Datasheet

LOAD
V
SUPPLY
f-3dB =
f
-3dB
1
2 (2R )Cp
FILT FILT
C
FILT
R << R
SHUNT FILTER
R <100W
FILT
R <100W
FILT
VIN+
VIN-
+5V
V+
OUT
INA193-INA198
R
1
5kW
R
L
R
1
5kW
GainError%=100 -
5kW
5k +RW
FILT
´ 100
INA193, INA194
INA195, INA196
INA197, INA198
www.ti.com SBOS307F –MAY 2004–REVISED FEBRUARY 2010
RFI/EMI Note that the specified accuracy of the
INA193-INA198 must then be combined in addition to
Attention to good layout practices is always
these tolerances. While this discussion treated
recommended. Keep traces short and, when
accuracy worst-case conditions by combining the
possible, use a printed circuit board (PCB) ground
extremes of the resistor values, it is appropriate to
plane with surface-mount components placed as
use geometric mean or root sum square calculations
close to the device pins as possible. Small ceramic
to total the effects of accuracy variations.
capacitors placed directly across amplifier inputs can
reduce RFI/EMI sensitivity. PCB layout should locate
the amplifier as far away as possible from RFI
sources. Sources can include other components in
the same system as the amplifier itself, such as
inductors (particularly switched inductors handling a
lot of current and at high frequencies). RFI can
generally be identified as a variation in offset voltage
or dc signal levels with changes in the interfering RF
signal. If the amplifier cannot be located away from
sources of radiation, shielding may be needed.
Twisting wire input leads makes them more resistant
to RF fields. The difference in input pin location of the
INA193-INA195 versus the INA196-INA198 may
provide different EMI performance.
INPUT FILTERING
An obvious and straightforward location for filtering is
at the output of the INA193-INA198; however, this
location negates the advantage of the low output
impedance of the internal buffer. The only other
option for filtering is at the input pins of the
INA193-INA198, which is complicated by the internal
5kΩ + 30% input impedance; this is illustrated in
Figure 22. Using the lowest possible resistor values
minimizes both the initial shift in gain and effects of
tolerance. The effect on initial gain is given by
Equation 3:
(3)
Figure 22. Input Filter (Gain Error − 1.5% to
Total effect on gain error can be calculated by
−2.2%)
replacing the 5kΩ term with 5kΩ − 30%, (or 3.5kΩ) or
5kΩ + 30% (or 6.5kΩ). The tolerance extremes of
R
FILT
can also be inserted into the equation. If a pair
of 100Ω 1% resistors are used on the inputs, the
initial gain error will be approximately 2%. Worst-case
tolerance conditions will always occur at the lower
excursion of the internal 5kΩ resistor (3.5kΩ), and the
higher excursion of R
FILT
− 3% in this case.
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