OPA683ID datasheet(17/31 Pages) TI

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OPA683

SBOS221E

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OUTPUT CURRENT AND VOLTAGE

The OPA683 provides output voltage and current capabilities

that can support the needs of driving doubly-terminated 50

Ω

lines. Changing the 1k

Ω load in Figure 1 to a 100Ω will give

a total load that is the parallel combination of the 100

Ω load

and the 2.4k

Ω total feedback network impedance. This 96Ω

load will require no more than 36mA output current to support

±3.5V output voltage swing. This is within the specified

minimum output current of +58mA/–45mA over the full tem-

perature range.

The specifications described above, though familiar in the

industry, consider voltage and current limits separately. In

many applications, it is the voltage • current, or V-I product,

which is more relevant to circuit operation. Refer to the

“Output Voltage and Current Limitations” plot in the Typical

Characteristics. The X and Y axes of this graph show the

zero-voltage output current limit and the zero-current output

voltage limit, respectively. The four quadrants give a more

detailed view of the OPA683’s output drive capabilities.

Superimposing resistor load lines onto the plot shows the

available output voltage and current for specific loads.

The minimum specified output voltage and current over

temperature are set by worst-case simulations at the cold

temperature extreme. Only at cold startup will the output

current and voltage decrease to the numbers shown in the

Electrical Specifications. As the output transistors deliver

power, their junction temperatures will increase, decreasing

their VBE’s (increasing the available output voltage swing)

and increasing their current gains (increasing the available

output current). In steady state operation, the available

output voltage and current will always be greater than that

shown in the over-temperature specifications since the out-

put stage junction temperatures will be higher than the

minimum specified operating ambient.

To maintain maximum output stage linearity, no output short

circuit protection is provided. This will not normally be a

problem since most applications include a series matching

resistor at the output that will limit the internal power dissipa-

tion if the output side of this resistor is shorted to ground.

However, shorting the output pin directly to the adjacent

positive power-supply pin (8-pin packages) will, in most

cases, destroy the amplifier. If additional short-circuit protec-

tion is required, consider a small series resistor in the power-

supply leads. This will, under heavy output loads, reduce the

available output voltage swing. A 5

Ω series resistor in each

power-supply lead will limit the internal power dissipation to

less than 1W for an output short circuit while decreasing the

available output voltage swing only 0.25V for up to 50mA

desired load currents. Always place the 0.1

µF power-supply

decoupling capacitors after these supply current limiting

resistors directly on the supply pins.

DRIVING CAPACITIVE LOADS

One of the most demanding and yet very common load

conditions for an op amp is capacitive loading. Often, the

capacitive load is the input of an ADC—including additional

external capacitance which may be recommended to im-

prove ADC linearity. A high-speed, high open-loop gain

amplifier like the OPA683 can be very susceptible to de-

creased stability and closed-loop response peaking when a

capacitive load is placed directly on the output pin. When the

amplifier’s open-loop output resistance is considered, this

capacitive load introduces an additional pole in the signal

path that can decrease the phase margin. Several external

solutions to this problem have been suggested. When the

primary considerations are frequency response flatness, pulse

response fidelity and/or distortion, the simplest and most

effective solution is to isolate the capacitive load from the

feedback loop by inserting a series isolation resistor between

the amplifier output and the capacitive load. This does not

eliminate the pole from the loop response, but rather shifts it

and adds a zero at a higher frequency. The additional zero

acts to cancel the phase lag from the capacitive load pole,

thus increasing the phase margin and improving stability.

The Typical Characteristics show the recommended “RS vs

Capacitive Load” and the resulting frequency response at the

load. The 1k

Ω resistor shown in parallel with the load

capacitor is a measurement path and may be omitted.

Parasitic capacitive loads greater than 3pF can begin to

degrade the performance of the OPA683. Long PC board

traces, unmatched cables, and connections to multiple de-

vices can easily cause this value to be exceeded. Always

consider this effect carefully, and add the recommended

series resistor as close as possible to the OPA683 output pin

(see Board Layout Guidelines).

DISTORTION PERFORMANCE

The OPA683 provides low distortion in a very low power

amplifier. The CFBplus architecture also gives two significant

areas of distortion improvement. First, in operating regions

where the 2nd-harmonic distortion due to output stage

nonlinearities is very low (frequencies < 1MHz, low output

swings into light loads) the linearization at the inverting node

provided by the CFBplus design gives 2nd-harmonic distor-

tions that extend into the –90dBc region. Previous current

feedback amplifiers have been limited to approximately

–85dBc due to the nonlinearities at the inverting input. The

second area of distortion improvement comes in a distortion

performance that is more gain independent than prior solu-

tions. To the extent that the distortion at a particular output

power is output stage dependent, 2nd-harmonic particularly,

and to a lesser extend 3rd-harmonic distortion, is constant as

the gain is increased. This is due to the constant loop gain

versus signal gain provided by the CFBplus design. As shown

in the Typical Characteristics, while the 2nd-harmonic is

constant with gain, the 3rd-harmonic degrades at higher

gains.

Relative to alternative amplifiers with < 1mA supply current,

the OPA683 holds much lower distortion at higher frequen-

cies (> 5MHz) and to higher gains. Generally, until the

fundamental signal reaches very high frequency or power

levels, the 2nd-harmonic will dominate the distortion with a

lower 3rd-harmonic component. Focusing then on the 2nd-

harmonic, increasing the load impedance improves distortion

slightly for the OPA683. Remember that the total load in-

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