[FAQ] INA190: How Offset (Vos) and Bias Currents (IB) Affect Low Current Measurements

There are several error sources to consider when sensing current (or using amplifiers in general) such as CMR, PSR, noise, gain error, and voltage offset drift, but two device properties (input bias current (I_B) and input offset voltage (V_OS) ) are critical for DC accuracy, especially in low-current applications. Of these two error sources, the dominant error source is usually I_B for using current-shunt monitors (CSMs) in low-current applications*.

Consider Error from V_os

• Limits the minimum measurable V_SHUNT
• V_os is modeled as positive voltage source at the non-inverting terminal.
• V_os in datasheet is usually characterized by shorting inputs together (V_SENSE = 0) and measuring output.
• % Error from V_os:  e_Vos = V_{os_max}/V_{shunt_min} × 100             (Equation 1)

Figure 1: Typical High-Side Current Sensing Application with Vos of CSM Modeled

Keep in mind, the V_os will vary additionally with the common-mode voltage (Vcm) due to CMR limitations. Vcm is defined as the average voltage common to both input pins, but for this analysis is approximately V_BUS since shunt voltages are small. You can learn how to calculate this error in a complete error analysis seen in the following reference design as an example or of course with our current sense amplifier training video series.

Overall, the higher the V_os, the higher your V_{SHUNT_MIN} will have to be in order to meet an error requirement for some minimum current. Which means you could either increase R_SHUNT, (despite cost, power dissipation, and board space) or choose a part with lower V_os. Additionally, you can perform a 1-point, system calibration to calibrate out the offset of a specific device. This would require measuring the offset of the device, storing this value in memory, and subtracting out this value when converting your output measurement into a current value. This is possible because the offset for an individual part will remain the same under similar condition (temperature, power, V_cm, etc.)

Consider Error from I_B

• Directly limits the minimum measurable current (or  I_SENSE).
• Limits the resistor values when implementing an input PI filter. Go here for more info.
• Along with I_Q, contributes to power consumption.
• I_B+ and I_B- are modeled as current sources into the amp and I_OS is the difference between them.
• I_B in datasheet is usually characterized from shorting inputs together (V_SENSE = 0).
• e_IB = I_B × R_SHUNT/V_{SHUNT_MIN_IDEAL} × 100 = I_B / I_{SENSE_MIN} × 100           (Equation 2)

Figure 2: Typical High-Side Current Sensing Application with Ib of CSM Modeled

While it is easy to calculate the e_Ib, what current you are actually trying to measure will dictate if you see the measurement higher or lower than what it should be. For example if you are pursuing a high-side scheme as in Figure 2,

V_SHUNT = (I_SOURCE - I_B+)×R_SHUNT  =  (I_LOAD + I_B-)×R_SHUNT

Thus according to this equation, if I_B >0, then your measurement of I_SOURCE will be lower than the true I_SOURCE value and your measurement of I_LOAD will be higher than the true I_LOAD value.

Similarly to V_os, I_B can also vary with V_cm. For example, in the INA240 datasheet the following plots of I_B vs. V_cm (see Figure 3) are shown and can shed light on what the designer can expect from the I_B for various V_cm, even when part is powered off.

Figure 3: Most datasheet IB specs are defined when common mode voltage is greater than supply voltage.

Also note that IB will change with the input voltage V_SHUNT (V_diff) see INA240’s IB behavior versus V_diff below in Figure 4. This behavior is mostly dictated by the differential input impedance of the device. At one point IB- can become negative for positive V_diff, but this will occur outside the dynamic range of the device. 

Figure 4: How the IB+ and IB- change as VDIFF increases

Ok finally… let us show how these errors present themselves in a low-current application. Say we wanted to sense high-side current that could range from 250µA to 1mA at a common mode of 12V using the INA240 powered with a 5V supply. 

Immediately, using Equation 2 we can quickly calculate that I_B (90µA typical) alone will contribute to roughly 36% error when sensing 250µA and 9% when sensing 1mA. Assuming this error is acceptable, you might determine the most optimal R_SHUNT should be somewhere between 5Ω and 25Ω. Let’s theoretically calculate the numbers using I_B values from Figure 4 and for shunt resistors of 5Ω and 25Ω. Results are in Table 1.

Table 1: Determining error and output voltage for low-current ranges and two different shunt resistor values. Green voltage output means VOUT is in output swing range, red means value is outside output swing range.

It’s clear that I_B is the dominant source of error no matter the R_SHUNT value. If you increase R_SHUNT, you’ll not only increase V_DIFF and thus I_B according to Figure 4, but you will begin to lose dynamic range, since your V_OUT cannot exceed 5V. If you decrease R_SHUNT, e_Vos will begin to contribute to the total error.

What to do if we need way less error?

Alternative 1 – Choose a CSM with lower I_B (Hint: INA190)

The INA240 is better suited for in-line motor control because of its enhanced CMR rejection. Other current sensors are in Table 2. Pay attention to the INA190 which has an I_B that is lower than all of the other devices by three orders of magnitude. This is because of its capacitively coupled input stage, which gives it much higher input differential impedance.

Table 2: Current Sense Amplifiers Sorted According to Typical I_B

Please also look at the low-current measurement designs TI has to offer below.

TIPD135 - 10uA-100mA, 0.05% Error, High-Side Current Sensing Solution Reference Design

TIPD156 - Low-Drift Bidirectional Single-Supply Low-Side Current Sensing Reference Design

TIPD104 - Current Sensing Solution, 10uA-10mA, Low-side, Single Supply

TIPD129 - 0-1A, Single-Supply, Low-Side, Current Sensing Solution

Alternative 2 – Implement an Instrumentation Amplifier

Instrumentation amplifiers (in-amps) might seem to accomplish the same task as current shunt monitors, and they can even have the same device name prefix ‘INA’ e.g., INA240 (a current shunt monitor) and INA121 (an instrumentation amplifier). However their differing internal topologies beget differing advantages.

In-amps do possess very high input impedance, so their bias currents can be extremely low (~pA), but in-amps are constrained to their V_CM vs. V_OUT curves, which you can calculate here given the supply and gain. On the other hand, CSMs can inherently possess a very large common-mode range. Try substituting an in-amp for the previous example and you will realize it will only work if power supply is >12V (see Figure 5 below), which adds cost, board space, and power consumption. You can download this tool here.

Figure 5: Showing the rail limitation of an instrumentation amplifier on common mode

Alternative 3 – Implement a Low-Side Current Sense Scheme with an Operation Amplifier (OPA)

This would require an amplifier with a true rail-to-rail input (RRI) common-mode range, unless the designer grants this OPA a negative power supply, which would increase cost and board space. While you can certainly choose an RRI amplifier with low I_B, low V_OS, and a decent GBW, you should in return expect reduced gain accuracy, higher I_Q, higher drift, increased board space/cost for external resistors, and a ground potential that is not truly 0 Volts anymore. Additionally, low-side current shunt schemes cannot deterministically distinguish between the system being powered off and a fault condition where bus power supply that you are measuring is shorted to ground. In both scenarios, the OPA will read zero-current conditions (V_DIFF = 0V).