OPA690 datasheet(14/27 Pages) TI

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OPA3692

SBOS228E

www.ti.com

OPERATING SUGGESTIONS

GAIN SETTING

Setting the gain with the OPA3692 is very easy. For a gain

of +2, ground the –IN pin and drive the +IN pin with the

signal. For a gain of +1, leave the –IN pin open and drive the

+IN pin with the signal. For a gain of –1, ground the +IN pin

and drive the –IN pin with the signal. As the internal resistor

values (not their ratios) change significantly over tempera-

ture and process, external resistors should not be used to

modify the gain.

OUTPUT CURRENT AND VOLTAGE

The OPA3692 provides output voltage and current capabili-

ties 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 tested 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 previously, 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 OPA3692 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 OPA3692 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 Electrical Character-

istics.

The minimum specified output voltage and current over-

temperature are set by worst-case simulations at the cold

temperature extreme. Only at cold start-up does the output

current and voltage decrease to the numbers shown in the

Electrical Characteristic tables. As the output transistors

deliver power, their junction temperatures increase, decreas-

ing their VBEs (increasing the available output voltage swing)

and increasing their current gains (increasing the available

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

put voltage and current is always greater than that shown in

the over-temperature specifications because the output stage

junction temperatures are 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 OPA3692. This circuit acts to limit the maxi-

mum source or sink current to approximately 250mA.

DRIVING CAPACITIVE LOADS

One of the most demanding, but 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 that may be recommended to improve

ADC linearity. A high-speed amplifier like the OPA3692 can be

very susceptible to decreased stability and closed-loop re-

sponse peaking when a capacitive load is placed directly on

the output pin. When the amplifier 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 sug-

gested. When the primary considerations are frequency re-

sponse 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 capaci-

tive load pole, thus increasing the phase margin and improv-

ing 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 OPA3692. Long PCB

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 OPA3692 output pin

(see the Board Layout Guidelines section).

DISTORTION PERFORMANCE

The OPA3692 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 dominates the distortion with a negligible 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

RF + RG, whereas in the inverting configuration, it is just RF.

Also, providing an additional supply decoupling capacitor (0.1

µF)

between the supply pins (for bipolar operation) improves the

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

In most op amps, increasing the output voltage swing increases

harmonic distortion directly. The Typical Characteristics 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 only by less than the expected 6dB, whereas the 3rd-

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