9.1.1 Basic Connections

Figure 30 shows the basic connections of the INA203-Q1. Connect the input pins, V_IN+ and V_IN–, as closely as possible to the shunt resistor to minimize any resistance in series with the shunt resistance.

Power-supply bypass capacitors are required for stability. Applications with noisy or high-impedance power supplies may require additional decoupling capacitors to reject power-supply noise. Connect bypass capacitors close to the device pins.

Figure 30. INA203-Q1 Basic Connection

9.1.2 Selecting R_SHUNT

The value chosen for the shunt resistor, R_SHUNT, depends on the application and is a compromise between small-signal accuracy and maximum permissible voltage loss in the measurement line. High values of R_SHUNT provide better accuracy at lower currents by minimizing the effects of offset, while low values of R_SHUNT minimize voltage loss in the supply line. For most applications, best performance is attained with an R_SHUNT value that provides a full-scale shunt voltage range of 50 mV to 100 mV. Maximum input voltage for accurate measurements is (V_SHUNT – 0.25)/Gain.

9.1.3 Input Filtering

An obvious and straightforward location for filtering is at the output of the INA203-Q1 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 INA203-Q1, which is complicated by the internal 5 kΩ + 30% input impedance; this configuration is illustrated in Figure 31. Using the lowest possible resistor values minimizes both the initial shift in gain and effects of tolerance. Equation 2 gives the effect on initial gain:

Equation 2.

To calculate the total effect on gain error, replace the 5-kΩ term with 5 kΩ – 30%, (or 3.5 kΩ) or 5 kΩ + 30% (or 6.5 kΩ). The tolerance extremes of R_FILT can also be inserted into the equation. If a pair of 100-Ω 1% resistors are used on the inputs, then the initial gain error will be 1.96%. Worst-case tolerance conditions will always occur at the lower excursion of the internal 5-kΩ resistor (3.5 kΩ), and the higher excursion of R_FILT + 3% in this case.

Figure 31. Input Filter

9.1.4 Accuracy Variations as a Result of V_SENSE and Common-Mode Voltage

The accuracy of the INA203-Q1 current-shunt monitors is a function of two main variables: V_SENSE (V_IN+ – V_IN–) and common-mode voltage, V_CM, relative to the supply voltage, V_S. V_CM is expressed as (V_IN+ + V_IN–)/2; however, in practice, V_CM is seen as the voltage at V_IN+ because the voltage drop across V_SENSE is usually small.

This section addresses the accuracy of these specific operating regions:

• Normal Case 1: V_SENSE ≥ 20 mV, V_CM ≥ V_S
• Normal Case 2: V_SENSE ≥ 20 mV, V_CM < V_S
• Low V_SENSE Case 1: V_SENSE < 20 mV, –16 V ≤ V_CM < 0
• Low V_SENSE Case 2: V_SENSE < 20 mV, 0 V ≤ V_CM ≤ V_S
• Low V_SENSE Case 3: V_SENSE < 20 mV, V_S < V_CM ≤ 80 V

9.1.4.1 Normal Case 1: V_SENSE ≥ 20 mV, V_CM ≥ V_S

This region of operation provides the highest accuracy. Here, the input offset voltage is characterized and measured using a two-step method. First, the gain is determined by Equation 3.

Equation 3.

where

• V_OUT1 = output voltage with V_SENSE = 100 mV
• V_OUT2 = output voltage with V_SENSE = 20 mV

Then the offset voltage is measured at V_SENSE = 100 mV and referred to the input (RTI) of the current shunt monitor, as shown in Equation 4.

Equation 4.

In the Typical Characteristics section, the Output Error vs Common-Mode Voltage curve (Figure 7) shows the highest accuracy for this region of operation. In this plot, V_S = 12 V; for V_CM ≥ 12 V, the output error is at its minimum. This case is also used to create the V_SENSE ≥ 20 mV output specifications in the Electrical Characteristics table.

