Datasheet

AD8295
Rev. A | Page 22 of 28
R
R
AD8295
IN-AMP
+V
S
+IN
–IN
0.1µF
10µF
10µF
0.1µF
REF
V
OUT
–V
S
C
D
C
C
C
C
10nF
1nF
1nF
07343-007
4.02kΩ
4.02kΩ
R
G
Figure 63. RFI Suppression
Lower cutoff frequencies improve RFI robustness. Accuracy of
the C
C
capacitors is important, because any mismatch between
the R × C
C
at the positive input and the R × C
C
at the negative
input degrades the CMRR of the AD8295. Keeping C
D
at least
10 times larger than C
C
is recommended.
DIFFERENTIAL OUTPUT
The AD8295 can be pin-strapped to provide a differential
output; the simplified schematic is shown in Figure 64 and the
full pin connection is shown in Figure 65. This configuration
uses the instrumentation amplifier to maintain the differential
voltage, while the op amp maintains the common-mode voltage.
Because the in-amp precisely controls the output relative to its
reference pin, this circuit has the same excellent dc performance
as the single-ended output configuration. The transfer function
for the differential and common-mode outputs are as follows:
V
DIFF_OUT
= V
OUT+
− V
OUT−
= G × (V
IN+
− V
IN−
)
V
CM_OUT
= (V
OUT+
+ V
OUT−
)/2 = V
REF
where:
GR
G
k4.49
1 +=
This configuration is fully specified (see Table 2, Figure 55, and
Figure 56). DC performance is the same as for the single-ended
configuration; ac performance is slightly different.
+IN
–IN
REF
IN-AMP
V
REF
INPUT
20kΩ
+
–
A1
+OUT
–OUT
07343-018
20kΩ
+
–
Figure 64. Differential Output Configuration Using an Op Amp
A1
OUT
A1
R2
A2
+IN
A2
–IN
–IN
–INPUT
+INPUT
R
G
+V
S
+V
S
0.1µF
–V
S
–V
S
OUT
REF
R
G
+IN
1
2
3
4
8765
13141516
A1
IA
A2
R1
20kΩ
R2
20kΩ
A2
OUT
AD8295
A1
+IN
V
REF
INPUT
+OUT
–OUT
A1
R1
A1
–IN
12
11
10
9
0.1µF
NOTES
1. CONNECT AS SHOWN IF A2 IS NOT BEING USED.
07343-008
Figure 65. Minimum Component Connections for Differential Output
An alternative differential output configuration, which also
requires no external components, is shown in Figure 66. Unlike
the circuits shown in Figure 64 and Figure 65, this configuration
uses an inverting op amp configuration to double the gain from
the instrumentation amplifier. Because this configuration requires
less gain from the instrumentation amplifier, it can have a wider
frequency response and a wider input common-mode range vs.
output voltage. However, because it does not take advantage of
feedback at the reference pin of the instrumentation amplifier,
dc performance includes the errors from the op amp and the
resistor network. When using the internal precision components
of the AD8295, these errors have a minimal effect on overall
accuracy. This configuration is not specified in this data sheet.
REF
V
REF
INPUT
A1
+IN
–IN
R2
20kΩ
+OUT
–OUT
–
+
IN-AMP
+
–
R1
20kΩ
R
G
07343-043
Figure 66. Alternative Differential Output Configuration