INA193,INA194

INA195,INA196

INA197,INA198

SBOS307B − MAY 2004 − REVISED JUNE 2004

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8

INPUT FILTERING

An obvious and straightforward location for filtering is at

the output of the INA19x series; 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 INA19x, which is complicated by the

internal 5k

Ω + 30% input impedance; see Figure 2. 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:

GainError%

+ 100 * 100

5k

W

5k

W ) R

FILT

Total effect on gain error can be calculated by replacing the

5k

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

6.5k

inserted into the equation. If a pair of 100

Ω 1% resistors

are used on the inputs, the initial gain error will be 1.96%.

Worst-case tolerance conditions will always occur at the

lower excursion of the internal 5k

Ω resistor (3.5kΩ), and

Note that the specified accuracy of the INA19x must then

be 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.

LOAD

V

SUPPLY

1

2

π (2 R

C

FILTER

R

R

R

V

IN+

V

IN

−

+5V

V+

OUT

INA193 −INA198

5k

Ω

5k

Ω

Figure 2. Input Filter (Gain Error − 1.5% to −2.2%)

(1)