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Caution: Wide Load!

Welcome to the new Precision Hub blog on Planet Analog. In the weeks and months ahead, precision analog experts from TI will share tools, tips, and tricks on a variety of amplifier and data converter topics. We hope you find the blog to be a center of knowledge and a resource to help you solve design challenges, and we welcome your feedback and comments!

Direct current sensing is one of the most frequent topics I get questions on when supporting customers on TI's Engineer-to-Engineer (E2E) forum. Direct current sensing is straightforward: Place a resistor in series with the load (shunt resistor) and measure the voltage across it (shunt voltage). This method works great for load currents that span 1 to 1.5 decades.

However, low-power applications require current sensing solutions spanning three or more decades. Such wide-load current ranges can be difficult when using linear devices to measure the shunt voltage.

The output swing of the amplifier limits the measurable load current range. For example, an output swing from 100mV to 4.9-V equates to ~1.5 decades of linear output range. So, what do you do when you want to measure three decades of load current? Adjust the gain!

Figure 1 shows how two gains can increase the measurable load current range.

Figure 1

Current sensing with two gain ranges

Current sensing with two gain ranges

Log amplifiers and programmable gain amplifiers are one option, but they're overkill if you only need to measure two to three decades of load current.

Another approach uses an op amp with a switch to control the gain, as shown in Figure 2.

Figure 2

Low-side, op amp, adjustable gain

Low-side, op amp, adjustable gain

This creates inaccuracy if there is any parasitic impedance between the shunt resistor and ground — a big drawback. Figure 3 shows that the parasitic voltage (VPAR ) is gained by the op amp when Rg is referred to ground.

Figure 3

Parasitic voltage error, Rg=GND

Parasitic voltage error, Rg =GND

To reduce this error, connect Rg to V par (Kelvin-connection). Figure 4 below shows that the op amp does not apply gain to the parasitic voltage, but it's still present at the output. Vpar varies with load current and PCB manufacturing tolerances.

Figure 4

Parasitic voltage error, Rg= Vpar

Parasitic voltage error, Rg = Vpar

To remove this error term, use a device that only amplifies differential voltages — for example, an instrumentation amplifier.

Figure 5 shows how an instrumentation amplifier removes the error when Kelvin-connected to the shunt resistor. The equation in Figure 5 simplifies to Vout =Vref +Vshunt (notice VPAR =0).

Figure 5

No parasitic error instrumentation amplifier

No parasitic error instrumentation amplifier

Many system designers want single-supply solutions. Traditional instrumentation amplifiers don't meet this need because they have output swing limitations based on the input common mode voltage, power supply, reference voltage, and gain.

Figure 6 depicts such a relationship for the INA333 instrumentation amplifier.

Figure 6

INA333 Single-Supply Operation

INA333 Single-Supply Operation

For example, the output swings from ~0V to ~2V if the input common-mode voltage is 1V. In low-side sensing, the common-mode voltage is 0V, so the output has little or no swing.

To overcome this, the INA326 instrumentation amplifier uses a unique current topology that provides true rail-to-rail input and output.

Combining the uniqueness of the INA326 with a switch to control its gain yields an elegant single-supply current sensing solution that detects up to 3 decades of load current.

Figure 7 is a schematic of an example design.

Figure 7

10μA to 10mA, single-supply current sensing solution

10μA to 10mA, single-supply current sensing solution

Figure 7 depicts a TI Precision Design for a 10μA-10mA single-supply current sensing solution. The design includes theory, calculations, and TINA-TI simulations.

Next time you're designing a current sensing solution, make sure you understand the limitations of the amplifier. Before jumping to complex solutions, realize that a simple switch can significantly increase the range!

About the author:
Pete Semig is an Analog Applications Engineer in the Precision Linear group at Texas Instruments. He supports Texas Instruments’ difference amplifiers & instrumentation amplifiers and specializes in current sensing. Prior to joining Texas Instruments in 2007 he earned his B.S.E.E. and M.S.E.E. from Michigan State University in 1998 & 2001, respectively. From 2001-2007 he was a faculty member in Michigan State University’s Department of Electrical & Computer Engineering where he taught a variety of courses and laboratories. Pete had the honor of receiving multiple annual service awards for teaching excellence, as recognized by his students. Related posts:

1 comment on “Caution: Wide Load!

  1. Brad_Albing
    August 6, 2013

    Clever idea to add a second range in the current monitoring circuitry. Why didn't I think of that? And you made some good points there regarding the use of differential measuring methods (instrumentation amp) to eleiminate the IR drop across those pesky parasitic elements.

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