THS4150 Datasheet, PDF (22/27 Pages) Texas Instruments – HIGH-SPEED DIFFERENTIAL I/O AMPLIFIERS

THS4150, THS4151
HIGHĆSPEED DIFFERENTIAL I/O AMPLIFIERS
SLOS321D − MAY 2000 − REVISED JANUARY 2004
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
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 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. The following graph 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.
VCC
50 kΩ
To Internal Bias
Circuitry Control
PD
VCC−
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.
1000
OUTPUT IMPEDANCE (SHUTDOWN)
vs
FREQUENCY
VCC = ±5 V
Rf = R(g) = 500 Ω
100
10
100 k
1M
10 M
100 M
1G
f Frequency − Hz
Figure 43
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