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 5V 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 Analog engineer's calculator 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 are 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 = 200kΩ and C₂ = 510pF.
   $\mathrm{G}=2\times \left(\frac{{\mathrm{R}}_2}{{\mathrm{R}}_1}\right)$
   ${\mathrm{R}}_1=\frac{2\times {\mathrm{R}}_2}{\mathrm{G}}$
   ${\mathrm{R}}_1=\left(\frac{2\times \text{200}\mathrm{k\Omega }}{4.9}\right)=\text{81.6327}\mathrm{k\Omega }\approx \text{81.6}\mathrm{k\Omega }$
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

Texas Instruments, SBOMAT8 TINA-TI™ circuit simulation, file download

Texas Instruments, Low-Level V-to-I Converter Reference Design, 0V to 5V Input and 0µA to 5µA Output, product page

Texas Instruments, Solving Op Amp Stability Issues, E2E^TM amplifiers forum

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