Datasheet

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SBOS317D − SEPTEMBER 2004 − REVISED AUGUST 2008
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15
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
OPA4820. Each has its advantages and disadvantages.
Figure 5 shows a basic starting point for noninverting
differential I/O applications.
R
F
402
Ω
R
F
402
Ω
1/4
OPA4820
+V
CC
−
V
CC
R
G
V
O
1/4
OPA4820
V
I
Figure 5. 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 5 is:
V
O
V
I
+ A
D
+ 1 ) 2
R
F
R
G
Figure 5 shows the recommended value of 402Ω.
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 5. 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 6 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.
R
F
402
Ω
R
F
402
Ω
R
G
R
G
1/4
OPA4820
+V
CC
−
V
CC
V
CM
V
CM
V
O
1/4
OPA4820
V
I
Figure 6. Inverting Differential I/O Amplifier
The two noninverting inputs provide an easy
common-mode control input. This 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 will also be
changing the input impedance as well. The differential gain
for this circuit is:
V
O
V
I
+*
R
F
R
G
(1)
(2)