Datasheet

AD8251
Rev. B | Page 21 of 24
APPLICATIONS
DIFFERENTIAL OUTPUT
In certain applications, it is necessary to create a differential
signal. High resolution ADCs often require a differential input.
In other cases, transmission over a long distance can require
differential signals for better immunity to interference.
Figure 61 shows how to configure the AD8251 to output a
differential signal. An op amp, the AD817, is used in an
inverting topology to create a differential voltage. V
REF
sets the
output midpoint according to the equation shown in the figure.
Errors from the op amp are common to both outputs and are
thus common mode. Likewise, errors from using mismatched
resistors cause a common-mode dc offset error. Such errors are
rejected in differential signal processing by differential input
ADCs or instrumentation amplifiers.
When using this circuit to drive a differential ADC, V
REF
can be
set using a resistor divider from the ADC reference to make the
output ratiometric with the ADC.
SETTING GAINS WITH A MICROCONTROLLER
+15
V
MICRO-
CONTROLLER
–15V
A0
A1
WR
+IN
–
IN
10F0.1µF
10F0.1µF
REF
AD8251
+
–
DGNDDGND
06287-059
Figure 60. Programming Gain Using a Microcontroller
+12
V
–12V
A0
A1
WR
+IN
10F
0.1F
10F
0.1F
AD8251
REF
G = 1
0.1µF
4.99k
4.99k
AD817
0.1µF
+12V
–12V
V
REF
0V
V
OUT
A = V
IN
+ V
REF
2
2
V
OUT
B = –V
IN
+ V
REF
+2.5V
–2.5V
0V
+2.5V
–2.5V
0V
TIME
AMPLITUDE
0V
TIME
AMPLITUDE
+5
V
–5V
AMPLITUDE
10pF
+12V
–12V
V
IN
+
–
+–
DGND
DGND
06287-060
Figure 61. Differential Output with Level Shift