The capacitive load of an ADC, or some other next-stage device, is commonly required to be driven. Directly connecting a capacitive load to the output pins of a closed-loop amplifier such as the OPA1637 can lead to an unstable response. One typical remedy to this instability is to add two small series resistors (R_ISO) at the outputs of the OPA1637 before the capacitive load. Good practice is to leave a place for the R_ISO elements in a board layout (a 0-Ω value initially) for later adjustment, in case the response appears unacceptable.

For applications where the OPA1637 is used as an output device to drive an unknown capacitive load, such as a cable, R_ISO is required. Figure 9-1 shows the required R_ISO value for a 40-degree phase-margin response. The peak required R_ISO value occurs when C_L is between 500 pF and 1 nF. As C_L increases beyond 1 nF, the bandwidth response of the device reduces, resulting in a slower response but no major degradation in phase margin. For a typical cable type, such as Belden 8451, capacitive loading can vary from 340 pF (10-foot cable) to 1.7 nF (50-foot cable). Selecting R_ISO to be 100 Ω provides sufficient phase margin regardless of the cable length. R_ISO can also be used within the loop feedback of the amplifier; however, simulation must be used to verify the stability of the system.

Figure 9-1 Required Isolation Resistance vs Capacitive Load for a 40° Phase Margin