Datasheet

AD8556
Rev. A | Page 24 of 28
EMI/RFI PERFORMANCE
Real-world applications must work with ever increasing
radio/magnetic frequency interference (RFI and EMI). In
situations where signal strength is low and transmission lines
are long, instrumentation amplifiers, such as the AD8556, are
needed to extract weak, small differential signals riding on
common-mode noise and interference. Additionally, wires and
PCB traces act as antennas and pick up high frequency EMI
signals. The longer the wire, the larger the voltage it picks up.
The amount of voltages picked up is dependent on the impedances
at the wires, as well as the EMI frequency. These high frequency
voltages are then passed into the in-amp through its pins. All
instrumentation amplifiers can rectify high frequency out-of-
band signals. Unfortunately, the EMI/RFI rectification occurs
because amplifiers do not have any significant common-mode
rejection above 100 kHz. Once these high frequency signals are
rectified, they appear as dc offset errors at the output.
The AD8556 features internal EMI filters on the VNEG, VPOS,
FILT, and VCLAMP pins. These built-in filters on the pins limit
the interference bandwidth and provide good RFI suppression
without reducing performance within the pass-band of the
instrumentation amplifier. A functional diagram of the
AD8556 along with its EMI/RFI filters is shown in
Figure 51.
The AD8556 has built-in filters on its inputs, VCLAMP, and
filter pins. The first-order, low-pass filters inside the AD8556
are useful to reject high frequency EMI signals picked up by
wires and PCB traces outside the AD8556. The most sensitive
pin of any amplifier to RFI/EMI signal is the noninverting pin.
Signals present at this pin appear as common-mode signals and
create problems.
The filters built at the input of the AD8556 have two different
bandwidths: common mode and differential mode. The common-
mode bandwidth defines what a common-mode RF signal sees
between the two inputs tied together and ground. The EMI
filters placed on the input pins of the AD8556 reject EMI/RFI
suppressions that appear as common-mode signals.
05448-053
VDD
VSS
1
2
3
+IN
–IN
OUT
A3
VDD
VSS
1
2
3
+IN
–IN
OUT
A4
V
OUT
VDD
VSS
1
2
3
+IN
–IN
OUT
A2
VDD
VSS
1
2
3
+IN
–IN
OUT
A1
VSS
1
2
3
+IN
–IN
OUT
A5
R7P4R5
VDDDIGIN
VCLAMP
R3
P2
P1
R1
R2
R4 R5
P3
VNEG
VPOS
DAC
LOGIC
EMI
FILTER
EMI
FILTER
EMI
FILTER
RF
EMI
FILTER
FILT/DIGOUT
AD8556
VSS
EMI
FILTER
Figure 51. Block Diagram Showing EMI/RFI Built-In Filters