THS7001_16 Datasheet, PDF (23/41 Pages) Texas Instruments – 70-MHz PROGRAMMABLE-GAIN AMPLIFIERS

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THS7001, THS7002
70ĆMHz PROGRAMMABLEĆGAIN AMPLIFIERS
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SLOS214C − OCTOBER 1998 − REVISED MARCH 2007
APPLICATION INFORMATION
shutdown control
There are two shutdown pins which control the shutdown for each half of the THS7002 and one shutdown pin
for the THS7001. When the shutdown pins signals are low, the THS7001 and THS7002 is active. But, when a
shutdown pin is high (+5 V), a preamplifier and the respective PGA section is turned off. Just like the Gain
controls, the shutdown logic is not latched and should always have a signal applied to them. A 3.3-kΩ resistor
to ground is usually applied to ensure a fixed logic state. A simplified circuit can be seen in Figure 60.
+VCC
To Internal
Bias Circuitry
Control
Gain
53 kΩ
DGND
−VCC
DGND
Figure 60. Simplified THS7001 and THS7002 Shutdown Circuit
One aspect of the shutdown feature, which is often over-looked, is that the PGA section will still have an output
while in shutdown mode. This is due to the internally fixed R_F and R_G resistors. This effect is true for any amplifier
connected as an inverter. The internal circuitry may be powered down and in a high-impedance state, but the
resistors are always there. This will then allow the input signal current to flow through these resistors and into
the output. The equivalent resistance of R_F and R_G is approximately 3 kΩ. To minimize this effect, a shunt
resistor to ground may be utilized, This will act as a classic voltage divider and will reduce the feed-through
voltage seen at the PGA output. The drawback to this is the increased load on the PGA while in the active state.
driving a capacitive load
Driving capacitive loads with high-performance amplifiers is not a problem as long as certain precautions are
taken. The first is to realize that the THS7001 and THS7002 has been internally compensated to maximize its
bandwidth and slew rate performance. When an amplifier is compensated in this manner, capacitive loading
directly on the output will decrease the device’s phase margin leading to high frequency ringing or oscillations.
Therefore, for capacitive loads of greater than 10 pF, it is recommended that a resistor be placed in series with
the output of the amplifier, as shown in Figure 61. A minimum value of 20 Ω should work well for most
applications. For example, in 75-Ω transmission systems, setting the series resistor value to 75 Ω both isolates
any capacitance loading and provides the proper line impedance matching at the source end.
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