www.ti.com

Output Voltage Range

RFI/EMI

Input Filtering

GainError% + 100 * 100

5k W

(3)

INA193A-EP,, INA194A-EP,, INA195A-EP

INA196A-EP, INA197A-EP, INA198A-EP

SBOS400 – MAY 2007

APPLICATION INFORMATION (continued)

The output of the INA193A–INA198A is accurate within the output voltage swing range set by the power-supply

pin, V+. This is best illustrated when using the INA195A or INA198A (which are both versions using a gain of

100), where a 100 mV full-scale input from the shunt resistor requires an output voltage swing of 10 V, and a

power-supply voltage sufficient to achieve 10 V on the output.

Attention to good layout practices is always recommended. Keep traces short and, when possible, use a printed

circuit board (PCB) ground plane with surface-mount components placed as close to the device pins as

possible. Small ceramic 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 INA193A–INA195A versus the INA196A–INA198A may provide different

EMI performance.

An obvious and straightforward location for filtering is at the output of the INA193A–INA198A; 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 INA193A–INA198A, which is complicated by the internal 5 k

Ω + 30% input

impedance; this is illustrated in Figure 5. 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:

Total effect on gain error can be calculated by replacing the 5-k

Ω term with 5 kΩ – 30%, (or 3.5 kΩ) or

5 k

Ω + 30% (or 6.5 kΩ). The tolerance extremes of R

100-

Ω 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 5 k

Ω resistor (3.5 kΩ), and the higher

combined in addition to these tolerances. While this discussion treated accuracy worst-case conditions by

combining the extremes of the resistor values, it is appropriate to use geometric mean or root sum square

calculations to total the effects of accuracy variations.

12

Submit Documentation Feedback