Datasheet

 
  ±     
SLOS264G − MARCH 2000 − REVISED DECEMBER 2001
27
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
shutdown control
The THS6043 is essentially the same amplifier as the THS6042. The only difference is the added flexibility of
a shutdown circuit. When the shutdown pin signal is low, the THS6043 is active. But, when a shutdown pin is
high (≥2 V), the THS6043 is turned off. The shutdown logic is not latched and should always have a signal
applied to it. To help ensure a fixed logic state, an internal 50 kΩ resistor to GND is utilized. An external resistor,
such as a 3.3 kΩ, to GND may be added to help improve noise immunity within harsh environments. If no
external resistor is utilized and SHDN pin is left unconnected, the THS6043 defaults to a power-on state. A
simplified circuit can be seen in Figure 47.
+V
CC
To Internal
Bias Circuitry
Control
−V
CC
GND
GND
50 kΩ
SHDN
Figure 47. Simplified THS6043 Shutdown Control Circuit
One aspect of the shutdown feature, which is often over-looked, is that the amplifier does not have a large output
impedance while in shutdown mode. This is due to the R
F
and R
G
resistors. This effect is true for any amplifier
connected as an amplifier with gains >1. The internal circuitry may be powered down and in a high-impedance
state, but the resistors are always there. This allows the signal to flow through these resistors and into the ground
connection. Figure 48 shows the results of the output impedance with no feedback resistor and a typically
configured amplifier.
0.01
0.1
1
10
100
1000
10 K 100 K
1 M
10 M 100 M
1 G
Shutdown Mode Impedance − k
f − Frequency − Hz
Ω
Open Loop
V
CC
= ±5 V to ±15 V
Gain = −8
R
F
= 750 Ω
Figure 48. Output Impedance In Shutdown Mode