Datasheet

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POWER-DOWN MODE
V
CC
PD
V
CC-
To Internal Bias
Circuitry Control
50 kΩ
200
1200
2200
10 k 100 k 1 M 10 M 100 M
f - Frequency - Hz
Output Impedance -
OUTPUT IMPEDANCE (IN SHUTDOWN)
vs
FREQUENCY
Ω
V
CC
= ±5 V,
V
I
= 0.8 V
PP
RMS
PD = V
CC-
THS4140
THS4141
SLOS320F – MAY 2000 – REVISED JANUARY 2006
PRINCIPLES OF OPERATION (continued)
The power-down mode is used when power saving is required. The power-down terminal ( PD) found on the
THS414x 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 33 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 33. 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 34 .
Figure 34.
19
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