8.3.1 Basic Connection

Figure 20 shows the basic connection of the INA193-INA198. To minimize any resistance in series with the shunt resistance, connect the input pins, V_IN+ and V_IN−, as closely as possible to the shunt resistor.

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 20. INA193-INA198 Basic Connection

8.3.2 Selecting R_S

The value chosen for the shunt resistor, R_S, 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_S provide better accuracy at lower currents by minimizing the effects of offset, while low values of R_S minimize voltage loss in the supply line. For most applications, best performance is attained with an R_S value that provides a full-scale shunt voltage range of 50 mV to 100 mV. Maximum input voltage for accurate measurements is 500 mV.

8.3.3 Inside the INA193-INA198

The INA193-INA198 devices use a new, unique internal circuit topology that provides common-mode range extending from −16 to 80 V while operating from a single power supply. The common-mode rejection in a classic instrumentation amplifier approach is limited by the requirement for accurate resistor matching. By converting the induced input voltage to a current, the INA193-INA198 devices provide common-mode rejection that is no longer a function of closely matched resistor values, providing the enhanced performance necessary for such a wide common-mode range. A simplified diagram (shown in Figure 21) shows the basic circuit function. When the common-mode voltage is positive, amplifier A2 is active.

The differential input voltage, (V_IN+) − (V_IN−) applied across R_S, is converted to a current through a resistor. This current is converted back to a voltage through R_L, and then amplified by the output buffer amplifier. When the common-mode voltage is negative, amplifier A1 is active. The differential input voltage, (V_IN+) − (V_IN−) applied across R_S, is converted to a current through a resistor. This current is sourced from a precision current mirror whose output is directed into R_L converting the signal back into a voltage and amplified by the output buffer amplifier. Patent-pending circuit architecture ensures smooth device operation, even during the transition period where both amplifiers A1 and A2 are active.

1.

NOINDENT:

Nominal resistor values are shown. ±15% variation is possible. Resistor ratios are matched to ±1%.

Figure 21. INA193-INA198 Simplified Circuit Diagram

Figure 22. Monitor Bipolar Output Power-Supply Current

Figure 23. Inductive Current Monitor Including Flyback

Figure 24. INA193-INA198 with Comparator

8.4.1 Input Filtering

An obvious and straightforward location for filtering is at the output of the INA193-INA198 devices; 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 INA193-INA198 devices, which is complicated by the internal 5-kΩ + 30% input impedance; this is illustrated in Figure 25. 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 Equation 1:

Equation 1.

Total effect on gain error can be calculated by replacing 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, the initial gain error will be approximately 2%. 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.

Note that the specified accuracy of the INA193-INA198 devices 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.

Figure 25. Input Filter (Gain Error − 1.5% To −2.2%)

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

The accuracy of the INA193−INA198 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 ≥ 20mV, V_CM ≥ V_S

Normal Case 2: V_SENSE ≥ 20mV, V_CM < V_S

Low V_SENSE Case 1: V_SENSE < 20mV, −16V ≤ V_CM < 0

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

Low V_SENSE Case 3: V_SENSE < 20mV, V_S < V_CM ≤ 80V

8.4.2.1 Normal Case 1: V_SENSE ≥ 20mv, 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 2.

Equation 2.

where

Then the offset voltage is measured at V_SENSE = 100mV and referred to the input (RTI) of the current shunt monitor, as shown in Equation 3.

Equation 3.

In the Typical Characteristics, the Output Error vs Common-Mode Voltage curve (Figure 6) 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.

8.4.2.3 Low V_SENSE Case 1: V_SENSE < 20mV, −16v ≤ V_CM < 0; and Low V_SENSE Case 3: V_SENSE < 20mV, V_S < V_CM ≤ 80V

Although the INA193−INA198 family of devices are 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 INA193−INA198 devices. 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 26 illustrates this effect using the INA195 and INA198 devices (Gain = 100).

Figure 26. Example for Low V_SENSE Cases 1 and 3 (INA195, INA198: Gain = 100)

8.4.2.4 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 INA193−INA198 family of devices. To achieve the wide input common-mode voltage range, these devices use two op amp front ends in parallel. One op amp 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 dominates 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 27 illustrates this behavior for the INA195 device. 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.

1.

NOINDENT:

INA193, INA196 V_OUT Tested Limit = 0.4V. INA194, INA197 V_OUT Tested Limit = 1V.

Figure 27. Example for Low V_SENSE Case 2 (INA195, INA198: Gain = 100)

8.4.3 Shutdown

Because the INA193-INA198 devices consume a quiescent current less than 1 mA, they can be powered by either the output of logic gates or by transistor switches to supply power. Use a totem-pole output buffer or gate that can provide sufficient drive along with 0.1-μF bypass capacitor, preferably ceramic with good high-frequency characteristics. This gate should have a supply voltage of 3 V or greater because the INA193-INA198 devices require a minimum supply greater than 2.7 V. In addition to eliminating quiescent current, this gate also turns off the 10-μA bias current present at each of the inputs. An example shutdown circuit is shown in Figure 28.

Figure 28. INA193-INA198 Example Shutdown Circuit

8.4.4 Transient Protection

The −16-V to +80-V common-mode range of the INA193-INA198 devices 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 INA193-INA198 devices are exposed to transients on the inputs in excess of its ratings, then external transient absorption with semiconductor transient absorbers (zeners or Transzorbs) will be necessary. Use of MOVs or 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 INA193-INA198 devices to be exposed to transients greater than +80 V (that is, allow for transient absorber tolerance, as well as additional voltage due to transient absorber dynamic impedance). Despite the use of internal zener-type ESD protection, the INA193-INA198 devices do not lend themselves to using external resistors in series with the inputs because the internal gain resistors can vary up to ±30%. (If gain accuracy is not important, then resistors can be added in series with the INA193-INA198 inputs with two equal resistors on each input.)

8.4.5 Output Voltage Range

The output of the INA193-INA198 devices are accurate within the output voltage swing range set by the power-supply pin, V+. This is best illustrated when using the INA195 or INA198 devices (which are both versions using a gain of 100), where a 100-mV full-scale input from the shunt resistor requires an output voltage swing of +10 V, and a power-supply voltage sufficient to achieve +10 V on the output.