SBOA270C March 2019 – December 2020 LMV358 , TLV170

**Design Goals**

Input | Output | Freq. | Supply | |||
---|---|---|---|---|---|---|

V_{iMin} | V_{iMax} | V_{oMin} | V_{oMax} | f | V_{cc} | V_{ee} |

–7V | 7V | –14V | 14V | 3kHz | 15V | –15V |

**Design Description**

This design inverts the input signal,
V_{i}, and applies a signal gain of –2V/V. The input signal typically
comes from a low-impedance source because the input impedance of this circuit is
determined by the input resistor, R_{1}. The common-mode voltage of an
inverting amplifier is equal to the voltage connected to the non-inverting node,
which is ground in this design.

**Design Notes**

- Use the op amp in a linear
operating region. Linear output swing is usually specified under the
A
_{OL}test conditions. The common-mode voltage in this circuit does not vary with input voltage. - The input impedance is determined by the input resistor. Make sure this value is large when compared to the source output impedance.
- 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.
- Small-signal bandwidth is
determined by the noise gain (or non-inverting gain) and op amp gain-bandwidth
product (GBP). Additional filtering can be accomplished by adding a capacitor in
parallel to R
_{2}. Adding a capacitor in parallel with R_{2}improves stability of the circuit if high value resistors are used. - Large signal performance can be limited by slew rate. Therefore, check the maximum output swing versus frequency plot in the data sheet to minimize slew-induced distortion.
- 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. ${V}_{o}={V}_{i}\times \left(-\frac{{R}_{2}}{{R}_{1}}\right)$

- Determine the starting value of
R
_{1}. The relative size of R_{1}to the signal source impedance affects the gain error. Assuming the impedance from the signal source is low (for example, 100Ω), set R_{1}= 10kΩ for 1% gain error.Equation 1. ${R}_{1}=10\mathrm{k\Omega}$ - Calculate the gain required for the
circuit. Since this is an inverting amplifier, use V
_{iMin}and V_{oMax}for the calculation.Equation 1. $G=\frac{{V}_{\mathrm{oMax}}}{{V}_{\mathrm{iMin}}}=\frac{14V}{-7V}=-2\frac{V}{V}$ - Calculate R
_{2}for a desired signal gain of –2 V/V.Equation 1. $G=-\frac{{R}_{2}}{{R}_{1}}\to {R}_{2}=-G\times {R}_{1}=-\left(-2\frac{V}{V}\right)\times 10\mathrm{k\Omega}=20\mathrm{k\Omega}$ - Calculate the small signal circuit
bandwidth to ensure it meets the 3-kHz requirement. Be sure to use the noise
gain, or non-inverting gain, of the circuit. Equation 1. ${\mathrm{GBP}}_{\mathrm{TLV}170}=1.2\mathrm{MHz}$Equation 1. $\mathrm{NG}=\left(1+\frac{{R}_{2}}{{R}_{1}}\right)=3\frac{V}{V}$Equation 1. $\mathrm{BW}=\frac{\mathrm{GBP}}{\mathrm{NG}}=\frac{1.2\mathrm{MHz}}{3V/V}=400\mathrm{kHz}$
- Calculate the minimum slew rate
required to minimize slew-induced distortion. Equation 1. ${V}_{p}=\frac{\mathrm{SR}}{2\times \pi \times f}\to \mathrm{SR}>2\times \pi \times f\times {V}_{p}$Equation 1. $\mathrm{SR}>2\times \pi \times 3\mathrm{kHz}\times 14V=263.89\frac{\mathrm{kV}}{s}=0.26\frac{V}{\mathrm{\mu s}}$
- SR
_{TLV170}= 0.4V/µs, therefore, it meets this requirement.

- SR
- To avoid stability issues, ensure
that the zero created by the gain setting resistors and input capacitance of the
device is greater than the bandwidth of the circuit. Equation 1. $\frac{1}{2\times \pi \times \left({C}_{\mathrm{cm}}+{C}_{\mathrm{diff}}\right)\times \left({R}_{2}\parallel {R}_{1}\right)}>\frac{\mathrm{GBP}}{\mathrm{NG}}$Equation 1. $\frac{1}{2\times \pi \times \left(3\mathrm{pF}+3\mathrm{pF}\right)\times}$
20 kΩ × 10 kΩ 20 kΩ + 10 kΩ > 1 . 2 MHz 3 V / V

Equation 1.

- C
_{cm}and C_{diff}are the common-mode and differential input capacitance of the TLV170, respectively. - Since the zero frequency is greater than the bandwidth of the circuit, this requirement is met.

**Design Simulations**

**DC Simulation Results**

**AC Simulation Results**

The bandwidth of the circuit depends on the noise gain, which is 3V/V. The bandwidth is determined by looking at the –3-dB point, which is located at 3dB given a signal gain of 6dB. The simulation sufficiently correlates with the calculated value of 400kHz.

**Transient Simulation Results**

The output is double the magnitude of the input and inverted.

**References:**

- Analog Engineer's Circuit Cookbooks
- SPICE Simulation File SBOC492
- TI Precision Labs

**Design Featured Op Amp**

TLV170 | |
---|---|

V_{ss} | ±18 V (36 V) |

V_{inCM} | (Vee-0.1 V) to (Vcc-2 V) |

V_{out} | Rail-to-rail |

V_{os} | 0.5 mV |

I_{q} | 125 µA |

I_{b} | 10 pA |

UGBW | 1.2 MHz |

SR | 0.4 V/µs |

#Channels | 1, 2, 4 |

www.ti.com/product/tlv170 |

**Design Alternate Op Amp**

LMV358A | |
---|---|

V_{ss} | 2.5 V to 5.5 V |

V
_{inCM}_{} | (V_{ee}–0.1 V) to (V_{cc}–1 V) |

V_{out} | Rail-to-rail |

V_{os} | 1 mV |

I_{q} | 70 µA |

I_{b} | 10 pA |

UGBW | 1 MHz |

SR | 1.7 V/µs |

#Channels | 1 (LMV321A), 2 (LMV358A), 4 (LMV324A) |

www.ti.com/product/lmv358A |

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

C | December 2020 | Updated result for Design Step 6. |

B | March 2019 | Changed LMV358 to LMV358A in the Design Alternate Op Amp section. |

A | January 2019 | Downstyle title. Added link to circuit cookbook landing page. |