THS4131 Datasheet, PDF (21/37 Pages) National Semiconductor (TI) – HIGH-SPEED, LOW-NOISE, FULLY-DIFFERENTIAL I/O AMPLIFIERS

THS4130
THS4131
www.ti.com
SLOS318H – MAY 2000 – REVISED MAY 2011
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 always stays 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 41 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 41. 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 42.
2200
V_CC = ±5 V
G=1
R_f = 1 kΩ
PD = V_CC−
1200
200
100 k
1M
10 M
100 M
1G
f − Frequency − Hz
Figure 42. Output Impedance (In Power-Down) vs Frequency
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