OPA690IDBVT datasheet(15/22 Pages) TI

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OPA690

SBOS223A

www.ti.com

INVERTING AMPLIFIER OPERATION

Since the OPA690 is a general-purpose, wideband voltage

feedback op amp, all of the familiar op amp application

circuits are available to the designer. Inverting operation is

one of the more common requirements and offers several

performance benefits. Figure 8 shows a typical inverting

configuration where the I/O impedances and signal gain from

Figure 1 are retained in an inverting circuit configuration.

FIGURE 8. Gain of –2 Example Circuit.

The second major consideration, touched on in the previous

paragraph, is that the signal source impedance becomes

part of the noise gain equation and hence influences the

bandwidth. For the example in Figure 8, the RM value

combines in parallel with the external 50

Ω source imped-

ance, yielding an effective driving impedance of 50

Ω || 67Ω

= 28.6

Ω. This impedance is added in series with R

G for

calculating the noise gain (NG). The resultant NG is 2.8 for

Figure 8, as opposed to only 2 if RM could be eliminated as

discussed above. The bandwidth will therefore be slightly

lower for the gain of –2 circuit of Figure 8 than for the gain

of +2 circuit of Figure 1.

The third important consideration in inverting amplifier design

is setting the bias current cancellation resistor on the

noninverting input (RB). If this resistor is set equal to the total

DC resistance looking out of the inverting node, the output

DC error, due to the input bias currents, will be reduced to

(Input Offset Current) • RF. If the 50Ω source impedance is

DC-coupled in Figure 8, the total resistance to ground on the

inverting input will be 228

Ω. Combining this in parallel with

the feedback resistor gives the RB = 146Ω used in this

example. To reduce the additional high frequency noise

introduced by this resistor, it is sometimes bypassed with a

capacitor. As long as RB < 350Ω, the capacitor is not required

since the total noise contribution of all other terms will be less

than that of the op amp’s input noise voltage. As a minimum,

the OPA690 requires an RB value of 50Ω to damp out

parasitic-induced peaking—a direct short to ground on the

noninverting input runs the risk of a very high frequency

instability in the input stage.

OUTPUT CURRENT AND VOLTAGE

The OPA690 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 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 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 OPA690’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 OPA690 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.

OPA690

Ω

402

Ω

200

Ω

146

Ω

Source

DIS

+5V

–5V

Ω

0.1

µF

6.8

µF

0.1

µF

0.1

µF

6.8

µF

Ω Load

In the inverting configuration, three key design consider-

ations must be noted. The first is that the gain resistor (RG)

becomes part of the signal channel input impedance. If input

impedance matching is desired (which is beneficial when-

ever the signal is coupled through a cable, twisted-pair, long

PC board trace, or other transmission line conductor), RG

may be set equal to the required termination value and RF

adjusted to give the desired gain. This is the simplest

approach and results in optimum bandwidth and noise per-

formance. However, at low inverting gains, the resultant

feedback resistor value can present a significant load to the

amplifier output. For an inverting gain of 2, setting RG to 50Ω

for input matching eliminates the need for RM but requires a

100

Ω feedback resistor. This has the interesting advantage

that the noise gain becomes equal to 2 for a 50

Ω source

impedance—the same as the noninverting circuits consid-

ered above. However, the amplifier output will now see the

100

Ω feedback resistor in parallel with the external load. In

general, the feedback resistor should be limited to the 200

Ω

to 1.5k

Ω range. In this case, it is preferable to increase both

the RF and RG values, as shown in Figure 8, and then

achieve the input matching impedance with a third resistor

(RM) to ground. The total input impedance becomes the

parallel combination of RG and RM.

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