LINEAR Design Note DN473: Using a Differential I/O Amplifier in Single-Ended Applications

Using a Differential I/O Amplifi er in Single-Ended Applications Design Note DN473 Glen Brisebois Introduction the voltage applied at VOCM. One fi nal observation is that

Recent advances in low voltage silicon germanium there is no longer a single inverting input: both inputs are and Bi CMOS processes have allowed the design and inverting and noninverting depending on which output production of very high speed amplifi ers. Because the is considered. For the purposes of circuit analysis, the processes are low voltage, most of the amplifi er designs inputs are labeled with “+” and “–” in the conventional have incorporated differential inputs and outputs to regain manner and one output receives a dot, denoting it as the and maximize total output signal swing. Since many low- inverted output for the “+” input.

voltage applications are single-ended, the questions arise,

Anybody familiar with conventional op amps knows that

“How can I use a differential I/O amplifi er in a single-ended

noninverting applications have inherently high input

application?” and “What are the implications of such

impedance at the noninverting input, approaching GΩ or

use?” This Design Note addresses some of the practical

even TΩ. But in the case of the fully differential op amp

implications and demonstrates specifi c single-ended

in Figure 1, there is feedback to both inputs, so there is

applications using the 3GHz gain-bandwidth LTC6406

no high impedance node. Fortunately this diffi culty can

differential I/O amplifi er.

be overcome.

Background

RI2 RF2

A conventional op amp has two differential inputs and an output. The gain is nominally infi nite, but control

is maintained by virtue of feedback from the output

LTC6406

to the negative “inverting” input. The output does not

go to infi nity, but rather the differential input is kept to

zero (divided by infi nity, as it were). The utility, variety and beauty of conventional op amp applications are

RI1 RF1 0.1μF

well documented, yet still appear inexhaustible. Fully

DN4GB F01

differential op amps have been less well explored.

Figure 1 shows a differential op amp with four feedback Figure 1. Fully Differential I/O Amplifi er Showing Two Outputs and an Additional VOCM Pin

resistors. In this case the differential gain is still nominally infi nite, and the inputs kept together by feedback, but this is not adequate to dictate the output voltages. The

Simple Single-Ended Connection of a Fully

reason is that the common mode output voltage can be

Differential Op amp

anywhere and still result in a “zero” differential input

Figure 2 shows the LTC6406 connected as a single-ended

voltage because the feedback is symmetric. Therefore, for

op amp. Only one of the outputs has been fed back and

any fully differential I/O amplifi er, there is always another

only one of the inputs receives feedback. The other input

control voltage to dictate the output common mode

is now high impedance. The LTC6406 works fi ne in this

voltage. This is the purpose of the VOCM pin, and explains

circuit and still provides a differential output. However, a

why fully differential amplifi ers are at least 5-pin devices

simple thought experiment reveals one of the downsides

(not including supply pins) rather than 4-pin devices. The

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differential gain equation is VOUT(DM) = VIN(DM) • R2/R1.

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The common mode output voltage is forced internally to their respective owners.