OPA690 datasheet(12/30 Pages) TI1

12 / 30 page

OPA2691

SBOS224D

www.ti.com

SINGLE-SUPPLY DIFFERENTIAL ADC DRIVER

Figure 3 shows a gain of +10 Single-Ended In/Diff. Out single-

supply ADC driver. Using a dual amplifier like the OPA2691

helps reduce 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 perfor-

mance matching (for example, 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 noninverting inputs of each of the two amplifiers in

the OPA2691. 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 gives

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 OPA2691, the ADC ADS823 operates on

a single +5V supply. Its internal common-mode voltage is

typically +2.5V which equals the required bias voltage for the

OPA2691.

Connecting two resistors between the top reference

(REFT = +3.5V) and bottom reference (REFB = +1.5V)

develops a +2.5V voltage level at their midpoint. Applying

that to the center tap of the transformer biases the ampli-

fiers appropriately. Sufficient bypassing 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 1st-order, low-

pass filter with capacitor C1 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

OPA2691I-14D, see 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 OPA2691 ensures that there is always

one amplifier controlling the line when using a wired-OR

circuit like that presented 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

resistors 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 (see 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 (see 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

0.1

µF

GND

200

Ω

BIAS = +2.5V

REFT

(+3.5V)

REFB

(+1.5V)

10pF

1/2

OPA2691

1/2

OPA2691

Ω

0.1

µF

0.1

µF

+5V

1:1

0.1

µF

G = 1 +

= 10

2 RF

4.7

µF

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