Datasheet

DIFFERENTIAL INTERFACE APPLICATIONS
V
O
V
I
=A =1+2 ´
D
R
F
R
G
(6)
R
F
750W
R
F
750W
R
G
R
G
1/4
OPA4830
+V
CC
-V
CC
V
CM
V
CM
V
O
1/4
OPA4830
V
I
R
F
750W
R
F
750W
1/4
OPA4830
+V
CC
-V
CC
R
G
V
O
1/4
OPA4830
V
I
V
O
V
I
= -
R
F
R
G
(7)
OPA4830
SBOS350A – DECEMBER 2006 – REVISED MAY 2008 .................................................................................................................................................... www.ti.com
Various combinations of single-supply or ac-coupled
gains can also be delivered using the basic circuit of
Dual and quad op amps are particularly suitable to
Figure 80 . Common-mode bias voltages on the two
differential input to differential output applications.
noninverting inputs pass on to the output with a gain
Typically, these op amps fall into either ADC input
of 1V/V because an equal dc voltage at each
interface or line driver applications. Two basic
inverting node creates no current through R
G
, giving
approaches to differential I/O are noninverting or
that voltage a common-mode gain of 1 to the output.
inverting configurations. Because the output is
differential, the signal polarity is somewhat Figure 81 shows a differential I/O stage configured as
meaningless — the noninverting and inverting an inverting amplifier. In this case, the gain resistors
terminology applies here to where the input is brought (R
G
) become the input resistance for the source. This
into the OPA4830. Each has its advantages and configuration provides a better noise performance
disadvantages. Figure 80 shows a basic starting point than the noninverting configuration, but does limit the
for noninverting differential I/O applications. flexibility in setting the input impedance separately
from the gain.
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 :
Figure 80 shows the recommended value of 750 Ω .
However, the gain may be adjusted using just the R
G
resistor.
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 :
Figure 80. Noninverting Differential I/O Amplifier
26 Submit Documentation Feedback Copyright © 2006 – 2008, Texas Instruments Incorporated
Product Folder Link(s): OPA4830