7.4 Device Functional Modes

7.4.1 Input Filtering

An obvious and straightforward filtering location is at the device output. However, this location negates the

advantage of the low output impedance of the internal buffer. The only other filtering option is at the device input

pins. This location, though, does require consideration of the ±30% tolerance of the internal resistances. Figure

7-2 shows a filter placed at the inputs pins.

CBYPASS

0.1µF

Output

-

+

IN-

IN+

GND

V+

OUT

REF

Power Supply

Bus Supply

Load

Bias

RS < 10 Ÿ

RS < 10 Ÿ

CF

RSHUNT

RINT

RINT

Figure 7-2. Filter at Input Pins

The addition of external series resistance, however, creates an additional error in the measurement so the value

of these series resistors must be kept to 10 Ω (or less if possible) to reduce any affect to accuracy. The internal

bias network shown in Figure 7-2 present at the input pins creates a mismatch in input bias currents when a

differential voltage is applied between the input pins. If additional external series filter resistors are added to

the circuit, the mismatch in bias currents results in a mismatch of voltage drops across the filter resistors. This

mismatch creates a differential error voltage that subtracts from the voltage developed at the shunt resistor. This

error results in a voltage at the device input pins that is different than the voltage developed across the shunt

resistor. Without the additional series resistance, the mismatch in input bias currents has little effect on device

operation. The amount of error these external filter resistor add to the measurement can be calculated using

Equation 2 where the gain error factor is calculated using Equation 1.

The amount of variance in the differential voltage present at the device input relative to the voltage developed

at the shunt resistor is based both on the external series resistance value as well as the internal input resistors,

R3 and R4 (or RINT as shown in Figure 7-2). The reduction of the shunt voltage reaching the device input pins

appears as a gain error when comparing the output voltage relative to the voltage across the shunt resistor.

A factor can be calculated to determine the amount of gain error that is introduced by the addition of external

series resistance. The equation used to calculate the expected deviation from the shunt voltage to what is seen

at the device input pins is given in Equation 1:

Gain Error Factor =

(1250 ´ INT

R

)

(1250

S

´

´

´

R ) + (1250

R

) + (R

R

)

INT

S

INT

(1)

where:

• RINT is the internal input resistor (R3 and R4).

• RS is the external series resistance.

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INA199

SBOS469H – APRIL 2009 – REVISED OCTOBER 2023

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