AD834

Data Sheet

Rev. F | Page 12 of 20

BIASING THE OUTPUT

The AD834 has two open collector outputs as shown in Figure 13.

transistors. The following general guidelines maximize

performance of the AD834.

MULTIPLIER

CORE

W1

W2

+5V

+5V

+5V

RW

49.9

Ω

RW

49.9

Ω

75

Ω

BIAS

–5V

OUTPUT OF AD834

AD834

5

4

6

Figure 13. Output Stage Block Diagram

12.5mA

RW

W COLLECTOR

HEADROOM

–

+

W BASE

+5V

+5V

NEGATIVE OUTPUT

VOTLAGE SWING

SITUATION 1

8.0mA TO 14mA

(GENERALLY 10.5mA)

Figure 14. Negative Swing

Figure 14 shows the currents at the input when the AD834

higher. For best performance, use resistor values that do not

saturate the output transistors. Allowing for adequate transistor

headroom reduces distortion.

When either output swings negative, the maximum current

flows through the RW resistors. It is in this situation that

headroom is at a minimum.

Try to keep headroom at or above 200 mV to maintain adequate

range. Headroom ≥ 200 mV.

This recommendation addresses the positive swing of the

output as shown in Figure 15. It is sometimes difficult to meet

this for negative output swing.

4.5mA

RW

W COLLECTOR

HEADROOM

–

+

W BASE

+5V

+5V

POSITIVE OUTPUT

VOTLAGE SWING

8.0mA TO 14mA

(GENERALLY 10.5mA)

SITUATION 2

Figure 15. Positive Output Swing

The current through RW is smaller for positive output swings.

For dc applications or applications where distortion is not a

concern, the headroom may be zero or as low as −200 mV.

However, for most cases, size the resistors to give the output

adequate headroom.

TRANSFORMER COUPLING

In many high frequency applications where baseband operation

is not required at either inputs or the output, transformer coupling

can be used. Figure 16 shows the use of a center-tapped output

transformer, which provides the necessary dc load condition

at the outputs, W1 and W2, and is designed to match into the

desired load impedance by appropriate choice of turns ratio.

The specific choice of the transformer design depends entirely

on the application. Transformers can also be used at the inputs.

Center-tapped transformers can reduce high frequency distortion

and lower HF feedthrough by driving the inputs with balanced

signals.

8

7

6

5

1

2

3

4

X2

Y1

Y2

AD834

X-INPUT

±1V FS

Y-INPUT

±1V FS

TERMINATION

RESISTOR

TERMINATION

RESISTOR

49.9Ω

+5V

4.7Ω

–5V

LOAD

1µF

CERAMIC

1µF

CERAMIC

Figure 16. Transformer-Coupled Output

A particularly effective type of transformer is the balun1, which

is a short length of transmission line wound onto a toroidal

ferrite core. Figure 17 shows this arrangement used to convert

the bal(anced) output to an un(balanced) one (therefore, the

use of the term). Although the symbol used is identical to that

for a transformer, the mode of operation is quite different. First,

the load should now be equal to the characteristic impedance of

the line (although this is usually not critical for short line lengths).

terminate the line, but again this is only necessary when an

as the dc conditions allow to minimize power loss to the load.

The line can be a miniature coaxial cable or a twisted pair.

Sevick; American Radio Relay League publication.