1213 datasheet(14/20 Pages) LINEAR

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LT1213/LT1214

APPLICATI

I FOR ATIO

positive rail, is about 100

Ω as the output starts to source

current; this resistance drops to about 20

Ω as the current

increases. Therefore when the output sources 1mA, the

output will swing to within 0.7V of the positive supply.

While sourcing 30mA, it is within 1.25V of the positive

supply.

The output of the LT1213/LT1214 will swing to within 4mV

of the negative supply while sinking zero current. Thus, in

a typical single supply application with the load going to

ground, the output will go to within 4mV of ground. The

open-loop output resistance when the output is driven

hard into the negative rail is about 29

Ω at low currents and

reduces to about 23

Ω at high currents. Therefore when

the output sinks 1mA, the output is about 33mV above the

negative supply and while sinking 30mA, it is about

690mV above it.

The output of the LT1213/LT1214 has reverse-biased

diodes to each supply. If the output is forced beyond either

supply, unlimited currents will flow. If the current is

transient and limited to several hundred mA, no damage

will occur.

Feedback Components

Because the input currents of the LT1213/LT1214 are less

than 200nA, it is possible to use high value feedback

resistors to set the gain. However, care must be taken to

insure that the pole that is formed by the feedback resis-

tors and the input capacitance does not degrade the

stability of the amplifier. For example, if a single supply,

noninverting gain of two is set with two 10k resistors, the

LT1213/LT1214 will probably oscillate. This is because

the amplifier goes open-loop at 6MHz (6dB of gain) and

has 45

° of phase margin. The feedback resistors and the

10pF input capacitance generate a pole at 3MHz that

introduces 63

° of phase shift at 6MHz! The solution is

simple, lower the values of the resistors or add a feedback

capacitor of 10pF or more.

Inputs

Typically at room temperature, the inputs of the LT1213/

LT1214 can common mode 400mV below ground (V –)

and to within 1.2V of the positive supply with the amplifier

still functional. However, the input bias current and offset

voltage will shift as shown in the characteristic curves. For

full precision performance, the common mode range

should be limited between ground (V –) and 1.5V below the

positive supply.

When either of the inputs is taken below ground (V –) by

more than about 700mV, that input current will increase

dramatically. The current is limited by internal 100

Ω

resistors between the input pins and diodes to each

supply. The output will remain low (no phase reversal) for

inputs 1.3V below ground (V –). If the output does not have

to sink current, such as in a single supply system with a 1k

load to ground, there is no phase reversal for inputs up to

8V below ground.

There are no clamps across the inputs of the LT1213/

LT1214 and therefore each input can be forced to any

voltage between the supplies. The input current will re-

main constant at about 100nA over most of this range.

When an input gets closer than 1.5V to the positive supply,

that input current will gradually decrease to zero until the

input goes above the supply, then it will increase due to the

previously mentioned diodes. If the inverting input is held

more positive than the noninverting input by 200mV or

more, while at the same time the noninverting input is

within 300mV of ground (V –), then the supply current will

increase by 2mA and the noninverting input current will

increase to about 10

µA. This should be kept in mind in

comparator applications where the inverting input stays

above ground (V –) and the noninverting input is at or near

ground (V –).

Output

The output of the LT1213/LT1214 will swing to within

0.61V of the positive supply with no load. The open-loop

output resistance, when the output is driven hard into the

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