OPA684 datasheet(21/35 Pages) TI2

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OPA2684

SBOS239D

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the 1.6k

Ω total feedback network impedance. This 94Ω load

will require no more than 40mA output current to support the

±3.8V minimum output voltage swing specified for 100Ω

loads. This is well under the specified minimum +120/–90mA

specifications over the full temperature 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 OPA2684’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 Characteristic tables. As the output transistors

deliver power, their junction temperatures will increase, de-

creasing their VBE’s (increasing the available output voltage

swing) and increasing their current gains (increasing the

available output current). In steady-state operation, the avail-

able output voltage and current will always be greater than

that shown in the over-temperature specifications since the

output 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) can destroy the

amplifier. If additional short-circuit protection 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 OPA2684 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

CLOAD and the resulting frequency response at the load. The

Ω resistor shown in parallel with the load capacitor is a

measurement path and may be omitted. The required series

resistor value may be reduced by increasing the feedback

resistor value from its nominal recommended value. This will

increase the phase margin for the loop gain, allowing a lower

series resistor to be effective in reducing the peaking due

capacitive load. SPICE simulation can be effectively used to

optimize this approach. Parasitic capacitive loads greater

than 5pF can begin to degrade the performance of the

OPA2684. Long PCB traces, unmatched cables, and con-

nections to multiple devices 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

OPA2684 output pin (see Board Layout Guidelines).

DISTORTION PERFORMANCE

The OPA2684 provides very low distortion in a low-power

part. 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 largely gain independent. To the extent

that the distortion at a particular output power is output stage

dependent, 3rd-harmonics particularly, and to a lesser ex-

tend 2nd-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 3rd-harmonic is constant

with gain, the 2nd-harmonic degrades at higher gains. This

is largely due to board parasitic issues. Slightly imbalanced

load return currents will couple into the gain resistor to cause

a portion of the 2nd-harmonic distortion. At high gains, this

imbalance has more gain to the output giving increased

2nd-harmonic distortion.

Relative to alternative amplifiers with < 2mA supply current,

the OPA2684 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

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