As the load resistance decreases, the useful input range

is effectively limited by the output drive capability, since

the buffers have a fixed voltage gain of +2V/V or -1V/V.

For example, a 50

Ω load can typically be driven from

when operating from a single +5V supply. If the buffer

is operated in the noninverting, gain of +2V/V configu-

ration with the inverting input grounded, the useful input

voltage range becomes 20mV to 1.7V instead of the

-100mV to 2.75V indicated by the

Electrical Charac

teristics. Beyond the useful input range, the buffer out-

put is a nonlinear function of the input, but it will not

undergo phase reversal or latchup.

Enable

The MAX4215 and MAX4219 have an enable feature

(EN_) that allows the buffer to be placed in a low-power

state. When the buffers are disabled, the supply current

is reduced to 400µA per buffer.

As the voltage at the EN_ pin approaches the negative

supply rail, the EN_ input current rises. Figure 2 shows

a graph of EN_ input current versus EN_ pin voltage.

Figure 3 shows the addition of an optional resistor in

series with the EN pin, to limit the magnitude of the cur-

rent increase. Figure 4 displays the resulting EN pin

input current to voltage relationship.

Disabled Output Resistance

The MAX4214/MAX4215/MAX4217/MAX4219/MAX4222

include internal protection circuitry that prevents dam-

age to the precision input stage from large differential

input voltages (Figure 5). This protection circuitry con-

sists of five back-to-back Schottky diodes between

IN_+ and IN_-. These diodes reduce the disabled out-

put resistance from 1k

Ω to 500Ω when the output volt-

age is 3V greater or less than the voltage at IN_+.

Under these conditions, the input protection diodes will

be forward biased, lowering the disabled output resis-

tance to 500

Ω.

Output Capacitive Loading and Stability

The MAX4214 family provides maximum AC perfor-

mance with no load capacitance. This is the case when

the load is a properly terminated transmission line.

These devices are designed to drive up to 20pF of load

capacitance without oscillating, but AC performance

will be reduced under these conditions.

High-Speed, Single-Supply, Gain of +2,

Closed-Loop, Rail-to-Rail Buffers with Enable

10

______________________________________________________________________________________

20

-160

0

100

300

500

-100

-120

0

200

400

-60

-140

-20

-40

-80

Figure 2. Enable Logic-Low Input Current vs. Enable Logic-

Low Threshold

OUT

IN-

EN_

IN+

10k

ENABLE

500

Ω

500

Ω

MAX42_ _

Figure 3. Circuit to Reduce Enable Logic-Low Input Current

0

-10

0

100

300

500

-7

-8

-1

200

400

-3

-5

-9

-2

-4

-6

Figure 4. Enable Logic-Low Input Current vs. Enable Logic-

Low Threshold with 10k

Ω Series Resistor