Design Goals

Design Description

This single-supply, low-side, current-sensing solution accurately detects load current between 10 μA and 10 mA. A unique yet simple gain switching network was implemented to accurately measure the three-decade load current range.

Design Notes

1. Use a maximum shunt resistance to minimize relative error at minimum load current.
2. Select 0.1% tolerance resistors for R₁, R₂, R₃, and R₄ in order to achieve approximately 0.1% FSR gain error.
3. Use a switch with low on-resistance (R_on) to minimize interaction with feedback resistances, preserving gain accuracy.
4. Minimize capacitance on INA326 gain setting pins.
5. Scale the linear output swing based on the gain error specification.

Design Steps

1. Define full-scale shunt resistance.
   ${\mathrm{R}}_1=\frac{{\mathrm{V}}_{\mathrm{iMax}}}{{\mathrm{I}}_{\mathrm{iMax}}}=\frac{250\mathrm{mV}}{10\mathrm{mA}}=25\mathrm{\Omega }$
2. Select gain resistors to set output range.
   ${\mathrm{G}}_{\mathrm{IiMax}}=\frac{{\mathrm{V}}_{\mathrm{oMax}}}{{\mathrm{V}}_{\mathrm{iMax}}}=\frac{{\mathrm{V}}_{\mathrm{oMax}}}{{\mathrm{R}}_1\times {\mathrm{I}}_{\mathrm{iMax}}}=\frac{4.9\mathrm{V}}{25\mathrm{\Omega }\times 10\mathrm{mA}}=19.6\frac{\mathrm{V}}{\mathrm{V}}$
   ${\mathrm{G}}_{\mathrm{IiMin}}=\frac{{\mathrm{V}}_{\mathrm{oMin}}}{{\mathrm{V}}_{\mathrm{iMin}}}=\frac{{\mathrm{V}}_{\mathrm{oMin}}}{{\mathrm{R}}_1\times {\mathrm{I}}_{\mathrm{iMin}}}=\frac{100\mathrm{mV}}{25\mathrm{\Omega }\times 10\mathrm{\mu A}}=400\frac{\mathrm{V}}{\mathrm{V}}$
   ${\mathrm{R}}_2=\frac{{\mathrm{R}}_4\times {\mathrm{G}}_{\mathrm{IiMin}}}{2}=\frac{50\mathrm{k\Omega }\times 400\frac{\mathrm{V}}{\mathrm{V}}}{2}=10\mathrm{M\Omega }$
   ${\mathrm{R}}_2\parallel {\mathrm{R}}_3=\frac{{\mathrm{R}}_4\times {\mathrm{G}}_{\mathrm{IiMax}}}{2}=\frac{50\mathrm{k\Omega }\times 19.6\frac{\mathrm{V}}{\mathrm{V}}}{2}=490\mathrm{k\Omega }$
   ${\mathrm{R}}_3=\frac{490\mathrm{k\Omega }\times {\mathrm{R}}_2}{{\mathrm{R}}_2-490\mathrm{k\Omega }}=515.25\mathrm{k\Omega }\approx 511\mathrm{k\Omega }\mathrm{}\text{ (Standard Value)}$
3. Select a capacitor for the output filter.
   ${\mathrm{f}}_{\mathrm{p}}=\frac{1}{2\times \mathrm{\pi }\times {\mathrm{R}}_5\times {\mathrm{C}}_4}=\frac{1}{2\times \mathrm{\pi }\times 100\mathrm{\Omega }\times 1 \mathrm{\mu F}}=1.59\mathrm{kHz}$
4. Select a capacitor for gain and filtering network.
   ${\mathrm{C}}_2=\frac{1}{2\times \mathrm{\pi }\times {\mathrm{R}}_2\times {\mathrm{f}}_{\mathrm{p}}}=\frac{1}{2\times \mathrm{\pi }\times 10\mathrm{M\Omega }\times 1.59\mathrm{kHz}}=10\mathrm{pF}$
   $\begin{array}{l}{\mathrm{C}}_3=\frac{1}{2\times \mathrm{\pi }\times \left({\mathrm{R}}_2\left|\right|{\mathrm{R}}_3\right)\times {\mathrm{f}}_{\mathrm{p}}}-{\mathrm{C}}_2=\frac{1}{2\times \mathrm{\pi }\times \left(10\mathrm{M\Omega }\left|\right|511\mathrm{k\Omega }\right)\times 1.59\mathrm{kHz}}-10\mathrm{pF}\\ \end{array}$
   ${\mathrm{C}}_3=196\mathrm{pF}\approx 194\mathrm{pF}\text{ (Standard Value)}$

Design Simulations

DC Simulation Results

AC Simulation Results

Design References

See Analog Engineer's Circuit Cookbooks for TI's comprehensive circuit library.

See circuit SPICE simulation file SBOC498.

See TIPD104, Current Sensing Solution, 10 µA-10 mA, Low-Side, Single Supply.

Design Featured Op Amp