Datasheet

OPA692
13
SBOS236E
www.ti.com
APPLICATIONS INFORMATION
WIDEBAND BUFFER OPERATION
The OPA692 gives the exceptional AC performance of a
wideband current-feedback op amp with a highly linear, high-
power output stage. It features internal R
F
and R
G
resistors
that make it easy to select a gain of +2, +1, or –1 without any
external resistors. Requiring only 5.1mA quiescent current, the
OPA692 will swing to within 1V of either supply rail and deliver
in excess of 160mA at room temperature. This low output
headroom requirement, along with supply voltage indepen-
dent biasing, gives remarkable single (+5V) supply operation.
The OPA692 will deliver greater than 200MHz bandwidth
driving a 2V
PP
output into 100Ω on a single +5V supply.
Previous boosted output stage amplifiers have typically suf-
fered from very poor crossover distortion as the output current
goes through zero. The OPA692 achieves a comparable
power gain with much better linearity. The primary advantage
of a current-feedback op amp over a voltage-feedback op amp
is that AC performance (bandwidth and distortion) is relatively
independent of signal gain.
Figure 1 shows the DC-coupled, gain of +2, dual power-supply
circuit configuration used as the basis of the ±5V Electrical and
Typical Characteristics. For test purposes, the input imped-
ance is set to 50Ω with a resistor to ground and the output
impedance is set to 50Ω with a series output resistor. Voltage
swings reported in the specifications are taken directly at the
input and output pins while load powers (dBm) are defined at
a matched 50Ω load. For the circuit of Figure 1, the total
effective load will be 100Ω || 804Ω = 89Ω. The disable control
line (
DIS ) is typically left open to ensure normal amplifier
operation. In addition to the usual power-supply decoupling
capacitors to ground, a 0.1µF capacitor can be included
between the two power-supply pins. This optional added
capacitor will typically improve the 2nd-harmonic distortion
performance by 3dB to 6dB.
Figure 2 shows the AC-coupled, gain of +2, single-supply
circuit configuration used as the basis of the +5V Electrical
and Typical Characteristics. Though not a
rail-to-rail design,
the OPA692 requires minimal input and output voltage head-
room compared to other very wideband current-feedback op
amps. It will deliver a 3V
PP
output swing on a single +5V
supply with greater than 150MHz bandwidth. The key re-
quirement of broadband single-supply operation is to main-
tain input and output signal swings within the usable voltage
ranges at both the input and the output. The circuit of Figure
2 establishes an input midpoint bias using a simple resistive
divider from the +5V supply (two 806Ω resistors). The input
signal is then AC-coupled into this midpoint voltage bias. The
input voltage can swing to within 1.5V of either supply pin,
giving a 2V
PP
input signal range centered between the supply
pins. The input impedance matching resistor (57.6Ω) used
for testing is adjusted to give a 50Ω input match when the
parallel combination of the biasing divider network is in-
cluded. The gain resistor (R
G
) is AC-coupled, giving the
circuit a DC gain of +1—which puts the input DC bias voltage
(2.5V) on the output as well. Again, on a single +5V supply,
the output voltage can swing to within 1V of either supply pin
while delivering more than 120mA output current. A demand-
ing 100Ω load to a midpoint bias is used in this characteriza-
tion circuit. The new output stage used in the OPA692 can
deliver large bipolar output currents into this midpoint load
with minimal crossover distortion, as shown by the +5V
supply, 3rd-harmonic distortion typical characteristics.
FIGURE 1. DC-Coupled, G = +2, Bipolar Supply, Specifica-
tion and Test Circuit.
FIGURE 2. AC-Coupled, G = +2, Single-Supply Specification
and Test Circuit.
OPA692
+5V
–5V
50Ω Load
50Ω
50Ω
50Ω Source
R
G
402Ω
R
F
402Ω
+
6.8µF0.1µF
+
6.8µF0.1µF
DIS
V
IN
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
OPA692
+5V
+V
S
DIS
V
S
/2
806Ω
100ΩV
O
V
IN
806Ω
R
G
402Ω
R
F
402Ω
0.1µF
0.1µF
+
6.8µF0.1µF
50Ω Source
57.6Ω