Power-down mode is used when power saving is required. The THS4130 power-down (PD) pin is an active low input. If left unconnected, an internal 250‑kΩ resistor to V_CC+ keeps the device turned on. The threshold voltage for the power-down function is approximately 1.4 V greater than V_CC–. Therefore, if the PD pin is 1.4 V greater than V_CC–, then the device is active. If the PD pin is less than 1.4 V greater than V_CC–, then the device is off. Pull the pin to V_CC– to turn the device off. Figure 8-3 shows the simplified version of the power-down circuit. While in power-down mode, the amplifier goes into a high-impedance state. The amplifier output impedance is typically greater than 1 MΩ in power-down mode.

Figure 8-3 Simplified Power-Down Circuit

Similar to an op amp in an inverting configuration, the output impedance of an FDA is determined by the feedback network configuration. In addition, the THS4130 has an internal 10‑kΩ resistor at each output that is tied to the V_CM error amplifier (see Section 8.2). The differential output impedance is equal to [(2 × R_F + 2 × R_G) || 20 kΩ]. Figure 8-4 shows the closed=loop output impedance of the THS4130 when in power-down.

VCC = ±5 V, gain = 1 V/V, RF = 1 kΩ, PD = VCC–

Figure 8-4 Output Impedance (in Power-Down) vs Frequency