OPA698 datasheet(22/27 Pages) TI

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OPA698

SBOS258B

www.ti.com

FIGURE 22. Driving Capacitive Loads.

OPA698

L is optional

THERMAL CONSIDERATIONS

The OPA698 will not require heat sinking under most oper-

ating conditions. Maximum desired junction temperature will

set a maximum allowed internal power dissipation as de-

scribed below. In no case should the maximum junction

temperature be allowed to exceed 150

°C.

The total internal power dissipation (PD) is the sum of

quiescent power (PDQ) and the additional power dissipated in

the output stage (PDL) while delivering load power. PDQ is

simply the specified no-load supply current times the total

supply voltage across the part. PDL depends on the required

output signals and loads. For a grounded resistive load, and

equal bipolar supplies, it is at maximum when the output is

at 1/2 either supply voltage. In this condition, PDL = VS2/(4RL)

where RL includes the feedback network loading. Note that it

is the power in the output stage, and not in the load, that

determines internal power dissipation.

The operating junction temperature is: TJ = TA + PD x θJA,

where TA is the ambient temperature. For example, the

maximum TJ for a OPA698ID with G = +2, RF = 402Ω, RL =

100

Ω, and ±V

S = ±5V at the maximum TA = +85°C is

calculated as:

PmW

C W

=×

(

) =

( )

(

)

=° +

15 5

155

100

804

155

225

125

113

ΩΩ

This would be the maximum TJ from VO = ±2.5VDC. Most

applications will be at a lower output stage power and have

a lower TJ.

CAPACITIVE LOADS

Capacitive loads, such as the input to ADCs, will decrease

the amplifier phase margin, which may cause high-frequency

peaking or oscillations. Capacitive loads

≥ 2pF should be

isolated by connecting a small resistor in series with the

output, as shown in Figure 22. Increasing the gain from +2

will improve the capacitive drive capabilities due to increased

phase margin.

In general, capacitive loads should be minimized for optimum

high-frequency performance. The capacitance of coax cable

(29pF/ft for RG-58) will not load the amplifier when the

coaxial cable, or transmission line, is terminated in its char-

acteristic impedance.

FREQUENCY RESPONSE COMPENSATION

The OPA698 is internally compensated to be unity-gain

stable, and has a nominal phase margin of 60

° at a gain of

+2. Phase margin and peaking improve at higher gains.

Recall that an inverting gain of –1 is equivalent to a gain of

+2 for bandwidth purposes (that is, noise gain = 2). Standard

external compensation techniques work with this device.

For example, in the inverting configuration, the bandwidth

may be limited without modifying the inverting gain by placing

a series RC network to ground on the inverting node. This

has the effect of increasing the noise gain at high frequen-

cies, which limits the bandwidth.

To maintain a wide bandwidth at high gains, cascade several

op amps, or use the high-gain optimized OPA699.

In applications where a large feedback resistor is required,

such as photodiode transimpedance amplifier, the parasitic

capacitance from the inverting input to ground causes peak-

ing or oscillations. To compensate for this effect, connect a

small capacitor in parallel with the feedback resistor. The

bandwidth will be limited by the pole that the feedback

resistor and this capacitor create. In other high-gain applica-

tions, use a three-resistor

Tee network to reduce the RC time

constants set by the parasitic capacitances. Be careful not to

increase the noise generated by this feedback network too

much.

PULSE SETTLING TIME

The OPA698 is capable of an extremely fast settling time in

response to a pulse input. Frequency response flatness and

phase linearity are needed to obtain the best settling times.

For capacitive loads, such as an ADC, use the recom-

mended RS in the typical performance curve RS vs Capaci-

tive Load. Extremely fine-scale settling (0.01%) requires

close attention to ground return current in the supply

decoupling capacitors.

The pulse settling characteristics, when recovering from

overdrive, are very good.

DISTORTION

The OPA698 distortion performance is specified for a 500

Ω

load, such as an ADC. Driving loads with smaller resistance

will increase the distortion, as illustrated in Figure 23. Re-

member to include the feedback network in the load resis-

tance calculations.

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