5 Slew Rate Dependencies on Circuit Signal Levels and Op Amp Gain Set by Feedback Network

Figure 5-1 has two schematics that produce a 1-V output step. In the left circuit, the 1-V is amplified by a noise gain of 1. In the right circuit, 10 mV is amplified by a noise gain of 100. In these non-inverting circuits, signal gain and noise gain are equivalent.

Figure 5-1 Two Schematics With The Same 1-V Output Step

Figure 5-1 is the resulting simulation waveform from the Figure 5-1 schematics. The unity gain (black curve) rises at 2 V/µs, which is the same as the data sheet specification. The 100 gain (red curve) has a slower slew rate that continues to decrease as the voltage rises. The initial slew rate for a 10-mV input step, following the Figure 3-1 output, is 14% of the natural slew maximum of 0.5 V/µs, which is 70 mV/µs. By the time the waveform rises half way, the V_ID has dropped to 5 mV. Now, the SR is the Figure 3-1 output 7% (of 0.5 V/µs), which is 35 mV/µs.

Figure 5-2 Two Waveforms With 1-V Step

Even if the TLV9002 device did not have slew boost, the unity gain slew rate (0.5 V/µs natural) is much faster than a G=100 curve. Slew boost or not, a smaller input signal with higher gain must always have a slower observed slew rate than a larger input signal with lower gain.