SBOA284 March 2021 OPA375 , TLV9002

**Design Goals**

Input | Output | BW | Supply | |||
---|---|---|---|---|---|---|

I_{iMin} | I_{iMax} | V_{oMin} | V_{oMax} | f_{p} | V_{cc} | V_{ee} |

0A | 100nA | 0V | 3.2V | 10kHz | 3.3V | 0V |

**Design Description**

This transimpedance amplifier with a T-network feedback configuration converts an input current into an output voltage. The current-to-voltage gain is based on the T-network equivalent resistance which is larger than any of the resistors used in the circuit. Therefore, the T-network feedback configuration circuit allows for very high gain without the use of large resistors in the feedback or a second gain stage, reducing noise, stability issues, and errors in the system.

**Design Notes**

- C
_{1}and R_{1}set the input signal cutoff frequency, f_{p}. - Capacitor C
_{1}in parallel with R_{1}helps limit the bandwidth, reduce noise, and also improve the stability of the circuit if high-value resistors are used. - The common-mode voltage is the voltage at the non-inverting input and does not vary with input current.
- A bias voltage can be added to the non-inverting input to bias the output voltage above the minimum output swing for 0A input current.
- Using high-value resistors can degrade the phase margin of the circuit and introduce additional noise in the circuit.
- Avoid placing capacitive loads directly on the output of the amplifier to minimize stability issues.
- For more information on op amp linear operating region, stability, slew-induced distortion, capacitive load drive, driving ADCs, and bandwidth see the
*Design References*section.

**Design Steps**

The transfer function of this circuit follows:

Equation 1. ${V}_{o}={I}_{i}\times \left(\frac{{R}_{2}\times {R}_{1}}{{R}_{3}}+{R}_{1}+{R}_{2}\right)$

- Calculate the required gain: Equation 1. $\text{Gain =}\frac{{V}_{\mathrm{oMax}}}{{\text{I}}_{\text{oMax}}}=\frac{\text{3.2V}}{\text{100nA}}{\text{= 3.2\xd7 10}}^{\text{7}}\frac{\text{V}}{\text{A}}$
- Choose the resistor values to set the pass-band gain: Equation 1. $\text{Gain=}\left(\frac{{R}_{2}\times {R}_{1}}{{R}_{3}}{\text{+ R}}_{\text{1}}{\text{+ R}}_{2}\right)$
Since R

_{1}will be the largest resistor value in the system choose this value first then choose R_{2}and calculate R_{3}. Select R_{1}= 3.3MΩ and R_{2}= 13kΩ. R_{1}is very large due to the large transimpedance gain of the circuit. R_{2}is in the ~10k ohm range so the op amp can drive it easily.Equation 1. ${R}_{3}=\left(\frac{{R}_{2}\times {R}_{1}}{Gain\u2013{R}_{1}\u2013{R}_{2}}\right)=\left(\frac{\text{13k\u2126 \xd7 3.3M\u2126}}{{\text{3.2 \xd7 10}}^{\text{7}}\frac{\text{V}}{\text{A}}\text{\u2013 3.3M\u2126 \u2013 13k\u2126}}\right)\text{= 1.5k\u2126}$ - Calculate C
_{1}to set the location of f_{p}.Equation 1. ${C}_{1}=\frac{1}{2\pi \times {R}_{1}\times {f}_{p}}=\frac{1}{\text{2\pi \xd7 3.3M\u2126 \xd7 10kHz}}=\text{4.82pF}\text{\u2248 4.8pF(StandardValue)}$ - Run a stability analysis to make sure that the circuit is stable. For more information on how to run a stability analysis see the
*TI Precision Labs - Op amp: Stability*video.

**Design Simulations**

**DC Simulation Results**

**AC Simulation Results**

**Design References**

- See Analog Engineer's Circuit Cookbooks for TI's comprehensive circuit library.
- See SPICE file, SBOMB39.
- See TIPD176, www.ti.com/tool/tipd176.
- For more information on many op amp topics including common-mode range, output swing, bandwidth, and how to drive an ADC please visit TI Precision Labs.

**Design Featured Op Amp**

TLV9002 | |
---|---|

V_{cc} | 1.8V to 5.5V |

V_{inCM} | Rail-to-rail |

V_{out} | Rail–to–rail |

V_{os} | 0.4mV |

I_{q} | 60µA |

I_{b} | 5pA |

UGBW | 1MHz |

SR | 2V/µs |

#Channels | 1, 2, 4 |

www.ti.com/product/TLV9002 |

**Design Alternate Op Amp**

OPA375 | |
---|---|

V _{cc}_{} | 2.25V to 5.5V |

V _{inCM}_{} | V_{ee} to (V_{cc} –1.2V) |

V_{out} | Rail–to–rail |

V_{os} | 0.15mV |

I_{q} | 890µA |

I_{b} | 10pA |

UGBW | 10MHz |

SR | 4.75V/µs |

#Channels | 1 |

www.ti.com/product/OPA375 |