OPA830 datasheet(21/37 Pages) NSC

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OPA830

SBOS263F − AUGUST 2004 − REVISED AUGUST 2008

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OPA830

325

Ω

528

Ω

78.7

Ω

650

Ω

+5V

845

Ω

1.87k

Ω

µF

Ω

Video DAC

Ω Load

Figure 5. Video Line Driver with SAG Correction

The input is shifted slightly positive in Figure 5 using the

voltage divider from the positive supply. This gives about

a 200mV input DC offset that will show up at the output pin

as a 400mV DC offset when the DAC output is at zero

current during the sync tip portion of the video signal. This

acts to hold the output in its linear operating region. This

will pass on any power-supply noise to the output with a

gain of approximately −20dB, so good supply decoupling

is recommended on the power-supply pin. Figure 6 shows

the frequency response for the circuit of Figure 5. This plot

shows the 8Hz low-frequency high-pass pole and a

high-end cutoff at approximately 100MHz.

−3

−6

−9

−12

−15

−18

−21

Frequency (Hz)

110

102

103

104

105

106

107

108

109

Figure 6. Video Line Driver Response to Matched

Load

NONINVERTING AMPLIFIER WITH REDUCED

PEAKING

Figure 7 shows a noninverting amplifier that reduces

peaking at low gains. The resistor RC compensates the

OPA830 to have higher Noise Gain (NG), which reduces

the AC response peaking (typically 5dB at G = +1 without

RC) without changing the DC gain. VIN needs to be a low

impedance source, such as an op amp. The resistor

values are low to reduce noise. Using both RT and RF

helps minimize the impact of parasitic impedances.

OPA830

OUT

+5V

Figure 7. Compensated Noninverting Amplifier

The Noise Gain can be calculated as follows:

+ 1 )

)

+ G

A unity-gain buffer can be designed by selecting

RT =RF = 20.0Ω and RC = 40.2Ω (do not use RG). This

gives a noise gain of 2, so the response will be similar to

the Characteristics Plots with G = +2. Decreasing RC to

20.0

Ω will increase the noise gain to 3, which typically

gives a flat frequency response, but with less bandwidth.

The circuit in Figure 1 can be redesigned to have less

peaking by increasing the noise gain to 3. This is

accomplished by adding RC = 2.55kΩ across the op amp

inputs.

(1)

(2)

(3)

OPA830 Datasheet(PDF) 21 Page - National Semiconductor (TI)

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