OPA690 datasheet(29/45 Pages) TI

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DUAL-CHANNEL, DIFFERENTIAL ADC

VIDEO LINE DRIVING

+5V

1/4

OPA4830

1/4

OPA4830

800W

16.7dB

NoiseFigure

Gain=8V/V

18dB

200W

1kW

0.1 F

500W

DualADC

1of2

Channels

1:2

50W

Source

OPA4830

www.ti.com.................................................................................................................................................... SBOS350A – DECEMBER 2006 – REVISED MAY 2008

signal at the transformer input. The two pull-down

DRIVER

resistors do show a signal path ground and should be

connected at the same physical point to ground, in

Where a low-noise, single-supply, interface to a

order to eliminate imbalanced ground return currents

differential input +5V ADC is required, the circuit of

from degrading 2nd-harmonic distortion.

Figure 85 can provide a high dynamic range, medium

gain interface for dual high-performance ADCs. The

circuit of Figure 85 uses two amplifiers in the

differential inverting configuration. The common-mode

Most video distribution systems are designed with

voltage is set on the noninverting inputs to the supply

Ω series resistors to drive a matched 75Ω cable. In

midscale. In this example, the input signal is coupled

order to deliver a net gain of 1 to the 75

Ω matched

in through a 1:2 transformer. This design provides

load, the amplifier is typically set up for a voltage gain

both signal gain, single to differential conversion, and

of +2V/V, compensating for the 6dB attenuation of the

a reduction in noise figure. To show a 50

Ω input

voltage divider formed by the series and shunt 75

Ω

impedance at the input to the transformer, two 200

Ω

resistors at either end of the cable.

resistors are required on the transformer secondary.

The circuit of Figure 72 applies to this requirement if

These two resistors are also the amplifier gain

all references to 50

Ω resistors are replaced by 75Ω

elements. Because the same dc voltage appears on

values. Often, the amplifier gain is further increased

both inverting nodes in the circuit of Figure 85, no dc

to 2.2, which recovers the additional dc loss of a

current will flow through the transformer, giving a dc

typical long cable run. This change would require the

gain of 1 to the output for this common-mode voltage,

gain resistor (RG) in Figure 72 to be reduced from

VCM.

750

Ω to 625Ω. In either case, both the gain flatness

The circuit of Figure 85 is particularly suitable for a

and the differential gain/phase performance of the

moderate resolution dual ADC used as I/Q samplers.

OPA4830

provide

exceptional

results

video

The optional 500

Ω resistors to ground on each

distribution applications. Differential gain and phase

amplifier output can be added to improve the 2nd-

measure the change in overall small-signal gain and

and 3rd-harmonic distortion by >15dB if higher

phase for the color sub-carrier frequency (3.58MHz in

dynamic range is required.

NTSC systems) versus changes in the large-signal

output level (which represents luminance information

The 5mA added output stage current significantly

in a composite video signal). The OPA4830, with the

improves linearity if that is required. The measured

typical 150

Ω load of a single matched video cable,

2nd-harmonic distortion is consistently lower than the

shows less than 0.07%/0.17

° differential gain/phase

3rd-harmonics for this balanced differential design. It

errors over the standard luminance range for a

is particularly helpful for this low-power design if there

positive

video

(negative

sync)

signal.

Similar

are no grounds in the signal path after the low-level

performance would be observed for multiple video

signals (see Figure 86).

Figure 85. Single-Supply Differential ADC Driver (1 of 2 channels)

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OPA690 Datasheet(PDF) 29 Page - Texas Instruments

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