Datasheet

OPA693
19
SBOS285A
www.ti.com
The Typical Characteristics show a Recommended R
S
vs
Capacitive Load curve to help the designer pick a value to
give < 0.1dB peaking to the load. The resulting frequency
response curves show a 0.1dB peaked response for several
selected capacitive loads and recommended R
S
combina-
tions. Parasitic capacitive loads greater than 2pF can begin
to degrade the performance of the OPA693. Long PC board
traces, unmatched cables, and connections to other amplifier
inputs can easily exceed this value. Always consider this
effect carefully, and add the recommended series resistor as
close as possible to the OPA693 output pin (see the Board
Layout Guidelines section).
The criterion for setting this R
S
resistor is a maximum
bandwidth, flat frequency response at the load (< 0.1dB
peaking). For the OPA693 operating in a gain of +2, the
frequency response at the output pin is very flat to begin with,
allowing relatively small values of R
S
to be used for low
capacitive loads.
DISTORTION PERFORMANCE
The OPA693 provides good distortion performance into a
100Ω load on ±5V supplies. Relative to alternative solutions,
the OPA693 holds much lower distortion at higher frequencies
(> 20Mhz) than alternative solutions. Generally, until the
fundamental signal reaches very high frequency or power
levels, the 2nd harmonic will dominate the distortion with a
negligible 3rd harmonic component. Focusing then on the 2nd
harmonic, increasing the load impedance improves distortion
directly. Remember that the total load includes the feedback
network—in the noninverting configuration (see Figure 1) this
is the sum of R
F
+ R
G
, while in the inverting configuration it is
just R
F
(see Figure 3). Also, providing an additional supply de-
coupling capacitor (0.01µF) between the supply pins (for
bipolar operation) improves the 2nd-order distortion slightly
(3dB to 6dB).
The OPA693 has an extremely low 3rd-order harmonic
distortion. This also produces a high 2-tone, 3rd-order inter-
modulation intercept. Two graphs for this intercept are given
in the in the Typical Characteristics; one for ±5V and one for
+5V. The lower curve shown in each graph is defined at the
50Ω load when driven through a 50Ω matching resistor, to
allow direct comparisons to RF MMIC devices. The higher
curve in each graph shows the intercept if the output is taken
directly at the output pin with a 500Ω load, to allow prediction
of the 3rd-order spurious level when driving a lighter load,
such as an ADC input. The output matching resistor attenu-
ates the voltage swing from the output pin to the load by 6dB.
If the OPA693 drives directly into the input of a high-
impedance device, such as an ADC, this 6dB attenuation is
not taken and the intercept will increase a minimum of 6dB,
as shown in the 500Ω load typical characteristic.
The intercept is used to predict the intermodulation spurious
levels for two closely-spaced frequencies. If the two test fre-
quencies (f1 and f2) are specified in terms of average and delta
frequency, f
O
= (f1 + f2)/2 and ∆f = |f2 – f1|/2, then the two, 3rd-
order, close-in spurious tones will appear at f
O
±3 × ∆f. The
difference between two equal test tone power levels and these
intermodulation spurious power levels is given by
∆dBc = 2 × (IM3 – P
O
), where IM3 is the intercept taken from
the Typical Characteristics and P
O
is the power level in dBm at
the 50Ω load for one of the two closely-spaced test frequencies.
For instance, at 50MHz, the OPA693 at a gain of +2 has an
intercept of 44dBm at a matched 50Ω load. If the full envelope
of the two frequencies needs to be 2V
PP
at this load, this
requires each tone to be 4dBm (1V
PP
). The 3rd-order inter-
modulation spurious tones will then be 2 × (44 – 4) = 80dBc
below the test tone power level (–76dBm). If this same 2V
PP
2-tone envelope were delivered directly into a lighter 500Ω load,
the intercept would increase to the 52dBm shown in the Typical
Characteristics. With the same output signal and gain condi-
tions, but now driving directly into a light load with no matching
loss, the 3rd-order spurious tones will then be at least
2 × (52 – 4) = 96dBc below the 4dBm test tone power levels
centered on 50MHz (–92dBm). We are still using a 4dBm for the
1V
PP
output swing into this 500Ω load. While not strictly correct
from a power standpoint, this does give the correct prediction for
spurious level. The class AB output stage for the OPA693 is
much more voltage swing dependent on output distortion than
strictly power dependent. To use the 500Ω intercept curve, use
the single-tone voltage swing as if it were driving a 50Ω load to
compute the P
O
used in the intercept equation.
GAIN ACCURACY AND LINEARITY
The OPA693 provides improved absolute gain accuracy and
DC linearity over earlier fixed gain of two line drivers. Oper-
ating at a gain of +2V/V by tying the –IN pin to ground, the
OPA693 shows a maximum gain error of ±0.9% at 25°C. The
DC gain will therefore lie between 1.982V/V and 2.018V/V at
room temperature. Over the specified temperature ranges,
this gain tolerance expands only slightly due to the matched
temperature drift for R
F
and R
G
. Achieving this gain accuracy
requires a very low impedance ground at –IN. Typical pro-
duction lots show a much tighter distribution in gain than this
±0.9% specification. Figure 12 shows a typical distribution in
measured gain at the gain of +2V/V configuration, in this
case showing a slight drop in the mean (0.25%) from the
nominal but with a very tight distribution.
Figure 12. Typical +2V/V Gain Distribution.
Gain(V/V)
Mean = 1.995
σ = 0.005
Number of Units
600
500
400
300
200
100
0
1.980 1.982 1.984 1.986 1.988 1.990 1.992 1.994 1.996 1.998 2.000 2.002 2.004 2.006 2.008 2.010 2.012 2.014 2.016 2.018 2.020