OPA682 datasheet(12/22 Pages) TI1

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OPA2681

SBOS091A

SINGLE SUPPLY DIFFERENTIAL A/D

CONVERTER DRIVER

Figure 3 shows a gain of +10 Diff. In/Diff. Out single supply

ADC driver. Using a dual amplifier like the OPA2681 helps

reducing the necessary board space, as it also reduces the

amount of required supply bypassing components. From a

signal point of view, dual amplifiers provide excellent per-

formance matching, e.g., gain and phase matching. The

differential ADC driver circuit shown in Figure 3 takes

advantage of this fact. A transformer converts the single-

ended input signal into a low level differential signal which

is applied to the high impedance non-inverting inputs of

each of the two amplifiers in the OPA2681. Resistor RG

between the inverting inputs controls the ac-gain of this

circuit according to equation G = 1 + 2RF/RG. With the

resistor values shown the AC-gain is set to 10. Adding a

capacitor (0.1

µF) in series with R

G blocks the dc-path giving

a DC gain of +1 for the common-mode voltage. This allows,

in a very simple way, to apply the required DC bias voltage

of +2.5V to the inputs of the amplifiers, which will also

appear at their outputs. Like the OPA2681 the A/D converter

ADS823 operates on a single +5V supply. Its internal

common-mode voltage is typically +2.5V which equals the

required bias voltage for the OPA2681. Connecting two

resistors between the top-reference (REFT = +3.5V) and

bottom reference (REFB = +1.5V) develop a +2.5V voltage

level at their midpoint. Applying that to the center tap of the

transformer biases amplifiers appropriately. Sufficient by-

passing at the center tap must be provided to keep this point

at a solid AC ground. Resistors RS isolate the op amp output

from the capacitive input of the converter, as well as forming

a first order low-pass filter with capacitor CI to attenuate

some of the wideband noise. This interface will provide

> 150MHz full scale input bandwidth to the ADS823.

WIDEBAND VIDEO MULTIPLEXING

One common application for video speed amplifiers which

include a disable pin is to wire multiple amplifier outputs

together, then select which one of several possible video

inputs to source onto a single line. This simple “Wired-OR

Video Multiplexer” can be easily implemented using the

OPA2681N as shown in Figure 4.

Typically, channel switching is performed either on sync or

retrace time in the video signal. The two inputs are approxi-

mately equal at this time. The “make-before-break” disable

characteristic of the OPA2681 ensures that there is always

one amplifier controlling the line when using a wired-OR

circuit like that shown in Figure 4. Since both inputs may be

on for a short period during the transition between channels,

the outputs are combined through the output impedance

matching resistors (82.5

Ω in this case). When one channel is

disabled, its feedback network forms part of the output

impedance and slightly attenuates the signal in getting out

onto the cable. The gain and output matching resistor have

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

matched load and provide a 75

Ω output impedance to the

cable. The video multiplexer connection (Figure 4) also

insures that the maximum differential voltage across the

inputs of the unselected channel do not exceed the rated

±1.2V maximum for standard video signal levels.

The section on Disable Operation shows the turn-on and

turn-off switching glitches using a grounded input for a

single channel is typically less than

±50mV. Where two

outputs are switched (as shown in Figure 4), the output line

is always under the control of one amplifier or the other due

to the “make-before-break” disable timing. In this case, the

switching glitches for two 0V inputs drop to < 20mV.

FIGURE 3. Wideband, Single Supply, Differential ADC Driver.

200

Ω

24.9

Ω

ADS823

10-Bit

60MSPS

24.9

Ω

0.1µF

GND

200

Ω

BIAS = +2.5V

REFT

(+3.5V)

REFB

(+1.5V)

10pF

1/2

OPA2681

1/2

OPA2681

Ω

0.1µF

+5V

1:1

0.1µF

G = 1 +

= 10

2 RF

4.7µF

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