9.1.1 Input Filtering

Based strictly on user design requirements, external filtering of the current signal may be desired. The initial location that can be considered for the filter is at the output of the current amplifier. Although placing the filter at the output satisfies the filtering requirements, this location changes the low output impedance measured by any circuitry connected to the output voltage pin. The other location for filter placement is at the current amplifier input pins. This location also satisfies the filtering requirement, but carefully select the components to minimize the impact on device performance. Figure 39 shows a filter placed at the inputs pins.

Figure 39. Filter at Input Pins

External series resistance provides a source of additional measurement error. Therefore, keep the value of these series resistors to 10-Ω or less in order to reduce loss of accuracy. The internal bias network shown in Figure 39 creates a mismatch in input bias currents when a differential voltage is applied between the input pins (see Figure 40). If additional external series filter resistors are added to the circuit, a mismatch is created in the voltage drop across the filter resistors. This voltage is a differential error voltage in the shunt resistor voltage. In addition to the absolute resistor value, mismatch resulting from resistor tolerance can significantly impact the error because this value is calculated based on the actual measured resistance.

Figure 40. Input Bias Current vs Differential Input Voltage

Calculate the measurement error expected from the additional external filter resistors using Equation 1.

Equation 1.

where

• Gain Error Factor is calculated using Equation 2.

Equation 2.

where

• R_S is the external filter resistance value

Calculate the gain error factor, shown in Equation 2, in order to determine the gain error introduced by the additional external series resistance. Equation 1 calculates the deviation of the shunt voltage resulting from the attenuation and imbalance created by the added external filter resistance. Table 1 provides the gain error factor and gain error for several resistor values.