OPA690 datasheet(20/33 Pages) TI

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OPA2822

SBOS188E

www.ti.com

Inverting operation offers some interesting opportunities to

increase the available signal bandwidth. When the source

impedance is matched by the gain resistor (Figure 10 for

example), the signal gain is (1 + RF/RG) while the noise gain

is (1 + RF/2RG). This reduces the noise gain almost by half,

extending the signal bandwidth and increasing the loop gain.

For instance, setting RF = 500Ω in Figure 10 will give a signal

gain for the amplifier of 5V/V. However, including the 50

Ω

source impedance reflected through the 1:2 transformer will

give an additional 100

Ω source impedance for the noise gain

analysis for each of the amplifiers. This reduces the noise gain

to 1 + 500

Ω/200Ω = 3.5V/V and results in an amplifier

bandwidth of at least 240MHz/3.5 = 68MHz.

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 OPA2822 can be very susceptible to de-

creased stability and closed-loop frequency response peak-

ing 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 sug-

gested. When the primary considerations are frequency

response flatness with low noise and 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

instead 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. For the OPA2822 operating at a gain of +2, the

frequency response at the output pin is already slightly

peaked without the capacitive load, requiring relatively high

values of RS to flatten the response at the load. One way to

reduce the required RS value is to use the noise gain

adjustment circuit of Figure 12.

The resistor across the two inputs, RNG, can be used to

increase the noise gain while retaining the desired signal

gain. This can be used either to improve flatness at low gains

or to reduce the required value of RS in capacitive load

driving applications. This circuit was used with RNG adjusted

to produce the gain flatness curve in the Typical Character-

istics. As shown in that curve, an RNG of 452Ω will give an NG

of 3 giving exceptional frequency response flatness at a

signal gain of +2. Equation 4 shows the calculation for RNG

given a target noise gain (NG) and signal gain (G):

RR G

NG G

−

(4)

where RS = Total Source Impedance on the Noninverting

Input [25

Ω in Figure 12]

G = Signal Gain [1 + (RF/RG)]

NG = Noise Gain Target

Using this technique to get initial frequency response flat-

ness will significantly reduce the required series resistor

value to get a flat response at the capacitive load. Using the

best-case noise gain of 3 with a signal gain of 2 allows the

required RS to be reduced, as shown in Figure 13. Here, the

required RS versus Capacitive Load is replotted along with

data from the Typical Characteristics. This demonstrates that

the use of RNG = 452Ω across the inputs results in much

lower required RS values to achieve a flat response.

FIGURE 12. Noise Gain Tuning for Noninverting Circuit.

1/2

OPA2822

Ω

402

Ω

402

Ω

Ω Source

FIGURE 13. Required RS vs Noise Gain.

100

Capacitive Load (pF)

100

1000

NG = 3, R

NG = 452Ω

NG = 2, R

NG = ∞

DISTORTION PERFORMANCE

The OPA2822 is capable of delivering exceptionally low

distortion through approximately 5MHz signal frequency.

While principally intended to provide very low noise and

distortion through the maximum ADSL frequency of 1.1MHz,

the OPA2822 in a differential configuration can deliver lower

than –85dBc distortions for a 4VPP swing through 5MHz. For

applications requiring extremely low distortion through higher

frequencies, consider higher slew rate amplifiers such as the

OPA687 or OPA2681.

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