OPA643 datasheet(10/18 Pages) TI1

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OPA643

A good rule of thumb is to target the parallel combination of

F and RG (Figure 1) to be less than about 200Ω. The

combined impedance RF RG interacts with the inverting

input capacitance, placing an additional pole in the feedback

network and thus a zero in the forward response. Assuming

a 3pF total parasitic on the inverting node, holding R

F RG

< 200

Ω will keep this pole above 250MHz. By itself, this

constraint implies that the feedback resistor R

F can increase

to several k

Ω at high gains. This is acceptable as long as the

pole formed by R

F and any parasitic capacitance appearing

in parallel with it is kept out of the frequency range of

interest. The exception to this is in wideband transimpedance

applications as described earlier. There, a feedback pole is

used to compensate for the zero formed by the input

capacitance and the feedback resistor.

In the inverting configuration, an additional design contraint

must be considered. RG becomes the input resistor and

therefore the load impedance to the driving source. If

impedance matching is desired, RG may be set equal to the

required termination value. However, at low inverting gains,

the resulting feedback resistor value can present a significant

load to the amplifier output. For example, an inverting gain

of –4 (noise gain of 5) with a 50

Ω input matching resistor

(= RG) would require a 200Ω feedback resistor, which would

increase output loading in parallel with the external load. To

decrease the added loading, it would be preferable to increase

both the RF and RG values, and then achieve the input

matching impedance with a third resistor to ground at the

input. The total input impedance becomes the parallel

combination of RG and this additional shunt input resistor.

BANDWIDTH VS GAIN

Voltage feedback op amps exhibit decreasing closed-loop

bandwidth as the signal gain is increased. In theory, this

relationship is described by the Gain Bandwidth Product

(GBP) shown in the Electrical Specifications. Ideally, dividing

GBP by the non-inverting signal gain (also called the noise

gain, or NG) will predict the closed-loop bandwidth. In

practice, this relationship only holds true when the phase

margin approaches 90

°, as it does in high gain configurations.

At low signal gains, most high speed amplifiers will exhibit

a more complex response with lower phase margin. The

OPA643 is optimized to give a maximally flat frequency

response at a gain of +5. Dividing the typical 800MHz gain

bandwidth product by the noise gain of 5 would predict a

closed-loop bandwidth of 160MHz. However, the actual

bandwidth is extended to > 200MHz due to the reduced

(< 90

°) phase margin at this noise gain. Increasing the gain

will increase the phase margin moving the closed-loop

bandwidth closer to that predicted by the gain bandwidth

product. The 40MHz bandwidth at a gain of +20, shown in

the Electrical Specifications, agrees with that predicted using

the 800MHz GBP.

LOW GAIN OPERATION

Decreasing the operating gain for the OPA643 from the

nominal design point of +5 will decrease the phase margin.

required 0.8pF value shown in Figure 3 to get the required

feedback pole.

This will set the –3dB bandwidth according to:

F–3dB ≅ √(GBP/2πRFCS) Hz

The example of Figure 3 will give approximately 23MHz

flat bandwidth using the 0.8pF feedback compensation.

WIDEBAND INVERTING SUMMING AMPLIFIER

One common application for a wideband op amp like the

OPA643 is to sum a number of signal sources together.

Figure 4 shows the inverting summing configuration that is

most often used. This circuit offers the benefit that each

input sees an input impedance set only by its individual input

resistor, since the summing junction (inverting op amp

node) is a virtual ground. Each input is non-interactive with

every other. However, the bandwidth from any input to the

summed output is set by the op amp noise gain (NG), equal

to the non-inverting voltage gain. So, even though each

inverting channel may have a low gain to the output (like the

–1 shown in Figure 4), the overall noise gain will set the

frequency response and the loop stability. The non-inverting

gain for Figure 4 is equal to +5 which will give a 200MHz

bandwidth at a gain of –1 for each of the input signals.

OPERATING SUGGESTIONS

OPTIMIZING RESISTOR VALUES

Since the OPA643 is a voltage feedback op amp, a wide

range of resistor values may be used for the feedback and

gain setting resistors (RF and RG in Figure 1). The primary

limits to these values are set by dynamic range (noise and

distortion) and parasitic capacitive considerations. Usually,

the feedback resistor value should be between 200

Ω and

Ω. Below 200Ω, the feedback network will present

additional output loading which can degrade the harmonic

distortion performance of the OPA643. Above 1k

Ω, the

typical parasitic capacitance (approximately 0.2pF) across

the feedback resistor may cause unintentional band-limiting

in the amplifier response.

FIGURE 4. Wideband Inverting Summing Amplifier.

402

Ω

Supply Decoupling

Not Shown

81.6

Ω

0.1µF

402

Ω

OPA643

+5V

–5V

O = – (V1 + V2 + V3 + V4)

402

Ω

402

Ω

402

Ω

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