OPA690 datasheet(17/39 Pages) TI1

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402W

OUT

1/2

OPA2690

OPA692

1/2

OPA2690

www.ti.com

SBOS238G – JUNE 2002 – REVISED MARCH 2010

Typically, channel switching is performed either on

Each of the two filter stages is a 1st-order filter with a

sync or retrace time in the video signal. The two

voltage gain of +1. The delay time through one filter

inputs are approximately equal at this time. The

is given by Equation 3:

make-before-break

disable

characteristic

the

tGR0 = 2RC

(3)

OPA2690 ensures that there is always one amplifier

For a more accurate analysis of the circuit, consider

controlling the line when using a wired-OR circuit like

the group delay for the amplifiers. For example, in the

that shown in Figure 41. As both inputs may be on for

case of the OPA2690, the group delay in the

a short period during the transition between channels,

bandwidth from 1MHz to 100MHz is approximately

the

outputs

are

combined

through

the

output

1.0ns. To account for this, modify the transfer

impedance matching resistors (82.5

Ω in this case).

function, which now comes out to be:

When one channel is disabled, its feedback network

tGR = 2(2RC + tD)

(4)

forms part of the output impedance and slightly

attenuates the signal in getting out onto the cable.

Where tD = (1/360) × (df/df) = delay of the op amp

The gain and output matching resistor have been

itself.

slightly increased to get a signal gain of +1 at the

matched load and provide a 75

Ω output impedance to

The values of resistors RF and RG should be equal

the cable. The video multiplexer connection (see

and low to avoid parasitic effects. If the all-pass filter

Figure 41) also ensures that the maximum differential

is designed for very low delay times, include parasitic

voltage across the inputs of the unselected channel

board capacitances to calculate the correct delay

does not exceed the rated ±1.2V maximum for

time. Simulating this application using the PSPICE

standard video signal levels.

model of the OPA2690 will allow this design to be

tuned to the desired performance.

See the Disable Operation section for the turn-on and

turn-off switching glitches using a 0V input for a

DIFFERENTIAL RECEIVER/DRIVER

single channel is typically less than ±50mV. Where

two outputs are switched (see Figure 41), the output

A very versatile application for a dual operational

line is always under the control of one amplifier or the

amplifier is the differential amplifier configuration

other due to the make-before-break disable timing. In

detailed in Figure 43. With both amplifiers of the

this case, the switching glitches for two 0V inputs

OPA2690 connected for noninverting operation, the

drops to < 20mV.

circuit provides a high input impedance whereas the

gain can easily be set by just one resistor, RG. When

HIGH-SPEED DELAY CIRCUIT

operated in low gains, the output swing may be

limited as a result of the common-mode input swing

The OPA2690 makes an ideal amplifier for a variety

limits of the amplifier itself. An interesting modification

of active filter designs. Shown in Figure 42 is a circuit

of this circuit is to place a capacitor in series with the

that uses the two amplifiers within the dual OPA2690

RG. Now the dc gain for each side is reduced to +1,

to design a two-stage analog delay circuit. For

whereas the ac gain still follows the standard transfer

simplicity, the circuit uses a dual-supply (±5V)

function of G = 1 + 2RF/RG. This might be

operation, but it can also be modified to operate on a

advantageous for applications processing only a

signal supply. The input to the first filter stage is

frequency band that excludes dc or very low

driven by the OPA692 wideband buffer amplifier to

frequencies. An input dc voltage resulting from input

isolate the signal input from the filter network.

bias currents is not amplified by the ac gain and can

be kept low. This circuit can be used as a differential

line receiver, driver, or as an interface to a differential

input ADC.

Figure 42. Two-Stage, All-Pass Network

Product Folder Link(s): OPA2690

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