OPA4830 Datasheet, PDF (26/45 Pages) National Semiconductor (TI) – Quad, Low-Power, Single-Supply, Wideband Operational Amplifier

OPA4830
SBOS350A – DECEMBER 2006 – REVISED MAY 2008.................................................................................................................................................... www.ti.com
DIFFERENTIAL INTERFACE APPLICATIONS
Dual and quad op amps are particularly suitable to
differential input to differential output applications.
Typically, these op amps fall into either ADC input
interface or line driver applications. Two basic
approaches to differential I/O are noninverting or
inverting configurations. Because 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 OPA4830. Each has its advantages and
disadvantages. Figure 80 shows a basic starting point
for noninverting differential I/O applications.
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 80 is
shown in Equation 6:
V_O
V_I
=
A_D
=
1
+
2
´
R_F
R_G
(6)
Figure 80 shows the recommended value of 750Ω.
However, the gain may be adjusted using just the R_G
resistor.
Various combinations of single-supply or ac-coupled
gains can also be delivered using the basic circuit of
Figure 80. Common-mode bias voltages on the two
noninverting inputs pass on to the output with a gain
of 1V/V because 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 81 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
configuration provides a better noise performance
than the noninverting configuration, but does limit the
flexibility in setting the input impedance separately
from the gain.
+V_CC
V_CM
1/4
OPA4830
R_F
R_G
750W
V_I
R_G
R_F
V_O
750W
+V_CC
1/4
OPA4830
R_F
750W
V_I
R_G
R_F
750W
V_O
1/4
OPA4830
-V_CC
Figure 80. Noninverting Differential I/O Amplifier
1/4
OPA4830
V_CM
-V_CC
Figure 81. Inverting Differential I/O Amplifier
The two noninverting inputs provide an easy
common-mode control input. This control is
particularly useful if the source is ac-coupled through
either blocking caps or a transformer. In either case,
the common-mode input voltages on the two
noninverting inputs again have a gain of 1 to the
output pins, giving an easy common-mode control for
single-supply operation. The input resistors may be
adjusted to the desired gain but also change the input
impedance as well. The differential gain for this circuit
is shown in Equation 7:
V_O
V_I
=
-
R_F
R_G
(7)
26
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