OPA690 datasheet(12/23 Pages) TI

12 / 23 page

OPA691

SBOS226A

www.ti.com

SINGLE-SUPPLY ADC INTERFACE

Most modern, high performance ADCs (such as the Texas

Instruments ADS8xx and ADS9xx series) operate on a single

+5V (or lower) power supply. It has been a considerable

challenge for single-supply op amps to deliver a low distor-

tion input signal at the ADC input for signal frequencies

exceeding 5MHz. The high slew rate, exceptional output

swing, and high linearity of the OPA691 make it an ideal

single-supply ADC driver. Figure 3 shows an example input

interface to a very high performance, 10-bit, 60MSPS CMOS

converter.

The OPA691 in the circuit of Figure 3 provides > 180MHz

bandwidth operating at a signal gain of +4 with a 2Vp-p

output swing. One of the primary advantages of the current

feedback internal architecture used in the OPA691 is that

high bandwidth can be maintained as the signal gain is

increased. The noninverting input bias voltage is referenced

to the midpoint of the ADC signal range by dividing off the top

and bottom of the internal ADC reference ladder. With the

gain resistor (RG) AC-coupled, this bias voltage has a gain of

+1 to the output, centering the output voltage swing as well.

Tested performance at a 20MHz analog input frequency and

a 60MSPS clock rate on the converter gives > 58dBc SFDR.

WIDEBAND INVERTING SUMMING AMPLIFIER

Since the signal bandwidth for a current feedback op amp may

be controlled independently of the noise gain (NG, which is

normally the same as the noninverting signal gain), very

broadband inverting summing stages may be implemented

using the OPA691. The circuit on the front page of this data

sheet shows an example inverting summing amplifier where

the resistor values have been adjusted to maintain both

maximum bandwidth and input impedance matching. If each

RF signal is assumed to be driven from a 50

Ω source, the NG

for this circuit will be (1 + 100

Ω/(100Ω/5)) = 6. The total

feedback impedance (from VO to the inverting error current) is

the sum of RF + (RI • NG) where RI is the impedance looking

into the inverting input from the summing junction (see the

Setting Resistor Values to Optimize Performance section).

Using 100

Ω feedback (to get a signal gain of –2 from each

input to the output pin) requires an additional 30

Ω in series

with the inverting input to increase the feedback impedance.

With this resistor added to the typical internal RI = 35Ω, the

total feedback impedance is 100

Ω + (65Ω • 6) = 490Ω, which

is equal to the required value to get a maximum bandwidth flat

frequency response for NG = 6. Tested performance shows

more than 200MHz small-signal bandwidth and a –1dBm

compression of 15dBm at the matched 50

Ω load through

100MHz.

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

OPA691, 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 OPA691 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

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 (see Figure 4)

FIGURE 3. Wideband, AC-Coupled, Single-Supply ADC Driver.

OPA691

360

Ω

+2.5V DC Bias

ADS823

10-Bit

60MSPS

Ω

2Vp-p

DIS

22pF

Input

120

Ω

REFB

REFT

Input

0.1

µF

0.1

µF

0.5Vp-p

Ω

0.1

µF

+3.5V

Ω

0.1

µF

+1.5V

+5V

Clock

+5V

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