OPA4684M Datasheet, PDF (16/32 Pages) Texas Instruments – QUAD LOW-POWER CURRENT-FEEDBACK OPERATIONAL AMPLIFIER

OPA4684M
QUAD LOWĆPOWER CURRENTĆFEEDBACK
OPERATIONAL AMPLIFIER
SGLS145B − AUGUST 2003 − REVISED FEBRUARY 2004
differential interface applications
Dual and quad op amps are particularly suitable to differential input to differential output applications. Typically,
these fall into either ADC input interface or line driver applications. Two basic approaches to differential I/O are
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
OPA4684. Each has its advantages and disadvantages. Figure 45 shows a basic starting point for noninverting
differential I/O applications.
Figure 45. Noninverting Differential I/O Amplifier
This approach provides 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 noninverting inputs without interacting
with the amplifier gain. The differential signal gain for the circuit of Figure 45 is:
V_O
V_I
+
A_D
+
1
)
2
R_F
R_G
Since the OPA4684 is a CFB_PLUS amplifier, its bandwidth is principally controlled with the feedback resistor
value; Figure 45 shows the recommended value of 800 Ω. However, the gain may be adjusted with considerable
freedom using just the R_G resistor. In fact, R_G may be a reactive network providing a very isolated shaping to
the differential frequency response. Since the inverting inputs of the OPA4684 are very low impedance
closed-loop buffer outputs, the R_G element does not interact with the amplifier’s bandwidth—wide ranges of
resistor values and/or filter elements may be inserted here with minimal amplifier bandwidth interaction.
Various combinations of single-supply or AC-coupled gains can also be delivered using the basic circuit of
Figure 45. 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 R_G, giving that voltage a
common-mode gain of 1 to the output.
Figure 46 shows a differential I/O stage configured as an inverting amplifier. In this case, the gain resistors (R_G)
become the input resistance for the source. This provides a better noise performance than the noninverting
configuration but does limit the flexibility in setting the input impedance separately from the gain.
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