Design Goals

Design Description

This circuit delivers a precise low-level current, I_L, to a load, R_L. The design operates on a single 5 V supply and uses one precision low-drift op amp and one instrumentation amplifier. Simple modifications can change the range and accuracy of the voltage-to-current (V-I) converter.

Design Notes

1. Voltage compliance is dominated by op amp linear output swing (see data sheet A_OL test conditions) and instrumentation amplifier linear output swing. See the Common-Mode Input Range Calculator for Instrumentation Amplifiers for more information.
2. Voltage compliance, along with R_LMin, R_LMax, and R_set bound the I_L range.
3. Check op amp and instrumentation amplifier input common-mode voltage range.
4. Stability analysis must be done to choose R₄ and C₁ for stable operation.
5. Loop stability analysis to select R₄ and C₁ will be different for each design. The compensation shown is only valid for the resistive load ranges used in this design. Other types of loads, op amps, or instrumentation amplifiers, or both will require different compensation. See the Design References section for more op amp stability resources.

Design Steps

1. Select R_set and check I_LMin based on voltage compliance.
   ${\mathrm{I}}_{\mathrm{LMax}}=\frac{{\mathrm{V}}_{\mathrm{oOPAMax}}}{{\mathrm{R}}_{\mathrm{set}}+{\mathrm{R}}_{\mathrm{LMax}}}$
   $\text{10}\mathrm{\mu A}=\frac{\text{4.9}\mathrm{V}}{{\mathrm{R}}_{\mathrm{set}}+\text{390}\mathrm{k\Omega }}\to {\mathrm{R}}_{\mathrm{set}}=\text{100}\mathrm{k\Omega }$
   ${\mathrm{I}}_{\mathrm{LMin}}=\frac{{\mathrm{V}}_{\mathrm{oOPAMin}}}{{\mathrm{R}}_{\mathrm{set}}+{\mathrm{R}}_{\mathrm{LMin}}}$
   ${\mathrm{I}}_{\mathrm{LMin}}=\frac{\text{0.1}\mathrm{V}}{\text{100}\mathrm{k\Omega }+\text{0}\mathrm{\Omega }}=\text{1}\mathrm{\mu A}$
   
2. Compute instrumentation amplifier gain, G.
   ${\mathrm{V}}_{\mathrm{setMin}}={\mathrm{I}}_{\mathrm{LMin}}\times {\mathrm{R}}_{\mathrm{set}}=\text{1}\mathrm{\mu A}\times \text{100}\mathrm{k\Omega }=\text{0.1}\mathrm{V}$
   ${\mathrm{V}}_{\mathrm{setMax}}={\mathrm{I}}_{\mathrm{LMax}}\times {\mathrm{R}}_{\mathrm{set}}=\text{10}\mathrm{\mu A}\times \text{100}\mathrm{k\Omega }=\text{1}\mathrm{V}$
   $\mathrm{G}=\frac{{\mathrm{V}}_{\mathrm{iMax}}-{\mathrm{V}}_{\mathrm{iMin}}}{{\mathrm{V}}_{\mathrm{setMax}}-{\mathrm{V}}_{\mathrm{setMin}}}$
   $\mathrm{G}=\frac{\text{4.9}\mathrm{V}-\text{0.49}\mathrm{V}}{\text{1}\mathrm{V}-\text{0.1}\mathrm{V}}=4.9$
3. Choose R₁ for INA326 instrumentation amplifier gain, G. Use data sheet recommended R₂ = 200 kΩ and C₂ = 510 pF.
   $\mathrm{G}=2\times \left(\frac{{\mathrm{R}}_2}{{\mathrm{R}}_1}\right)$
   ${\mathrm{R}}_1=\frac{2\times {\mathrm{R}}_2}{\mathrm{G}}$
   ${\mathrm{R}}_1=\left(\frac{2\times \text{200}\mathrm{k\Omega }}{4.9}\right)=\text{81.6327}\mathrm{k\Omega }\approx \text{81.6}\mathrm{k\Omega }$
4. The final transfer function of the circuit follows:
   ${\mathrm{I}}_{\mathrm{L}}=\frac{{\mathrm{V}}_{\mathrm{i}}}{\mathrm{G}\times {\mathrm{R}}_{\mathrm{set}}}$
   ${\mathrm{I}}_{\mathrm{L}}=\frac{{\mathrm{V}}_{\mathrm{i}}}{4.9\times \text{100}\mathrm{k\Omega }}=\frac{{\mathrm{V}}_{\mathrm{i}}}{\text{490}\mathrm{k\Omega }}$
   ${\mathrm{V}}_{\mathrm{i}}=\text{0.49}\mathrm{V}\to {\mathrm{I}}_{\mathrm{L}}=\text{1}\mathrm{\mu A}$
   ${\mathrm{V}}_{\mathrm{i}}=\text{4.9}\mathrm{V}\to {\mathrm{I}}_{\mathrm{L}}=\text{10}\mathrm{\mu A}$

Design Simulations

DC Simulation Results

Design References

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

See the TINA-TI™ circuit simulation file, SBOMAT8.

See TIPD107.

See Solving Op Amp Stability Issues - E2E FAQ.

See TI Precision Labs - Op Amps.

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