OPA690 datasheet(17/23 Pages) TI

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OPA691

SBOS226A

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on the noninverting input to achieve bias current error can-

cellation at the output. The input bias currents for a current

feedback op amp are not generally matched in either magni-

tude or polarity. Connecting a resistor to ground on the

noninverting input of the OPA691 in the circuit of Figure 9 will

actually provide additional gain for that input’s bias and noise

currents, but will not decrease the output DC error since the

input bias currents are not matched.

OUTPUT CURRENT AND VOLTAGE

The OPA691 provides output voltage and current capabilities

that are unsurpassed in a low-cost monolithic op amp. Under

no-load conditions at 25

°C, the output voltage typically swings

closer than 1V to either supply rail; the +25

°C swing limit is

within 1.2V of either rail. Into a 15

Ω load (the minimum tested

load), it is tested to deliver more than

±160mA.

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 OPA691’s output drive capabilities,

noting that the graph is bounded by a “Safe Operating Area”

of 1W maximum internal power dissipation. Superimposing

resistor load lines onto the plot shows that the OPA691 can

drive

±2.5V into 25Ω or ±3.5V into 50Ω without exceeding the

output capabilities or the 1W dissipation limit. A 100

Ω load

line (the standard test circuit load) shows the full

±3.9V

output swing capability, as shown in the Typical Specifica-

tions.

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 protect the output stage from accidental shorts to ground

and the power supplies, output short-circuit protection is

included in the OPA691. The circuit acts to limit the maximum

source or sink current to approximately 250mA.

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 OPA691 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 ver-

sus Capacitive Load and the resulting frequency response at

the load. Parasitic capacitive loads greater than 2pF can

begin to degrade the performance of the OPA691. Long PC

board traces, unmatched cables, and connections 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 OPA691 output pin

(see Board Layout Guidelines).

DISTORTION PERFORMANCE

The OPA691 provides good distortion performance into a

100

Ω load on ±5V supplies. Relative to alternative solutions,

it provides exceptional performance into lighter loads and/or

operating on a single +5V supply. Generally, until the funda-

mental signal reaches very high frequency or power levels,

the 2nd-harmonic will dominate the distortion with a negli-

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

harmonic, increasing the load impedance improves distortion

directly. Remember that the total load includes the feedback

network—in the noninverting configuration (see Figure 1) this

is the sum of RF + RG, while in the inverting configuration it

is just RF. Also, providing an additional supply decoupling

capacitor (0.1

µF) between the supply pins (for bipolar opera-

tion) improves the 2nd-order distortion slightly (3dB to 6dB).

In most op amps, increasing the output voltage swing in-

creases harmonic distortion directly. The Typical Character-

istics show the 2nd-harmonic increasing at a little less than

the expected 2x rate while the 3rd-harmonic increases at a

little less than the expected 3x rate. Where the test power

doubles, the 2nd-harmonic increases by less than the ex-

pected 6dB while the 3rd-harmonic increases by less than

the expected 12dB. This also shows up in the 2-tone,

3rd-order intermodulation spurious (IM3) response curves.

The 3rd-order spurious levels are extremely low at low output

power levels. The output stage continues to hold them low

even as the fundamental power reaches very high levels. As

the Typical Characteristics show, the spurious intermodulation

powers do not increase as predicted by a traditional intercept

model. As the fundamental power level increases, the

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