Current Measurement with a Shunt Resistor – 2

This is the third post of the current measurement series. You can read the previous version below, 👇🏼

In shunt resistor measurement technique, the voltage across the shunt resistor is proportional to the current. Therefore, if the voltage across the shunt resistor is measured, the current can be calculated. However, since the used shunt resistance values must be very low to be able to allow high current pass with a lower power dissipation, an amplification must be done. With these lower values of shunts, the voltage drop across the shunt is very small. When measuring this tiny voltage drop, in an ideal scenario the signal-conditioning front end has zero offset, zero input bias currents, zero gain error, and zero noise, which are the main parameters to consider when selecting an amplifier for battery current sensing. The amplifier specifications are critical because when lower currents flow through the lower shunt value, the resulting voltage drop is comparatively lower (in terms of µV). Differentiating the signal and noise is difficult when the offset and noise are also in the range of µV. In consideration of these requirements, this study selects the INA240 from Texas Instruments because it offers optimal performance in this application and provides accurate results.

INA240A1 has the following features;

For accurate current measurement, Input Offset, Input Offset Drift, Gain Error, Gain Error Drift and CMRR are very important. This current op-amp is specifically selected for its features. 

As below shows when there is 220A current flows through the 500µΩ shunt, the voltage drop across the shunt will be between -110mV to +110mV. The INA240A1 amplifier senses the voltage in between -110mV to +110mV in a differential configuration. INA240 has a 20V/V Gain. The voltage across the shunt resistor will be amplified by 20 times. And the INA240 will have an output of 0.3V to 4.7V for     -220A to +220A current. The output voltage is limited with the supply voltage which is 5V. Intentionally there is 0.3V margin left in order to eliminate unknown Gain Error. INA240A1 has a known Gain Error of:

GND+50mV ≤Vout ≤VS-200mV∶ ±0.05% Typical and ±0.20% Max
\\
Where
\\
VS=5V

So, keeping the output voltage of INA240 within these limits will have a lowest guaranteed Gain Error of ±0.05% to ±0.20%.

The INA240 device accepts both positive and negative currents because it has a reference voltage of 2.5V, which means the absolute zero output of the INA240 is 2.5V, the negative current falls below 2.5V, and the positive currents rise above 2.5V. The REF5025 external reference output generates this 2.5V.

The following figure shows the INA240 schematic. Two transient voltage suppression (TVS) diodes connect to the two ends of the shunt to protect the signal line from transients (TVS specifications vary depending on the chosen environment). Assuming up to 80-V system, select the breakdown voltage of the TVS as 85V to prevent it from clamping before this value. Each TVS diode is capable of handling 600 W, which means that they can withstand 1200 W of power.

Because a current shunt monitor is the first stage of amplification and offers 20 V/V, the current measurement accuracy is highly limited by the shunt amplifier. For this reason, an appropriate precision amplifier is necessary for a current shunt measurement with higher accuracy. The key specifications for the amplifier selection are as follows:

• Low input offset: Input offset voltage is typically the biggest factor that affects the system accuracy when measuring current. This offset occurs because the shunt output voltage is typically very small, generally in the order of µV, due to which the amplifier offset has a big impact on the measurement accuracy.
• Low offset drift: Offset drift is critical to maintain the system accuracy over temperature. Minimizing the drift is also important because calibrating drift errors is very complicated and may require additional hardware.
• Low bias current: The input bias current of the amplifier affects the current that flows through the shunt, and thus affects the voltage drop across the shunt. For this reason, use amplifiers with low input bias current for better system accuracy.
• Low noise: The inherent noise of an amplifier can also affect the measurement accuracy, especially when configuring the amplifier with a higher noise gain.
• Small-signal bandwidth: This bandwidth should guarantee no attenuation of the input signal.

Comparison of INA240

INA240A1 is compared with INA190A1 and INA303A1. Although they have very similar specs, all specifications are very important when it comes to practice.

INA190A1 has slightly less V_OS @25 ºC @125 ºC and V_OS drift than INA240A1. But it has more Gain Error Drift which is difficult to get rid of. INA240A1 has the widest Common Mode Voltage range among these three current opamps. Common Mode Voltage range is very crucial using the current sensor in High-Side or Low Side configuration. The higher Common Mode Voltage range allows higher battery stacks to be used in High-Side measurement configuration. Also, INA240A1 has the smallest V_OS Error and Gain Drift Error. V_OS Error can be calibrated in the software.

CMRR and PSRR values of INA240A1 are reasonable compared to others.

I will talk about the buffer stage in the next article. Stay tuned!