9.1.4.3 Low V_SENSE Case 1:

V_SENSE < 20 mV, –16 V ≤ V_CM < 0; and

Low V_SENSE Case 3:

V_SENSE < 20 mV, V_S < V_CM ≤ 80 V

Although the INA203-Q1 is not designed for accurate operation in either of these regions, some applications are exposed to these conditions; for example, when monitoring power supplies that are switched on and off while V_S is still applied to the INA203-Q1. It is important to know what the behavior of the devices will be in these regions.

As V_SENSE approaches 0 mV, in these V_CM regions, the device output accuracy degrades. A larger-than-normal offset can appear at the current shunt monitor output with a typical maximum value of V_OUT = 300 mV for V_SENSE = 0 mV. As V_SENSE approaches 20 mV, V_OUT returns to the expected output value with accuracy as specified in the Electrical Characteristics. Figure 32 illustrates this effect (Gain = 100).

Data taken from INA205, a device of INA20x family with Gain = 100

Figure 32. Example for Low V_SENSE Cases 1 and 3 (Gain = 100)

9.1.4.4 Low V_SENSE Case 2: V_SENSE < 20 mV, 0 V ≤ V_CM ≤ V_S

This region of operation is the least accurate for the INA203-Q1. To achieve the wide input common-mode voltage range, this device uses two operational amplifiers (Opamp) front ends in parallel. One Opamp front end operates in the positive input common-mode voltage range, and the other in the negative input region. For this case, neither of these two internal amplifiers dominate and overall loop gain is very low. Within this region, V_OUT approaches voltages close to linear operation levels for Normal Case 2. This deviation from linear operation becomes greatest the closer V_SENSE approaches 0 V. Within this region, as V_SENSE approaches 20 mV, device operation is closer to that described by Normal Case 2. Figure 33 illustrates this behavior. The V_OUT maximum peak for this case is tested by maintaining a constant V_S, setting V_SENSE = 0 mV, and sweeping V_CM from 0 V to V_S. The exact V_CM at which V_OUT peaks during this test varies from part to part, but the V_OUT maximum peak is tested to be less than the specified V_OUT Tested Limit.

Data taken from INA205, a device of INA20x family with Gain = 100

Figure 33. Example for Low V_SENSE Case 2 (Gain = 100)

9.1.5 Transient Protection

The –16 V to +80 V common-mode range of the INA203-Q1 is ideal for withstanding automotive fault conditions ranging from 12-V battery reversal up to 80-V transients, since no additional protective components are needed up to those levels. In the event that the INA203-Q1 is exposed to transients on the inputs in excess of their ratings, then external transient absorption with semiconductor transient absorbers (zeners or Transzorbs) are necessary. Use of metal oxide varistors (MOVs) or video disk recorders (VDRs) is not recommended except when they are used in addition to a semiconductor transient absorber. Select the transient absorber such that it will never allow the INA203-Q1 to be exposed to transients greater than 80 V (that is, allow for transient absorber tolerance, as well as additional voltage because of transient absorber dynamic impedance). Despite the use of internal zener-type ESD protection, the INA203-Q1 does not lend itself to using external resistors in series with the inputs because the internal gain resistors can vary up to ±30% but are closely matched. (If gain accuracy is not important, then resistors can be added in series with the INA203-Q1 inputs with two equal resistors on each input.)

9.2.1 Polyswitch Warning and Fault Detection Circuit

Figure 34. Polyswitch Warning and Fault Detection Circuit Schematic

9.2.1.3 Application Curves

Figure 35. Polyswitch Warning and Fault Detection Circuit Response

Figure 36. Polyswitch Warning and Fault Detection Circuit With Delay Response

9.2.2 Lead-Acid Battery Protection Circuit

See Figure 37 for a protection scheme using INA203-Q1 for a lead-acid battery application.

Figure 37. Lead-Acid Battery Protection Circuit Schematic