AD8189ARUZ-R7 datasheet(15/24 Pages) AD

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AD8188/AD8189

Rev. 0 | Page 15 of 24

IN0A

IN0B

IN1B

IN1A

IN2A

IN2B

OUT0

OUT1

OUT2

AD8188

MUX SYSTEM

“C_BYPASS”

VREF

INTERNAL CAP

BIAS REFERENCE

DIRECT CONNECTION TO ANY “QUIET” AC GROUND

(FOR EXAMPLE, GND, VCC, AND VEE).

Figure 45. VREF Pin Connection for AD8188 (Differs from AD8189)

The AD8189

The AD8189 uses on-chip feedback resistors to realize the gain-

of-two function. To provide low crosstalk and a high output

impedance when disabled, each set of 500 Ω feedback resistors

is terminated by a dedicated reference buffer. A reference buffer

is a high speed op amp configured as a unity-gain follower. The

three reference buffers, one for each channel, share a single,

high impedance input, the VREF pin (see Figure 46). VREF input

bias current is typically less than 2 μA.

OUT1

OUT0

OUT2

VREF

GBUF 0

GBUF 1

GBUF 2

500Ω

VF-2

VF-1

1×

VFO

Figure 46. Conceptual Diagram of a Single Multiplexer Channel, G = 2

This configuration has a few implications for single-supply

operation:

•

On the AD8189, VREF cannot be tied to the most negative

analog supply, VEE. The limits on reference voltage are (see

Figure 47):

VEE

+ 1.3 V < VREF × VCC − 1.6 V

1.3 V < VREF, 3.4 V on 0 V/5 V supplies

VREF

OUT0

VO_MAX = 3.7V

VO_MIN = 1.3V

VO_MAX = 3.4V

1.3V

1.6V

GND

VOUT

VREF

Figure 47. Output Compliance of Main Amplifier Channel and Ground Buffer

•

The signal at the VREF pin appears at each output.

Therefore, VREF should be tied to a well bypassed, low

impedance source. Using superposition, it is shown that

VOUT

= 2 × VIN − VREF

•

To maximize the output dynamic range, the reference

voltage should be chosen with care. For example, consider

amplifying a 700 mV video signal with a sync pulse

300 mV below black level. If the user decides to set VREF at

black level to preferentially run video signals on the faster

NPN transistor path, the AD8189 allows a reference

voltage as low as 1.3 V + 300 mV = 1.6 V. If the AD8189 is

used, the sync pulse is amplified to 600 mV. Therefore, the

lower limit on VREF becomes 1.3 V + 600 mV = 1.9 V. For

routing RGB video, an advantageous configuration is to

employ +3 V and −2 V supplies, in which case VREF can be

tied to ground.

If system considerations prevent running the multiplexer on

split supplies, a false ground reference should be employed. A

low impedance reference can be synthesized with a second

operational amplifier. Alternately, a well bypassed resistor

divider can be used. Refer to the Applications section for

further explanation and more examples.

VREF

0.022µF

GND

OP21

100kΩ

10kΩ

100Ω

FROM 1992 ADI AMPLIFIER

APPLICATIONS GUIDE

1µF

Figure 48. Synthesis of a False Ground Reference

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