[FAQ] INA193: Measuring Vsense < 20mV with the INA193-INA198 products

Current sense amplifiers are different from typical op amps and instrumentation amps because the common mode of the inputs can exceed the supply rail of the device.  These amplifiers have different architectures to make this happen, and part of the way this works is to actually siphon a little current from the inputs to power the first stage of the amplifier.  Because the first stage is no longer powered by the supply rail, the voltage can exceed the supply rail.  This is very convenient when measuring current from bus rails of 12V, or 24V, or 48V, or even 80V with a part that runs off a single +5V to GND supply.

When the voltage on the inputs is lower than the supply voltage of the device, the power for the front end is usually taken from the supply.  However, as the voltage on the inputs rises, there is a transition point where the supply is no longer able to power the front end and the voltage on the inputs is used.  You can often see this behavior evident in the input bias currents for a device.  Even more interesting, with devices that can accept significantly negative input voltages exceeding the GND rail, the device will actually drive bias current out of the amplifier instead of sinking it!

In the plot below, the bias current of the IN+ line is around 15uA when the input voltage is above around 10V, and the supply is at 12V.  As the voltage on the inputs gets lower, the supply takes over biasing the inputs, so the current drops.  As the voltage drops even lower, the input sources current out of the IN+ line into the bus.  This behavior is a little different across device families and architectures so be sure to check the data sheets and be aware of the currents as they pertain to different applications and implementation.

Input bias currents vary against supply and common mode voltage

The INA193-INA198, INA200-INA208, and INA270-INA271 products have the same general architecture for the amplifier, and so these products have similar behavior when small sense voltages are applied to the inputs.  The simplified architecture is shown below.  In this case, separate amplifiers (A1 and A2) handle the different ranges of operation of the device, defined primarily by the relation of the bus voltage and supply voltage.

Simplified schematic of the INA193-INA198 series of current sense amplifiers

In the family data sheets, there are five regions of operation, defined as:

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 < 0V

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

It is important to know which region the application is intended to operate in and design for it.  For Normal Cases 1 and 2, most of the electrical characteristics table and typical curves in the data sheet apply (they are generally called out as specified like so: V_SENSE = 20mV to 100mV).  The parts work well in this region, have a very linear output response to input voltages.  For Low V_SENSE Cases 1-3, the outputs vary considerably as the amplifiers are no longer operating in a linear and monotonic region.  The parameters in the electrical characteristics table in the OUTPUT section are almost exclusively about the expected output of the amplifier in the Low V_SENSE Cases, and are denoted as V_SENSE < 20mV.

So what exactly does the output look like with V_SENSE < 20mV?  There is a section in the data sheets of these devices called Accuracy Variations as a Result of V_SENSE and Common-Mode Voltage which explain in some detail observations of the expected behavior.  Low V_SENSE cases 1 and 3 have similar behavior with V_SENSE between 0mV and 20mV.  There are plots in the data sheet that show some measured behavior of the device, like the one below.  This data is not indicative of every device--just the average of a few units sampled at one time at room temperature.

Example output in Low VSENSE Cases 1 and 3

It is important to understand that the output voltage, while it does appear linear and just a little skewed, is not guaranteed.  There is actually nothing guaranteed about these regions of operation in the OUTPUT section of the electrical characteristics table.  There is only a 300mV typical number which essentially means that the lowest voltage output seen with a V_SENSE < 20mV is likely 300mV.  It could be less, it could be more.  For the INA195, the output could really be anything, typically between 0V and 2V (2V being where the device enters linear operation with V_SENSE > 20mV).  It is important to understand that designing a system to calibrate out this behavior is not advised.  The behavior varies from wafer lot to wafer lot, device to device, and putting in a gain or offset correction factor may work for one lot of parts but not the next lot.

Low V_SENSE Case 2 can show a different behavior, as in the example plot below.  This plot specifically shows how different parts behave and why it is important to understand the wide range of variance in this region of operation.  The only guarantee made in the electrical characteristics table is the voltage output in Low V_SENSE Case 2 will not exceed the listed threshold.  In the case of the INA195 and INA202, that limit is 2V.  For the INA193 and INA200, that limit is 0.4V.

Example output in Low VSENSE Case 2

It is best to design a robust system that does not rely this region of operation to make application-critical decisions with these device families.  There are many ways to accomplish this--here are a few.

