Designed as a low-power, low-voltage operational amplifier, the OPAx607 devices are capable of delivering a robust output drive. For resistive loads of 10 kΩ, the output swings to within a few millivolts of either supply rail, regardless of the applied power-supply voltage. Different load conditions change the ability of the amplifier to swing close to the rails. The OPAx607 devices drive up to a nominal capacitive load of 47 pF on the output with no special consideration and without the need of a series isolation resistor R_ISO while still being able to achieve 45° of phase margin. When driving capacitive loads greater than 47 pF, TI recommends using R_ISO as shown in Figure 8-1 in series with the output as close to the device as possible. Refer to Figure 7-18 for looking up different values of R_ISO required for C_L to achieve 45° phase margin. Without R_ISO, the external capacitance (C_L) interacts with the output impedance (Z_O) of the amplifier, resulting in stability issues. Inserting R_ISO isolates C_L from Z_O and restores the phase margin. Figure 8-1 shows the test circuit.

Figure 8-1 Input Current Protection and Driving Capacitive Loads

Figure 8-2 and Figure 8-3 show the phase margin achieved with varying R_ISO with different values of C_L.

Gain = 10 V/V,

Cf = 2.5 pF,

RL = 10 kΩ

Figure 8-2 Phase Margin vs. Series Isolation Resistor

Gain = 20 V/V,

Cf = 2.5 pF,

RL = 10 kΩ

Figure 8-3 Phase Margin vs. Series Isolation Resistor