13

LT6210/LT6211

62101f

Capacitive Loads

The LT6210/LT6211 are stable with any capacitive load.

Although peaking and overshoot may result in the AC

transient response, the amplifier’s compensation decreases

bandwidth with increasing output capacitive load to en-

sure stability. To maintain a response with minimal peak-

ing, the feedback resistor can be increased at the cost of

bandwidth as shown in the Typical Performance Charac-

teristics. Alternatively, a small resistor (5

Ω to 35Ω) can be

put in series with the output to isolate the capacitive load

from the amplifier output. This has the advantage that the

amplifier bandwidth is only reduced when the capacitive

load is present. The disadvantage of this technique is that

the gain is a function of the load resistance.

Power Supplies

The LT6210/LT6211 will operate on single supplies from

3V to 12V and on split supplies from

±1.5V to ±6V. If split

supplies of unequal absolute value are used, input offset

voltage and inverting input current will shift from the

values specified in the Electrical Characteristics table.

Input offset voltage will shift 2mV and inverting input

current will shift 0.5

µA for each volt of supply mismatch.

Slew Rate

Unlike a traditional voltage feedback op amp, the slew rate

of a current feedback amplifier is not independent of the

amplifier gain configuration. In a current feedback ampli-

fier, both the input stage and the output stage have slew

rate limitations. In the inverting mode, and for gains of 2

or more in the noninverting mode, the signal amplitude

between the input pins is small and the overall slew rate is

that of the output stage. For gains less than 2 in the

noninverting mode, the overall slew rate is limited by the

input stage. The input slew rate of the LT6210/LT6211 on

approximately 600V/

µs and is set by internal currents and

capacitances. The output slew rate is additionally con-

strained by the value of the feedback resistor and internal

capacitance. At a gain of 2 with 887

Ω feedback and gain

resistors,

±5Vsuppliesandthesamebiasingasabove,the

output slew rate is typically 700V/

µs. Larger feedback

resistors, lower supply voltages and lower supply current

levels will all reduce slew rate. Input slew rates signifi-

cantly exceeding the output slew capability can actually

decrease slew performance in a positive gain configura-

tion; the cleanest transient response will be obtained from

input signals with slew rates slower than 1000V/

µs.

Output Swing and Drive

The output stage of the LT6210/LT6211 consists of a pair

of class-AB biased common emitters that enable the

output to swing rail-to-rail. Since the amplifiers can poten-

tially deliver output currents well beyond the specified

minimum short-circuit current, care should be taken not

to short the output of the device indefinitely. Attention

must be paid to keep the junction temperature of the IC

below the absolute maximum rating of 150

°C if the output

is used to drive low impedance loads. See Note 5 for

details. Additionally, the output of the amplifier has re-

verse-biased ESD diodes connected to each supply. If the

output is forced beyond either supply, large currents will

flow through these diodes. If the current is limited to 80mA

or less, no damage to the part will occur.

APPLICATIO S I FOR ATIO

TYPICAL APPLICATIO

3V Cable Driver with Active Termination

Driving back-terminated cables on single supplies usually

results in very limited signal amplitude at the receiving end

of the cable. However, positive feedback can be used to

reduce the size of the series back termination resistor,

thereby decreasing the attenuation between the series and

shunt termination resistors while still maintaining con-

trolled output impedance from the line-driving amplifier.

Figure 3 shows the LT6210 using this “active termination”

scheme on a single 3V supply. The amplifier is AC-coupled

and in an inverting gain configuration to maximize the

looking into the amplifier circuit from the cable is 50

Ω

throughout the usable bandwidth.