8.2.1.1 Design Requirements

The device can be configured to monitor current flowing in one direction (unidirectional) or in both directions (bidirectional) depending on how the REF pin is configured. The most common case is unidirectional where the output is set to ground when no current is flowing by connecting the REF pin to ground, as shown in Figure 30. When the input signal increases, the output voltage at the OUT pin increases.

8.2.1.2 Detailed Design Procedure

The linear range of the output stage is limited in how close the output voltage can approach ground under zero input conditions. In unidirectional applications where measuring very low input currents is desirable, bias the REF pin to a convenient value above 50 mV to get the output into the linear range of the device. To limit common-mode rejection errors, TI recommends buffering the reference voltage connected to the REF pin.

A less frequently-used output biasing method is to connect the REF pin to the supply voltage, V+. This method results in the output voltage saturating at 200 mV below the supply voltage when no differential input signal is present. This method is similar to the output saturated low condition with no input signal when the REF pin is connected to ground. The output voltage in this configuration only responds to negative currents that develop negative differential input voltage relative to the device IN– pin. Under these conditions, when the differential input signal increases negatively, the output voltage moves downward from the saturated supply voltage. The voltage applied to the REF pin must not exceed the device supply voltage.

8.2.1.3 Application Curve

An example output response of a unidirectional configuration is shown in Figure 31. With the REF pin connected directly to ground, the output voltage is biased to this zero output level. The output rises above the reference voltage for positive differential input signals but cannot fall below the reference voltage for negative differential input signals because of the grounded reference voltage.

Figure 31. Unidirectional Application Output Response

8.2.2.1 Design Requirements

The device is a bidirectional, current-sense amplifier capable of measuring currents through a resistive shunt in two directions. This bidirectional monitoring is common in applications that include charging and discharging operations where the current flow-through resistor can change directions.

8.2.2.2 Detailed Design Procedure

The ability to measure this current flowing in both directions is enabled by applying a voltage to the REF pin, as shown in Figure 32. The voltage applied to REF (V_REF) sets the output state that corresponds to the zero-input level state. The output then responds by increasing above V_REF for positive differential signals (relative to the IN– pin) and responds by decreasing below V_REF for negative differential signals. This reference voltage applied to the REF pin can be set anywhere between 0 V to V+. For bidirectional applications, V_REF is typically set at midscale for equal signal range in both current directions. In some cases, however, V_REF is set at a voltage other than midscale when the bidirectional current and corresponding output signal do not need to be symmetrical.

8.2.2.3 Application Curve

An example output response of a bidirectional configuration is shown in Figure 33. With the REF pin connected to a reference voltage ( 2.5 V in this case) the output voltage is biased upwards by this reference level. The output rises above the reference voltage for positive differential input signals and falls below the reference voltage for negative differential input signals.

Figure 33. Bidirectional Application Output Response