OPA694 datasheet(12/27 Pages) TI2

12 / 27 page

OPA2694

SBOS320D − SEPTEMBER 2004 − REVISED APRIL 2013

www.ti.com

ADC DRIVER

Most

modern,

high-performance

analog-to-digital

converters (ADCs), such as Texas Instruments ADS522x

series, require a low-noise, low-distortion driver. The

OPA2694 combines low-voltage noise (2.1nV/√Hz) with

low harmonic distortion. Figure 3 shows an example of a

wideband, AC-coupled, 12-bit ADC driver.

One OPA2694 is used in the circuit of Figure 3 to form a

differential driver for the ADS5220. The OPA2694 offers

> 150MHz bandwidth at a differential gain of 5V/V, with a

2VPP output swing. A 2nd-order RLC filter is used in order to

limit the noise from the amplifier and provide some

attenuation for higher-frequency harmonic distortion.

WIDEBAND INVERTING SUMMING AMPLIFIER

Since the signal bandwidth for a current-feedback op amp

can be controlled independently of the noise gain (NG),

which is normally the same as the noninverting signal

gain), wideband inverting summing stages may be

implemented using the OPA2694. The circuit in Figure 4

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 50Ω source,

the NG for this circuit will be (1 + 100Ω/(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 100Ω feedback (to

get a signal gain of –2 from each input to the output pin)

requires an additional 30Ω in series with the inverting input

to increase the feedback impedance. With this resistor

added to the typical internal RI = 30Ω, the total feedback

impedance is 100Ω + (60Ω • 6) = 460Ω, which is equal to

the required value to get a maximum bandwidth flat

frequency response for NG = 6.

Single−to−Differential

Gain of 10

Power−supply decoupling not shown.

+5V

−

V+

12−Bit

40MSPS

ADS5220

500

−5V

0.1

μF

100

500

1:2

VCM

1/2

OPA2694

1/2

OPA2694

Figure 3. Wideband, AC-Coupled, Low-Power ADC Driver

100

+5V

DIS

−5V

VO = −(V1 +V2 +V3 +V4 +V5)

100MHz,

−1dB Compression = 15dBm

RG−58

1/2

OPA2694

Figure 4. 200MHz RF Summing Amplifier

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