OPA690 datasheet(17/33 Pages) TI2

17 / 33 page

OPA699

SBOS261D

www.ti.com

To choose the values for both CS and CF, two parameters and

only three equations need to be solved. The first parameter is

the target high-frequency noise gain (NG2), which should be

greater than the minimum stable gain for the OPA699. Here,

a target of NG2 = 26 is used. The second parameter is the

desired low-frequency signal gain, which also sets the low-

frequency noise gain (NG1). To simplify this discussion, we will

target a maximally flat 2nd-order low-pass Butterworth fre-

quency response (Q = 0.707). The signal gain shown in Figure

5 sets the low-frequency noise gain to NG1 = 1 + RF/RG (= 2

in this example). Then, using only these two gains and the

gain bandwidth product for the OPA699 (1000MHz), the key

frequency in the compensation is set by Equation1.

GBP

O =−









−−













11 2

(1)

Physically, this ZO (22.3MHz for the values shown above) is

set by 1/(2

πR

F(CF + CS)) and is the frequency at which the

rising portion of the noise gain would intersect the unity gain

if projected back to a 0dB gain. The actual zero in the noise

gain occurs at NG1 • ZO and the pole in the noise gain occurs

at NG2 • ZO. That pole is physically set by 1/(RFCF). Since

GBP is expressed in Hz, multiply ZO by 2π and use to get CF

by solving Equation 2.

RZ NG

(

)

(2)

Finally, since CS and CF set the high-frequency noise gain,

determine CS using Equation 3 (solving for CS by using

NG2 = 6):

CNG

=−

(

)

(3)

which gives CS = 15pF.

Both of these calculated values have been reduced slightly

in Figure 5 to account for parasitics. The resulting closed-

loop bandwidth is approximately equal to Equation 4.

GBP

–3

≅

•

(4)

For the values shown in Figure 5, f–3dB is approximately

149MHz. This is less than that predicted by simply dividing

the Gain Bandwidth Product (GBP) product by NG1. The

compensation network controls the bandwidth to a lower

value, while providing the full slew rate at the output and an

improved distortion performance due to increased loop gain

at frequencies below NG1 • ZO.

LOW DISTORTION, LIMITED OUTPUT,

ADC INPUT DRIVER

Figure 6 shows a simple ADC driver that operates on a single

supply, and gives excellent distortion performance. The limit

voltages track the input range of the converter, completely

protecting against input overdrive. Note that the limiting

voltages have been set 100mV above/below the correspond-

ing reference voltage from the converter. This circuit also

implements an improved distortion for an inverting gain of

–2 using external compensation.

OPA699

S = +5V

+3.5V

+1.5V

REFB

REFT

0.1

µF

100pF

H = +3.6V

L = +1.4V

0.1

µF

0.1

µF

18pF

1000pF

4pF

750

Ω

24.9

Ω

374

Ω

562

Ω

102

Ω

1.4k

Ω

1.4k

Ω

102

Ω

562

Ω

ADS822

10-Bit

40MSPS

10-Bit

Data

S = +5V

INT/EXT

RSEL

GND

FIGURE 6. Single Supply, Limiting ADC Input Driver.

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