1. Example 1: V_CM = 40V, V_S= 5V, current range of interest 1A to 10A
   1. Choose a shunt resistor big enough that the smallest current generates at least 20mV to be in Normal Case 1.  In this case, 1A * 20mΩ = 20mV.
   2. Make sure your amplifier gain isn't so large that your highest current exceeds the supply.  In this case, 10A * 20mΩ = 200mV.
      1. INA193 (G=20V/V) 200mV * 20 = 4V (Will work for 5V supply)
      2. INA194 (G=50V/V) 200mV * 50 = 10V (Too Large for 5V supply)
      3. INA195 (G=100V/V) 200mV * 100 = 20V (Too Large for 5V supply)
2. Example 2: V_CM = 5V, V_S = 12V, current range of interest 0A to 10A, Gain = 100, sense resistor = 10mΩ
   
   1. Calculating from the design specifications, the V_SENSE will be from 0mV to 100mV, and the desired output will be from 0V to 10V.
   2. Because this solution will attempt to measure V_SENSE = 0mV, the device will operate in Low V_SENSE Case 2. Look at the error as V_SENSE approaches 0mV and decide if it is acceptable.
      
      1. Every input voltage from 0mV to 20mV could produce a 2V output, which is 20% of the full output range, limiting the accurate and linear region of the device to just 2V-10V outputs.
      2. If we assume that the effects of gain and offset errors are dwarfed by the Low V_SENSECase 2 behaviors, then worst case, at 0mV input, the output could be 2V.
         1. Assuming a perfect part with 0 offset error and perfect gain error, then at 2A, error = 0%.
         2. At 1.5A, if the output is 2V, then the error is |(2V - (1.5A * 10mΩ * 100))/((1.5A * 10mΩ * 100)| = 33%
         3. At 1A, if the output is 2V, then the error is |(2V - (1A * 10mΩ * 100))/((1A * 10mΩ * 100)| = 100%
         4. At 500mA, if the output is 2V, then the error is |(2V - (500mA * 10mΩ * 100))/((500mA * 10mΩ * 100)| = 300%
         5. At 250mA, if the output is 2V, then the error is |(2V - (250mA * 10mΩ * 100))/((250mA * 10mΩ * 100)| = 700%
      3. Increasing the sense voltage by increasing the resistor is one option, but will require either a higher V_S or a lower gain.
         
         1. For instance, using a 20mΩ resistor and a gain = 50V/V, the input voltage is now from 0mV to 200mV, but the output is still 0V to 10V.
         2. The linear region of the amplifiers will now be from 1V-10V instead of 2V-10V, because V_SENSE= 20mV at 1A instead of 2A now.
         3. Assuming a perfect part with 0 offset error and perfect gain error, then at 1A, error = 0% (compared to 100% with 10mΩ, G=100V/V)
         4. At 500mA, if the output is 1V, then the error is |(1V - (500mA * 20mΩ * 50))/((500mA * 20mΩ * 50)| = 100%(compared to 300% with 10mΩ, G=100V/V)
         5. At 250mA, if the output is 1V, then the error is |(1V - (250mA * 20mΩ * 50))/((250mA * 20mΩ * 50)| = 300% (compared to 700% with 10mΩ, G=100V/V)
   3. Choosing a different device will also work - for instance the LMP8640-H will operate V_CM = 5V, V_S = 12V, current range of interest 0A to 10A, Gain = 100, sense resistor = 10mΩ and offers good offset and gain error specifications that include low V_SENSE conditions.
      
3. Example 3: V_SENSE = 0mV to 40mV, V_S = 5V, V_CM = 20V, reasonably good accuracy required down to V_SENSE = 0mV
   
   1. Because the V_SENSE is so low, there are many suitable options for devices.  Gains of up to 100 V/V with a V_S = 5V are acceptable.
      
      1. The LMP8640 has gains of 20, 50, and 100, and is in a very similar package and supports common mode voltages up to +42V.
      2. The INA213, INA214, INA215 have gains of 50, 100, and 75, are very small, high accuracy and support common modes up to +26V.
      3. More cost effective but less accurate devices like the INA199 and the INA181 are also excellent options that support up to +26V.
   2. Using a device with a reference voltage input pin will also help avoid swing-to-GND errors when V_SENSE = 0mV.
      
      1. For more information on swing-to-GND, see the Design and Debug Guide chapter: Understanding Swing-to-Rail specifications.
4. Example 4: Same as 3, but also with integrated comparators (compared to the INA200-INA208 product families)
   
   1. The INA301, INA302, and INA303 families have excellent specs, the same gain options, and support common modes up to 36V.
5. Example 5: A circuit with V_SENSE < 20mV, and V_CM > 42V.
   
   1. The INA282, INA240, and LMP8640HV, LMP8481 families support voltages with trimmed gain resistors and fixed gains, and support up to 76V or 80V common modes.
   2. The INA168 and INA169 have variable gains determined by an output resistor on the current output.  These devices operate up to 60V
   3. Switch to low-side sensing and most current sense amplifiers are supported.