AD8178 datasheet(31/40 Pages) AD

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AD8178

Rev. 0 | Page 31 of 40

small amounts of bias current, but noise appearing on these

pins is buffered to all the output stages. As such, they should

be connected to low noise, low impedance voltage references

to avoid being sources of noise, offset, and crosstalk in the

signal path.

Termination

The AD8178 is designed to drive 100 Ω terminated to ground

on each output (or an effective 200 Ω differential) while

meeting data sheet specifications over the specified operating

temperature range, if care is taken to observe the maximum

power derating curves.

Termination at the load end is recommended to shorten settling

time and provide for best signal integrity. In differential signal

paths, it is often desirable to series-terminate the outputs, with a

resistor in series with each output. A side effect of termination

is an attenuation of the output signal by a factor of two. In this

case, gain is usually necessary somewhere else in the signal path

to restore the signal level.

Whenever a differential output is used single-ended, it is desirable

to terminate the used single-ended output with a series resistor,

as well as to place a resistor on the unused output to match the

load seen by the used output.

When disabled, the outputs float to midsupply. A small current

is required to drive the outputs away from their midsupply state.

This current is easily provided by an AD8178 output (in its

enabled state) bussed together with the disabled output.

Exceeding the allowed output voltage range may saturate

internal nodes in the disabled output, and consequently,

an increase in disabled output current may be observed.

Single-Ended Output

Usage

The AD8178 output pairs can be used single-ended, taking only

one output and not using the second. This is often desired to

reduce the routing complexity in the design or because a single-

ended load is being driven directly. This mode of operation

produces good results but has some shortcomings when compared

to taking the output differentially. When observing the single-

ended output, noise that is common to both outputs appears in

the output signal.

When observing the output single-ended, the distribution of offset

voltages appears greater. In the differential case, the difference

between the outputs, when the difference between the inputs is

zero, is a small differential offset. This offset is created from

mismatches in devices in the signal path. In the single-ended

case, this differential offset is still observed, but an additional

offset component is also relevant. This additional component

is the common-mode offset, which is the difference between the

average of the outputs and the output common-mode reference.

This offset is created by mismatches that affect the signal path

in a common-mode manner. A differential receiver rejects this

common-mode offset voltage, but in the single-ended case, this

offset is observed with respect to the signal ground. The single-

ended output sums half the differential offset voltage and all

of the common-mode offset voltage for a net increase in

observed offset.

Single-Ended Gain

The AD8178 operates as a closed-loop differential amplifier.

The primary control loop forces the difference between the

output terminals to be a ratio of the difference between the

input terminals. One output increases in voltage, while the

other decreases an equal amount to make the total output

voltage difference correct. The average of these output voltages

is forced to the voltage on the common-mode reference terminal

(VOCM_CMENCOFF or VOCM_CMENCON) by a second

control loop. If only one output terminal is observed with respect

to the common-mode reference terminal, only half of the differ-

ence voltage is observed. This implies that when using only one

output of the device, half of the differential gain is observed.

An AD8178 taken with single-ended output appears to have

a gain of +2.

It is important to note that all considerations that apply to the

used output phase regarding output voltage headroom apply

unchanged to the complement output phase, even if this is not

actually used.

Termination

When operating the AD8178 with a single-ended output, the

preferred output termination scheme is to refer the load to the

output common mode. A series termination can be used, at an

additional cost of one half the signal gain.

In single-ended output operation, the complementary phase of

the output is not used and may or may not be terminated locally.

Although the unused output can be floated to reduce power dissi-

pation, there are several reasons for terminating the unused output

with a load resistance matched to the load on the signal output.

One component of crosstalk is magnetic coupling by mutual

inductance between output package traces and bond wires that

carry load current. In a differential design, there is coupling from

one pair of outputs to other adjacent pairs of outputs. The differ-

ential nature of the output signal simultaneously drives the

coupling field in one direction for one phase of the output and

in an opposite direction for the other phase of the output. These

magnetic fields do not couple equally into adjacent output pairs,

due to different proximities; but they do destructively cancel the

crosstalk to some extent. If the load current in each output is equal,

this cancellation is greater and less adjacent crosstalk is observed

(regardless of whether the second output is actually being used).

A second benefit of balancing the output loads in a differential

pair is to reduce fluctuations in current requirements from the

power supply. In single-ended loads, the load currents alternate

from the positive supply to the negative supply. This creates a

parasitic signal voltage in the supply pins due to the finite

resistance and inductance of the supplies. This supply fluctuation

appears as crosstalk in all outputs, attenuated by the power

supply rejection ratio (PSRR) of the device.

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