THS4140 Datasheet, PDF (19/33 Pages) Texas Instruments – HIGH SPEED FULLY DIFFERENTIAL I/O AMPLIFIERS

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THS4140
THS4141
SLOS320F – MAY 2000 – REVISED JANUARY 2006
POWER-DOWN MODE
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 kΩ 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 MΩ in the power-down state.
V_CC
50 kΩ
To Internal Bias
Circuitry Control
PD
V_CC-
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.
2200
OUTPUT IMPEDANCE (IN SHUTDOWN)
vs
FREQUENCY
V_CC = ±5 V,
V_I = 0.8 V_PP RMS
PD = V_CC-
1200
200
10 k
100 k
1M
10 M
f - Frequency - Hz
Figure 34.
100 M
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