REV. A

–12–

AD8186/AD8187

THEORY OF OPERATION

The AD8186 (G = +1) and AD8187 (G = +2) are single-supply,

triple 2:1 multiplexers with TTL compatible global input switch-

ing and output-enable control. Optimized for selecting between

two RGB (red, green, blue) video sources, the devices have high

peak slew rates, maintaining their bandwidth for large signals.

Additionally, the multiplexers are compensated for high phase

margin, minimizing overshoot for good pixel resolution. The

multiplexers also have respectable video specifications and are

superior for switching NTSC or PAL composite signals.

The multiplexers are organized as three independent channels,

each with two input transconductance stages and one output

transimpedance stage. The appropriate input transconductance

stages are selected via one logic pin (SEL A/

B) such that all

three outputs switch input connections simultaneously. The

unused input stages are disabled with a proprietary clamp cir-

cuit to provide excellent crosstalk isolation between “on” and

“off ” inputs while protecting the disabled devices from damag-

ing reverse base-emitter voltage stress. No additional input

buffering is necessary, resulting in low input capacitance and

high input impedance without additional signal degradation.

The transconductance stage, a high slew rate, class AB circuit,

sources signal current into a high impedance node. Each output

stage contains a compensation network and is buffered to the

output by a complementary emitter-follower stage. Voltage

feedback sets the gain, with the AD8186 configured as a unity

gain follower and the AD8187 as a gain-of-two amplifier with a

feedback network. This architecture provides drive for a reverse-

terminated video load (150

) with low differential gain and

phase errors while consuming relatively little power. Careful

chip layout and biasing result in excellent crosstalk isolation

between channels.

High Impedance, Output Disable Feature, and Off Isolation

The output-enable logic pin (OE) controls whether the three

outputs are enabled or disabled to a high impedance state.

The high impedance disable allows larger matrices to be built

by busing the outputs together. In the case of the AD8187

(G = +2), a feedback isolation scheme is used so that the

impedance of the gain-of-two feedback network does not load

the output. When not in use, the outputs can be disabled to

reduce power consumption.

The reader may have noticed that the off isolation performance of

the signal path is dependent upon the value of the load resistor,

as a simple high-pass network with an effective capacitance of

3 fF. Off isolation will improve as the load resistance is decreased. In

the case of the AD8186, off isolation is specified with a 1 k

Ω

load. However, a practical application would likely gang the

outputs of multiple muxes. In this case, the proper load resistance

for the off isolation calculation is the output impedance of an

enabled AD8186, typically less than a 10th of an ohm.

Full Power Bandwidth vs. –3 dB Large Signal Bandwidth

Note that full power bandwidth for an undistorted sinusoidal signal

is often calculated using the peak slew rate from the equation

Full Power Bandwidth =

Peak Slew Rate

Sinusoid Amplitude

2

π×

The peak slew rate is not the same as the average slew rate. The

average slew rate is typically specified as the ratio

∆

∆

V

t

OUT

measured between the 20% to 80% output levels of a suffi-

ciently large output pulse. For a natural response, the peak slew

rate may be 2.7 times larger than the average slew rate. There-

fore, calculating a full power bandwidth with a specified average

slew rate will give a pessimistic result. In specifying the large

signal performance of these multiplexers, we’ve published the

large-signal bandwidth, the average slew rate, and the measure-

ments of the total harmonic distortion. (Large signal bandwidth

is defined as the –3 dB point measured on a 2 V p-p output

sine wave.) Specifying these three aspects of the signal path’s

large signal dynamics allows the user to predict system behavior

for either pulse or sinusoid waveforms.

Single-Supply Considerations

DC-Coupled Inputs, Integrated Reference Buffers, and

The AD8186 and AD8187 offer superior large signal dynamics.

The trade-off is that the input and output compliance is limited

to ~1.3 V from either rail when driving a 150

load. These

sections address some challenges of designing video systems

within a single 5 V supply.

The AD8186

The AD8186 is internally wired as a unity-gain follower. Its

inputs and outputs can both swing to within ~1.3 V of either

rail. This affords the user 2.4 V of dynamic range at input and

output, which should be enough for most video signals, whether

the inputs are ac- or dc-coupled. In both cases, the choice of

output termination voltage will determine the quiescent load

current.

an ac ground (the more quiet supply is a good bet). Internally,

The capacitor’s other terminal connects to an internal node.

The consequence of building this bypass capacitor on-chip is

current. (Contrast this to the case of the AD8187, where the

µA of input bias current). Second,

the supply range.

IN0A

IN0B

IN1B

IN1A

IN2A

IN2B

AD8186

OUT0

OUT1

OUT2

MUX SYSTEM

BIAS REFERENCE

INTERNAL CAP

“C_BYPASS”

DIRECT CONNECTION TO ANY “QUIET” AC GROUND

from AD8187)