THS4130 Datasheet, PDF (22/27 Pages) Texas Instruments – HIGH SPEED LOW NOISE, FULLY DIFFERENTIAL I/O AMPLIFIERS

THS4130, THS4131
HIGHĆSPEED, LOW NOISE, FULLYĆDIFFERENTIAL I/O AMPLIFIERS
SLOS318E − 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
THS413x 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 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
OUTPUT IMPEDANCE (IN POWER DOWN)
vs
FREQUENCY
VCC = ±5 V
G=1
Rf = 1 kΩ
PD = VCC−
1200
200
100 k
1M
10 M
100 M
1G
f − Frequency − Hz
Figure 42
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