THS7001, THS7002

70-MHz PROGRAMMABLE-GAIN AMPLIFIERS

SLOS214B – OCTOBER 1998 – REVISED AUGUST 1999

23

POST OFFICE BOX 655303

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-k

Ω 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

–VCC

DGND

DGND

53 k

Ω

Gain

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

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

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.