SBOA258A February 2019 – December 2020 OPA322 , OPA354

**Design Goals**

Input | Output | Bandwidth | Supply | ||||
---|---|---|---|---|---|---|---|

V_{iMin} | V_{iMax} | V_{oMin} | V_{oMax} | BW | V_{cc} | V_{ee} | V_{ref} |

–0.24V | +0.24V | +0.1V | +4.9V | 10MHz | +2.5V | 0V | 2.5V |

**Design Description**

This design uses 3 op–amps to build a discrete wide bandwidth instrumentation amplifier. The circuit converts a differential, high frequency signal to a single–ended output.

**Design Notes**

- Reduce the capacitance on the output of each op amp to avoid stability issues.
- Use low gain configurations to maximize the bandwidth of the circuit.
- Use precision resistors to achieve high DC CMRR performance.
- Use small resistors in op–amp feedback to maintain stability.
- Set the reference voltage, V
_{ref}, at mid–supply to allow the output to swing to both supply rails. - Phase margin of 45° or greater is required for stable operation.
- R
_{7}sets the gain of the instrumentation amplifier. - Linear operation depends upon the input common–mode and the output swing ranges of the discrete op amps used. The linear output swing ranges are specified under the A
_{ol}test conditions in the op amps datasheets. - V
_{ref}also sets the common-mode voltage of the input, V_{i}, to ensure linear operation.

**Design Steps**

- The transfer function of the circuit is given below. Equation 1.
where V

_{i}is the differential input voltageVref is the reference voltage provided to the amplifier

Equation 1. - To maximize the usable bandwidth of design, set the gain of the diff amp stage to 1V/V. Use smaller value resistors to minimize noise.
Equation 1. $\mathrm{Choose}{R}_{3}={R}_{4}={R}_{5}={R}_{6}=500\Omega (\mathrm{Standard}\mathrm{value})$
- Choose values for resistors R
_{1}and R_{2}. Keep these values low to minimize noise.Equation 1. ${R}_{1}={R}_{2}=\text{500}\Omega (\mathrm{Standard}\mathrm{value})$ - Calculate resistor R
_{7}to set the gain of the circuit to 10V/VEquation 1.Equation 1. - Calculate the reference voltage to bias the input to mid-supply. This will maximize the linear output swing of the instrumentation amplifier. See References for more information on the linear operating region of instrumentation amplifiers.
Equation 1. ${V}_{\mathrm{ref}}=\frac{{V}_{s}}{2}=\frac{\text{5}V}{2}=\text{2.5}V$

**Design Simulations**

**DC Simulation Results**

**Transient Simulation Results**

**AC Simulation Results**

**References**

- Analog Engineer's Circuit Cookbooks
- SPICE Simulation File SBOMAU6
- TI Precision Labs
*Instrumentation Amplifier V*_{CM}vs. V_{OUT}Plots- Common-mode Range Calculator for Instrumentation Amplifiers

OPA354 | |
---|---|

V_{ss} | 2.5V to 5.5V |

V_{inCM} | Rail–to–rail |

V_{out} | Rail–to–rail |

V_{os} | 2mV |

I_{q} | 4.9mA/Ch |

I_{b} | 3pA |

UGBW | 250MHz |

SR | 150V/µs |

#Channels | 1,2,4 |

www.ti.com/product/opa354 |

**Design Alternate Op Amp**

OPA322 | |
---|---|

V_{ss} | 1.8V to 5.5V |

V_{inCM} | Rail–to–rail |

V_{out} | Rail–to–rail |

V_{os} | 500µV |

I_{q} | 1.6mA/Ch |

I_{b} | 0.2pA |

UGBW | 20MHz |

SR | 10V/µs |

#Channels | 1,2,4 |

www.ti.com/product/opa322 |

Revision | Date | Change |
---|---|---|

A | December 2020 | Updated R11 to R7 for resistor number consistency |