SBOA610 January   2025 OPA690 , OPA695

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Background
    1. 1.1 Slew Rate and Slew Boost Definitions
    2. 1.2 Slew Rate Limits for Voltage Feedback Amplifiers
    3. 1.3 Slew Rate Internals for Current Feedback
  5. 2Slew Rate Data Using the OPA690
  6. 3Slew Rate Data Using the OPA695
  7. 4Summary
  8. 5References

2 Slew Rate Data Using the OPA690

The OPA690 is a voltage feedback amplifier. It has a very high slew rate for a voltage feedback amplifier due to additional circuitry supplementing the slew. The typical value in the device-specific data sheet is 1800V/μs and minimum value is 1400V/μs. This is specified for a gain of 2V/V and an output step of 4V, meaning that the input was a 2V step function. This input is called VID, as it is the differential voltage applied instantaneously across the inputs of the amplifier.

The part is set up using the DEM-OPA-SOT-1B board, in a noninverting configuration, shown in Figure 2-1.

Setup for Lab Testing Figure 2-1 Setup for Lab Testing

The pulse response of the device-specific data sheet setup is shown in Figure 2-2. The slew rate can be calculated to be 1560V/us, which is within an acceptable margin of error based on the specification.

Pulse Response of the OPA690 in Data Sheet Conditions Figure 2-2 Pulse Response of the OPA690 in Data Sheet Conditions

Figure 2-3 shows the pulse response with the OPA690 gain increased to 5V/V. The measured slew rate was 573V/μs, which is much slower than the lower gain configuration due to the limited input differential voltage.

Pulse Response of the OPA690 G = 5. Figure 2-3 Pulse Response of the OPA690 G = 5.

Figure 2-4 shows a pulse response graph for VID = 2, 0.8, 0.4, and 0.2V, while adjusting the gain to achieve a 4V step at the output. As expected with the theory, the slew rate continuously decreases as the input differential voltage decreases.

Pulse Responses of the OPA690 With Different Input Voltages and Gains Figure 2-4 Pulse Responses of the OPA690 With Different Input Voltages and Gains

The slew rate response gets much slower for each decrease in VID. The overall relationship between slew rate and VID is shown in Figure 2-5. This direct relationship between slew rate and input differential voltage is what is expected based on the theoretical analysis.

Slew Rate vs. Input Differential Voltage Figure 2-5 Slew Rate vs. Input Differential Voltage

Figure 2-5 highlights the dramatic drop in slew rate as VID decreases. Ramping Up on Slew Rate shows a similar graph for the relationship between slew rate and VID. However, the relationship shown is much less extreme than our findings. For example, the paper's results show that 100% of a device's maximum slew rate can be achieved with as little as 0.1V input, whereas the OPA690 needs around 2V. This is common in amplifiers with slew rate boost. Although the slew rate decreases more steeply with decreases in VID, the value is still much higher than a comparable non slew boosted amplifier.