Figure 48. Test configuration for RISO

Please note that RISO = 47 Ω is sufficient to make the TSB952 stable whatever the capacitive load.

5.8

Resistor values for high speed op amp design

Due to its high gain bandwidth product (GBP), this op amp is particularly sensitive to parasitic impedances. Board

parasitic elements should be taken into account in any sensitive design. Indeed, excessive parasitic elements

(both capacitive and inductive) in the op amp frequency range can alter performance and stability. These issues

can often be mitigated by lowering the resistive impedances.

More specifically, the RC network created by the schematic resistors (Rf and Rg) and the parasitic capacitances

of both the op amp and the PCB can generate a pole below or in the same order of magnitude as the closed-loop

bandwidth of the circuit. In this case, the feedback circuit is not able to fully play its role at high frequency, and the

application can be unstable. This issue can happen when the schematic gain is low (typically < 5), or the device is

used in follower mode with a resistor in the feedback. In these cases, it is advised to use a low value feedback

resistor (Rf), typically 600 Ω.

Figure 49. Inverting amplifier configuration with parasitic input capacitances

Also, some designs use an input resistor on the positive input, generally of the same value as the input on the

negative resistor. This resistor can be useful to balance the input currents on the positive and negative inputs, and

reduce the impact of those input currents on precision. However, this is not useful with the TSB952 as the input

currents are very low. Furthermore, this resistor can also interact with the input capacitances to generate a pole.

The frequency of this pole should be kept higher than the closed-loop bandwidth frequency.

The macromodel provided takes into account the circuit parasitic capacitors. Thus, a transient Spice simulation

(100 mV step) is an easy way to evaluate the stability of the application. However, this cannot replace a hardware

evaluation of the application circuit.

TSB952

Application information

DS14576 - Rev 1

page 22/36