SBOA300A June 2018 – December 2020 INA181 , INA181-Q1 , INA186 , INA186-Q1 , INA190 , INA190-Q1 , INA199 , INA199-Q1 , INA210 , INA210-Q1 , INA211 , INA211-Q1 , INA212 , INA212-Q1 , INA213 , INA213-Q1 , INA214 , INA214-Q1 , INA215 , INA215-Q1 , INA2181 , INA2181-Q1 , INA4181 , INA4181-Q1

**Design Goals**

Input | Output | Supply | ||||
---|---|---|---|---|---|---|

I_{inMin} |
I_{inMax} |
V_{cm} |
V_{outMin} |
V_{outMax} |
V_{s} |
V_{ref} |

–4A | 4A | 12.5 V | 0.5 V | 4.5 V | 5 | 2.5 V |

**Design Description**

The low-side bidirectional current-shunt monitor solution illustrated in the following image can accurately measure currents from –4A to 4A, and the design parameters can easily be changed for different current measurement ranges. Current-shunt monitors from the INA21x family have integrated precision gain resistors and a zero-drift architecture that enables current sensing with maximum drops across the shunt as low as 10mV full-scale.

**Design Notes**

- To avoid additional error, use R
_{1}= R_{2}and keep the resistance as small as possible (no more than 10Ω, as stated in*INA21x Voltage Output, Low- or High-Side Measurement, Bidirectional, Zero-Drift Series, Current-Shunt Monitors*)_{.} - Low-side sensing should not be used in applications where the system load cannot withstand small ground disturbances or in applications that need to detect load shorts.
- The
*Getting Started with Current Sense Amplifiers*video series introduces implementation, error sources, and advanced topics that are good to know when using current sense amplifiers.

**Design
Steps**

- Determine V
_{ref}based on the desired current range:With a current range of –4A to 4A, then half of the range is below 0V, so set:

Equation 1. ${V}_{ref}=\frac{1}{2}{V}_{s}=\frac{5}{2}=2.5V$ - Determine the desired shunt
resistance based on the maximum current and maximum output voltage:
To not exceed the swing-to-rail and to allow for some margin, use V

_{outMax}= 4.5V. This, combined with maximum current of 4A and the V_{ref}calculated in step 1, can be used to determine the shunt resistance using the equation:Equation 1. ${R}_{1}=\frac{{V}_{\mathrm{outMax}}-{V}_{\mathrm{ref}}}{\mathrm{Gain}\times {I}_{\mathrm{loadMax}}}=\frac{4.5-2.5}{100\times 4}=5\mathrm{m\Omega}$ - Confirm V
_{out}will be within the desired range:At the maximum current of 4A, with Gain = 100V/V, R

_{1}= 5mΩ, and V_{ref}= 2.5V:Equation 1. ${V}_{\mathrm{out}}={I}_{\mathrm{load}}\times \mathrm{Gain}\times {R}_{1}+{V}_{\mathrm{ref}}=4\times 100\times 0.005+2.5=4.5V$At the minimum current of -4A, with Gain = 100V/V, R

_{1}= 5mΩ, and V_{ref}= 2.5V:Equation 1. ${V}_{\mathrm{out}}={I}_{\mathrm{load}}\times \mathrm{Gain}\times {R}_{1}+{V}_{\mathrm{ref}}=-4\times 100\times 0.005+2.5=0.5V$ - Filter cap selection:
To filter the input signal at 1kHz, using R

_{1}= R_{2}= 10Ω:Equation 1. ${C}_{1}=\frac{1}{2\pi ({R}_{1}+{R}_{2}){F}_{-3\mathrm{dB}}}=\frac{1}{2\pi (10+10)1000}=7.958\times {10}^{-6}\approx 8\mathrm{\mu F}$

For more information on signal
filtering and the associated gain error, see *INA21x Voltage Output, Low- or
High-Side Measurement, Bidirectional, Zero-Drift Series, Current-Shunt
Monitors*.

**Design
Simulations**

**DC Analysis Simulation
Results**

The following plot shows the simulated
output voltage V_{out} for the given input current I_{in}.

**AC Analysis Simulation
Results**

The following plot shows the simulated gain vs frequency, as designed for in the design steps.

**Transient
Analysis Simulation Results**

The following plot shows the simulated
delay and settling time of the output V_{out} for a step response in
I_{in} from –4A to 4A.