Datasheet

POWER-DOWN MODE
V
CC
PD
V
CC-
To Internal Bias
Circuitry Control
50 kΩ
10
100
1000
100 k 1 M 10 M
f Frequency - Hz
OUTPUT IMPEDANCE (SHUTDOWN)
vs
FREQUENCY
100 M 1 G
V
CC
= ±5 V
Output Impedance (Shutdown) -
Ω
R
f
= R
(g)
= 500 Ω
THS4150
THS4151
SLOS321G – MAY 2000 – REVISED MARCH 2009 ........................................................................................................................................................... www.ti.com
The power-down mode is used when power saving is required. The power-down terminal ( PD) found on the
THS415x 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 Ω resistor to V
CC
. The threshold voltage for this terminal is approximately 1.4 V above V
CC –
. This
means that if the PD terminal is 1.4 V above V
CC –
, the device is active. If the PD terminal is less than 1.4 V
above V
CC –
, the device is off. For example, if V
CC –
= – 5 V, then the device is on when PD reaches 3.6 V, ( – 5 V +
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 to
V
CC –
in order to turn the device off. Figure 42 shows the simplified version of the power-down 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.
Figure 42. Simplified Power-Down Circuit
Due to the similarity of the standard inverting amplifier configuration, the output impedance appears to be very
low while in the power-down state. This is because the feedback resistor (R
f
) and the gain resistor (R
(g)
) are 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 43 .
Figure 43.
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