AD8186 datasheet(13/20 Pages) AD

13 / 20 page

REV. A

AD8186/AD8187

–13–

The AD8187

The AD8187 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 4). VREF input bias

current is typically less than 2

µA.

VREF

GBUF 0

GBUF 1

GBUF 2

500

OUT 0

500

VF-1

VF-2

VFO

OUT1

OUT2

Figure 4. Conceptual Diagram of a Single

Multiplexer Channel, G = +2

This configuration has a few implications for single-supply

operation:

1) On the AD8187, VREF may not be tied to the most negative

analog supply, VEE.

Limits on Reference Voltage (AD8187, see Figure 5):

Von

V /

V Supplies

REF

.– .

VREF

OUT 0

1.3V

1.6V

1.3V

VOUT

VO_MAX = 3.7V

VO_MAX = 3.4V

VO_MIN = 1.3V

VREF

GND

Figure 5. Output Compliance of Main Amplifier

Channel and Ground Buffer

2) 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 easily shown that

OUT

REF

=×

2–

3) To maximize the output dynamic range, the reference voltage

should be chosen with some care.

For example, consider amplifying a 700 mV video signal with a

sync pulse 300 mV below black level. The user might decide to set

VREF at black level to preferentially run video signals on the faster

NPN transistor path. The AD8186 would, in this case, allow a

reference voltage as low as 1.3 V + 300 mV = 1.6 V. If the AD8187

is used, the sync pulse would be 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 would be to

employ +3 V and –2 V supplies, in which case VREF could 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 may be synthesized with a second opera-

tional amplifier. Alternately, a well bypassed resistor divider

may serve. Refer to the Application section for further explana-

tion and more examples.

VREF

1 F

GND

OP21

100k

10k

0.022 F

100

1 F

FROM 1992 ADI AMPLIFIER

APPLICATIONS GUIDE

Figure 6a. Synthesis of a False Ground Reference

VREF

1 F

10k

CAP MUST BE LARGE

ENOUGH TO ABSORB

TRANSIENT CURRENTS

WITH MINIMUM BOUNCE.

Figure 6b. Alternate Method for Synthesis of a

False Ground Reference

High Impedance Disable

Both the AD8186 and the AD8187 may have their outputs

disabled to a high impedance state. In the case of the AD8187,

the reference buffers also disable to a state of high output

impedance. This feature prevents the feedback network of a

disabled channel from loading the output, which is valuable

when busing together the outputs of several muxes.

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