In Figure 3 the input signal is swept between both supply

rails (0V - 2.8V). The linear part of the plot ‘V

covers approximately the voltage range between 1.0V and

2.0V. If a difference of 50mV between output and input is

acceptable, the output range is between 1.05V and 2.15V

(see curve V

swing can be determined by using Figure 4. In the plot ‘Gain

vs. V

from 1.15V till 2.1V. Outside this range the gain differs from

1. This will introduce distortion of the output signal.

Another point is the DC bias voltage necessary to get the

optimum output voltage swing. As discussed above, the

output voltage swing can be 1V

resistors are used, the DC bias will be 1.4V, which is half of

the supply voltage of 2.8V. In this situation the output swing

will exceed the lower limit of 1.15V, so it is necessary to

introduce a small DC offset of 200mV to make use of the full

output swing range of the output stage.

DRIVING RESISTIVE AND CAPACITIVE LOADS

The maximum output current of the LMV115 is about 200µA

which means the output can drive a maximum load of 1V/

200µA = 5k

Ω. Using lower load resistances will exceed the

maximum linear output current. The LMV115 can drive a

small capacitive load, but make sure that every capacitor

directly connected to the output becomes part of the loop of

the buffer and will reduce the gain/phase margin, increasing

the instability at higher capacitive values. This will lead to

peaking in the frequency response and in extreme situations

oscillations can occur. A good practice when driving larger

capacitive loads is to include a series resistor to the load

capacitor. A to D converters present complex and varying

capacitive loads to the buffer. The best value for this isolation

resistance is often found by experimentation.

SHUTDOWN MODE

LMV115 offers a shutdown function that can be used to

disable the device and to optimize current consumption.

Switching between the normal mode and the shutdown

mode requires connecting the shutdown pin either to the

negative or the positive supply rail. If directly connected to

one of the supply rails, the part is guaranteed in the correct

mode. But if the shutdown pin is driven by other output

stages, there is a voltage range in which the installed mode

is not certainly set and it is recommended not to drive the

shutdown pin in this voltage range. As can be seen in Figure

5 this hysteresis varies from 1V to 1.6V. Below 1V the

LMV115 is securely ‘ON’ and above 1.6V securely ‘OFF’

while using a supply voltage of 2.8V.

PRINTED CIRCUIT BOARD LAYOUT AND COMPONENT

VALUES SELECTION

For a good high frequency design both the active parts and

the passive ones should be suitable for the purpose they are

used for. Amplifying high frequencies is possible with stan-

dard through-hole components, but for frequencies above

several hundreds of MHz the best choice is using surface

mount devices. Nowadays designs are often assembled with

surface mount devices for the aspect of minimizing space,

but this also greatly improves the performance of designs,

handling high frequencies. Another important issue is the

PCB, which is no longer a simple carrier for all the parts and

a medium to interconnect them. The board becomes a real

20075133

FIGURE 3. V

20075134

FIGURE 4. Gain

20075135

FIGURE 5. Hysteresis

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