8

LT1395/LT1396/LT1397

Capacitance on the Inverting Input

Current feedback amplifiers require resistive feedback

from the output to the inverting input for stable operation.

Take care to minimize the stray capacitance between the

output and the inverting input. Capacitance on the invert-

ing input to ground will cause peaking in the frequency

response (and overshoot in the transient response).

Capacitive Loads

The LT1395/LT1396/LT1397 can drive many capacitive

loads directly when the proper value of feedback resistor

is used. The required value for the feedback resistor will

increase as load capacitance increases and as closed-loop

gain decreases. 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 capaci-

tive load is present. The disadvantage is that the gain is a

function of the load resistance. See the Typical Perfor-

mance Characteristics curves.

Power Supplies

The LT1395/LT1396/LT1397 will operate from single or

split supplies from

±2V (4V total) to ±6V (12V total). It

is not necessary to use equal value split supplies, how-

ever the offset voltage and inverting input bias current

will change. The offset voltage changes about 2.5mV per

volt of supply mismatch. The inverting bias current will

typically change about 10

µA per 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 LT1395/LT1396/LT1397 is

approximately 600V/

µsandissetbyinternalcurrentsand

capacitances. The output slew rate is set by the value of

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the feedback resistor and internal capacitance. At a gain

of 2 with 255

Ω feedback and gain resistors and ±5V

supplies, the output slew rate is typically 800V/

µs. Larger

feedback resistors will reduce the slew rate as will lower

supply voltages.

Differential Input Signal Swing

To avoid any breakdown condition on the input transis-

tors, the differential input swing must be limited to

±5V. In

normal operation, the differential voltage between the

input pins is small, so the

±5V limit is not an issue.

Buffered RGB to Color-Difference Matrix

An LT1397 can be used to create buffered color-differ-

ence signals from RGB inputs (Figure 1). In this applica-

tion, the R input arrives via 75

Ω coax. It is routed to the

noninverting input of LT1397 amplifier A1 and to a 845

Ω

resistor R8. There is also an 82.5

Ω termination resistor

R11, which yields a 75

Ω input impedance at the R input

when considered in parallel with R8. R8 connects to the

inverting input of a second LT1397 amplifier (A2), which

also sums the weighted G and B inputs to create a

–0.5 • Y output. LT1397 amplifier A3 then takes the

–0.5 • Y output and amplifies it by a gain of –2, resulting

in the Y output. Amplifier A1 is configured in a noninvert-

ing gain of 2 with the bottom of the gain resistor R2 tied

to the Y output. The output of amplifier A1 thus results in

the color-difference output R-Y.

The B input is similar to the R input. It arrives via 75

Ω

coax, and is routed to the noninverting input of LT1397

amplifier A4, and to a 2320

Ω resistor R10. There is also

a 76.8

Ω termination resistor R13, which yields a 75Ω

input impedance when considered in parallel with R10.

R10 also connects to the inverting input of amplifier A2,

adding the B contribution to the Y signal as discussed

above. Amplifier A4 is configured in a noninverting gain

of 2 configuration with the bottom of the gain resistor R4

tied to the Y output. The output of amplifier A4 thus

results in the color-difference output B-Y.

The G input also arrives via 75

Ω coax and adds its

contribution to the Y signal via a 432

Ω resistor R9, which

is tied to the inverting input of amplifier A2. There is also

a 90.9

Ω termination resistor R12, which yields a 75Ω