5.7

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 parasitics should be taken into account in any sensitive design. Indeed, excessive parasitic (both capacitive

and inductive) in the op amp frequency range can alter performances 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 (as documented in Table 6 to Table 8

and illustrated in Figure 48) and the PCB can generate a pole below or in the same order of magnitude than

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 1 kΩ.

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

resistance on the negative input. 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 on

the TSV7722 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.

TSV7721, TSV7722, TSV7723

Resistor values for high speed op amp design

DS13614 - Rev 3

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