/2

100⍀

AD815 9

11

7

0.1␮F 10␮F

–15V

Figure 53. Differential Driver with Transformer Input

Direct Single-Ended-to-Differential Conversion

Two types of circuits can create a differential output signal from

a single-ended input without the use of any other components

than resistors. The first of these is illustrated in Figure 54.

+15V

VIN

4

8

AMP 1

1/2

AD815

6

5

RF1

402⍀

RG

100⍀

RF2

10

499⍀

1/2

AMP 2 AD815 9

11

7

RL VOUT

–15V

Figure 54. Direct Single-Ended-to-Differential Conversion

Amp 1 has its + input driven with the input signal, while the

+ input of Amp 2 is grounded. Thus the – input of Amp 2 is

driven to virtual ground potential by its output. Therefore

Amp 1 is configured for a noninverting gain of five, (1 + RF1/RG),

because RG is connected to the virtual ground of Amp 2’s – input.

When the + input of Amp 1 is driven with a signal, the same

signal appears at the – input of Amp 1. This signal serves as an

input to Amp 2 configured for a gain of –5, (–RF2/RG). Thus the

two outputs move in opposite directions with the same gain and

create a balanced differential signal.

This circuit can work at various gains with proper resistor

selection. But in general, in order to change the gain of the

circuit, at least two resistor values will have to be changed. In

addition, the noise gain of the two op amps in this configuration

will always be different by one, so the bandwidths will not match.

A second circuit that has none of the disadvantages mentioned

in the above circuit creates a differential output voltage feedback

op amp out of the pair of current feedback op amps in the AD815.

This circuit, drawn in Figure 55, can be used as a high power

differential line driver, such as required for ADSL (asymmetrical

digital subscriber loop) line driving.

Each of the AD815’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 output of

Amp 2 driven to saturation in the negative direction. This is

similar to the way a conventional op amp behaves without any

feedback.

REV. C

–13–

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