OPA695 datasheet(22/32 Pages) TI

22 / 32 page

OPA695

SBOS293B

www.ti.com

DIFFERENTIAL I/O

APPLICATIONS

The OPA695 offers very low 3rd-order distortion terms with

a dominant 2nd-order distortion for the single amplifier op-

eration. For the lowest distortion, particularly where differen-

tial outputs are needed, operating two OPA695s in a differ-

ential I/O design will suppress these even-order terms, deliv-

ering extremely low harmonic distortion through high fre-

quencies and powers. Differential outputs are often preferred

for high performance ADCs, twisted-pair driving, and mixer

interfaces. Two basic approaches to differential I/Os are the

noninverting or inverting configurations. Since the output is

differential, the signal polarity is somewhat meaningless—

the noninverting and inverting terminology applies here to

where the input is brought into the two OPA695s. Each

approach has its advantages and disadvantages. Figure 13

shows a basic starting point for non-inverting differential I/O

applications.

applications to include a blocking capacitor in series with RG.

This reduces the gain to 1 at low frequency, rising to the AD

expression shown above at higher frequencies. The

noninverting input approach of Figure 13 can be used for

higher gains than the inverting input approach. It will, how-

ever, have a reduced full-power bandwidth due to the lower

slew rate of the OPA695 running noninverting vs inverting

input mode of operation.

Various combinations of single-supply or AC-coupled gain

can also be delivered using the basic circuit of Figure 13.

Common-mode bias voltages on the two noninverting inputs

pass on to the output with a gain of 1, since an equal DC

voltage at each inverting node creates no current through

RG. This circuit does show a common-mode gain of 1 from

input to output. The source connection should either remove

this common-mode signal if undesired (using an input trans-

former can provide this function), or the common-mode

voltage at the inputs can be used to set the output common-

mode bias. If the low common-mode rejection of this circuit

is a problem, the output interface may also be used to reject

that common-mode. For instance, most modern differential

input ADCs reject common-mode signals very well, while a

line driver application through a transformer will also remove

the common-mode signal at the secondary of the trans-

former.

Figure 14 shows a differential I/O stage configured as an

inverting amplifier. In this case, the gain resistors (RG)

become part of the input resistance for the source. This

provides a better noise performance than the non-inverting

configuration, but does limit the flexibility in setting the input

impedance separately from the gain.

500

Ω

500

Ω

OPA695

–V

OPA695

FIGURE 13. Noninverting Input Differential I/O Amplifier.

FIGURE 14. Inverting Input Differential I/O Amplifier.

500

Ω

500

Ω

OPA695

–V

OPA695

This approach allows for a source termination impedance

that is independent of the signal gain. For instance, simple

differential filters may be included in the signal path right up

to the non-inverting inputs without interacting with the

gain setting. The differential signal gain for the circuit of

Figure 13 is:

AD = 1 + 2 • RF /RG

Since the OPA695 is a current feedback amplifier, its band-

width is principally controlled with the feedback resistor

value—Figure 13 shows a typical value of 500

Ω. However,

the differential gain may be adjusted with considerable free-

dom using just the RG resistor. In fact, RG may be a reactive

network providing a very isolated shaping to the differen-

tial frequency response. It is common for AC-coupled

(6)

The two noninverting inputs provide an easy common-mode

control input. This is particularly easy if the source is AC-

coupled through either blocking caps or a transformer. In

either case, the common-mode input voltages on the two

OPA695 Datasheet(PDF) 22 Page - Texas Instruments

OPA695 Datasheet(HTML) 22 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL