THS4130, THS4131

HIGHSPEED, LOW NOISE, FULLY DIFFERENTIAL I/O AMPLIFIERS

SLOS318E − MAY 2000 − REVISED JANUARY 2004

22

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

power-down mode

The power-down mode is used when power saving is required. The power-down terminal (PD) found on the

THS413x is an active low terminal. If it is left as a no-connect terminal, the device will always stay on due to an

internal 50 k

1.4 V = −3.6 V). By the same calculation, the device is off below −3.6 V. It is recommended to pull the terminal

circuit. While in the power-down state, the amplifier goes into a high impedance state. The amplifier output

impedance is typically greater than 1 M

Ω in the power-down state.

VCC

PD

VCC−

To Internal Bias

Circuitry Control

50 k

Ω

Figure 41. Simplified Power-Down Circuit

Due to the similarity of the standard inverting amplifier configuration, the output impedance appears to be very

still connected to the circuit. Therefore, a current path is allowed between the input of the amplifier and the output

of the amplifier. An example of the closed loop output impedance is shown in Figure 42.

Figure 42

200

1200

2200

100 k

1 M

10 M

100 M

1 G

f − Frequency − Hz

OUTPUT IMPEDANCE (IN POWER DOWN)

vs

FREQUENCY

VCC = ±5 V

G = 1

Rf = 1 kΩ

PD = VCC−