Datasheet
OPA2694
SBOS320D − SEPTEMBER 2004 − REVISED APRIL 2013
www.ti.com
12
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
2V
PP
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 V
O
to the inverting error
current) is the sum of R
F
+ (R
I
• NG). where R
I
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 R
I
= 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.
L
+5V
L
R
1
R
1
R
2
R
2
C
V
−
V+
12−Bit
40MSPS
ADS5220
500
Ω
−
5V
0.1
μ
F
25
Ω
100
Ω
100
Ω
500
Ω
C
1
C
1
V
I
50
Ω
1:2
25
Ω
V
CM
1/2
OPA2694
1/2
OPA2694
Figure 3. Wideband, AC-Coupled, Low-Power ADC Driver
100
Ω
50
Ω
+5V
DIS
−
5V
V
O
=
−
(V
1
+V
2
+V
3
+V
4
+V
5
)
100MHz,
−
1dB Compression = 15dBm
50
Ω
V
2
50
Ω
V
3
50
Ω
V
4
50
Ω
V
1
50
Ω
RG−58
50
Ω
V
5
30
Ω
1/2
OPA2694
Figure 4. 200MHz RF Summing Amplifier