8

3550.6

February 8, 2006

The circuit shown in Figure 8 is a simplified schematic of the

enable/disable function. The large value resistors in series with

the DISABLE pin makes it appear as a current source to the

driver. When the driver pulls this pin low current flows out of the

pin and into the driver. This current, which may be as large as

350

µA when external circuit and process variables are at their

extremes, is required to insure that point “A” achieves the

proper potential to disable the output.The driver must have the

compliance and capability of sinking all of this current.

dedicated TTL gate. The maximum low level output voltage

of the TTL gate, 0.4V, has enough compliance to insure that

turn on, and the TTL gate will sink enough current to keep

the DISABLE pin should be driven with an open collector

where the enable/disable function is not required on all circuits

some circuits can be permanently enabled by letting the

DISABLE pin float. If a driver is used to set the enable/disable

level, be sure that the driver does not sink more than 20

µA

when the DISABLE pin is at a high level. TTL gates, especially

CMOS versions, do not violate this criteria so it is permissible to

control the enable/disable function with TTL.

Typical Applications

Four Channel Video Multiplexer

cable in its characteristic impedance of 75

terminates the cable in its characteristic impedance. The

amplifier is set up in a gain configuration of +2 to yield an

overall network gain of +1 when driving a double terminated

position 1. At position 1 the switch pulls the disable pin up to

the plus supply rail thereby enabling the amplifier. Since all

of the actual signal switching takes place within the amplifier,

its differential gain and phase parameters, which are 0.03%

and 0.03 degrees respectively, determine the circuit’s

operate in a similar manner.

When the plus supply rail is 5V the disable pin can be driven by

a dedicated TTL gate as discussed earlier. If a multiplexer IC or

its equivalent is used to select channels its logic must be break

before make. When these conditions are satisfied the

HA5024IP is often used as a remote video multiplexer, and the

multiplexer may be extended by adding more amplifier ICs.

Low Impedance Multiplexer

Two common problems surface when you try to multiplex

multiple high speed signals into a low impedance source such

as an A/D converter. The first problem is the low source

impedance which tends to make amplifiers oscillate and

causes gain errors. The second problem is the multiplexer

which supplies no gain, introduces all kinds of distortion and

limits the frequency response. Using op amps which have an

enable/disable function, such as the HA5024, eliminates the

multiplexer problems because the external mux chip is not

needed, and the HA5024 can drive low impedance (large

capacitance) loads if a series isolation resistor is used.

15K

15K

ENABLE/DISABLE INPUT

A

FIGURE 8. SIMPLIFIED SCHEMATIC OF ENABLE/DISABLE

FUNCTION

NOTES:

19. All resistors in

Ω.

21. Use ground plane.

FIGURE 9. FOUR CHANNEL VIDEO MULTIPLEXER

+

-

+

-

+

-

+

-

VIDEO

INPUT

#1

VIDEO OUTPUT

75

Ω LOAD

75

3

2

4

75

681

2000

681

TO

1

75

681

681

75

10

8

9

7

2000

100

2

3

4

VIDEO

INPUT

#3

75

13

12

14

15

-5V

11

681

681

2000

75

+5V

ALL

OFF

VIDEO

INPUT

#4

75

18

19

17

6

20

75

2000

681

681

+5V

+5V IN

+5V

0.1

µF

10

µF0.1µF10µF

-5V IN

-5V

100

Ω

(NOTE 17)

100

Ω

(NOTE 17)

100

Ω

(NOTE 17)

100

Ω

(NOTE 17)

HA5024