Design Goals

Design Description

The transimpedance op amp circuit configuration converts an input current source into an output voltage. The current to voltage gain is based on the feedback resistance. The circuit is able to maintain a constant voltage bias across the input source as the input current changes which benefits many sensors.

Design Notes

1. Use a JFET or CMOS input op amp with low bias current to reduce DC errors.
2. A bias voltage can be added to the non-inverting input to set the output voltage for 0 A input currents.
3. Operate within the linear output voltage swing (see A_ol specification) to minimize non-linearity errors.

Design Steps

1. Select the gain resistor.
   ${\mathrm{R}}_1=\frac{{\mathrm{V}}_{\mathrm{oMax}}-{\mathrm{V}}_{\mathrm{oMin}}}{{\mathrm{I}}_{\mathrm{iMax}}}=\frac{5\mathrm{V}-0\mathrm{V}}{50\mathrm{\mu A}}=100\mathrm{k\Omega }$
2. Select the feedback capacitor to meet the circuit bandwidth.
   ${\mathrm{C}}_1\le \frac{1}{2\times \mathrm{\pi }\times {\mathrm{R}}_1\times {\mathrm{f}}_{\mathrm{p}}}$
   ${\mathrm{C}}_1\le \frac{1}{2\times \mathrm{\pi }\times 100\mathrm{k\Omega }\times 10\mathrm{kHz}}\le 159\mathrm{pF}\approx 150\mathrm{pF}\text{ (Standard Value)}$
3. Calculate the necessary op amp gain bandwidth (GBW) for the circuit to be stable.
   $\mathrm{GBW}>\frac{{\mathrm{C}}_{\mathrm{i}}+{\mathrm{C}}_1}{2\times \mathrm{\pi }\times {\mathrm{R}}_1\times {{\mathrm{C}}_1}^2}>\frac{6\mathrm{pF}+150\mathrm{pF}}{2\times \mathrm{\pi }\times 100\mathrm{k\Omega }\times {\left(150\mathrm{pF}\right)}^2}>11.03\mathrm{kHz}$
   $\begin{array}{l}\text{where }{\mathrm{C}}_{\mathrm{i}}={\mathrm{C}}_{\mathrm{s}}+{\mathrm{C}}_{\mathrm{d}}+{\mathrm{C}}_{\mathrm{cm}}=0\mathrm{pF}+3\mathrm{pF}+3\mathrm{pF}=6\mathrm{pF}\text{ given }\end{array}$
   • C_s: Input source capacitance
   • C_d: Differential input capacitance of the amplifier
   • C_cm: Common-mode input capacitance of the inverting input

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 SBOC501.

See TIPD176.

Design Featured Op Amp

Design Alternate Op Amp