OPA2846_14 Datasheet, PDF (20/30 Pages) Texas Instruments – Dual, Wideband, Low-Noise, Voltage-Feedback Operational Amplifier

OPA2846
SBOS274C −JUNE 2003 − REVISED AUGUST 2008
DISTORTION PERFORMANCE
The OPA2846 is capable of delivering an exceptionally low
distortion signal at high frequencies over a wide range of
gains. The distortion plots in the Typical Characteristics
show the typical distortion under a wide variety of
conditions. Most of these plots are limited to 110dB
dynamic range.
Generally, until the fundamental signal reaches very high
frequencies or powers, 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, this is sum of (R_F + R_G), while
in the inverting configuration, it is just R_F (see Figure 1 and
Figure 2). Increasing output voltage swing increases
harmonic distortion directly. A 6dB increase in output
swing will generally increase the 2nd-harmonic to 12dB
and the 3rd-harmonic to 18dB. Increasing the signal gain
will also increase the 2nd-harmonic distortion. Again, a
6dB increase in gain will increase the 2nd and 3rd
harmonic by approximately 6dB each, even with constant
output power and frequency. Finally, the distortion
increases as the fundamental frequency increases, due to
the rolloff in the loop gain with frequency. Conversely, the
distortion will improve going to lower frequencies down to
the dominant open-loop pole at approximately 100kHz.
Starting from the −82dBc 2nd-harmonic for a 5MHz, 2V_PP
fundamental into a 200Ω load at G = +10 (from the Typical
Characteristics), the 2nd-harmonic distortion for frequen-
cies lower than 100kHz will approximately be:
−82dBc − 20 log(5MHz/100kHz) = −116dBc
The OPA2846 has extremely low 3rd-order harmonic
distortion. This also gives a high 2-tone, 3rd-order
intermodulation intercept as shown in the Typical
Characteristics. This intercept curve is defined at the 50Ω
load when driven through a 50Ω matching resistor to allow
direct comparisons to RF MMIC devices. This matching
network attenuates the voltage swing from the output pin
to the load by 6dB. If the OPA2846 drives directly into the
input of a high impedance device, such as an A/D
converter, the 6dB attenuation is not taken. Under these
conditions, the intercept will increase by a minimum 6dBm.
The intercept is used to predict the intermodulation
spurious for two, closely-spaced frequencies. If the two
test frequencies, f₁ and f₂, are specified in terms of
average and delta frequency, f_O = (f₁ + f₂)/2 and f = |f₂ −
f₁|/2, 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 Characteristic 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 10MHz,
the OPA2846 at a gain of +10 has an intercept of 44dBm
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at a matched 50Ω load. If the full envelope of the two
frequencies needs to be 2V_PP, this requires each tone to
be 4dBm. The 3rd-order intermodulation spurious tones
will then be 2 × (48 − 4) = 88dBc below the test-tone
power level (−84dBm). If this same 2V_PP, 2-tone envelope
were delivered directly into the input of an ADC—without
the matching loss or the loading of the 50Ω network—the
intercept would increase to at least 50dBm. With the same
signal and gain conditions, but now driving directly into a
light load, the spurious tones will then be at least
2 × (54 − 4) = 100dBc below the 4dBm test-tone power
levels centered on 10MHz.
DC ACCURACY AND OFFSET CONTROL
The OPA2846 can provide excellent DC signal accuracy
due to its high open-loop gain, high common-mode
rejection, high power-supply rejection, and low input offset
voltage and bias current offset errors. To take full
advantage of its low ±0.65mV input offset voltage, careful
attention to input bias current cancellation is also required.
The low noise input stage of the OPA2846 has a relatively
high input bias current (10µA typical into the pins), but with
a very close match between the two input currents—typi-
cally ±100nA input offset current. The total output offset
voltage may be reduced considerably by matching the
source impedances looking out of the two inputs. For
example, one way to add bias current cancellation to the
circuit of Figure 1 (page 12) would be to insert a 20Ω
series resistor into the noninverting input from the 50Ω
terminating resistor. When the 50Ω source resistor is
DC-coupled, this will increase the source resistances for
the noninverting input bias current to 45Ω. Since this is
now equal to the resistance looking out of the inverting
input (R_F || R_G), the circuit will cancel the gains for the bias
currents to the output, leaving only the offset current times
the feedback resistor as a residual DC error term at the
output. Using the 453Ω feedback resistor, this output error
will now be less than ±0.6µA × 453Ω = ±0.27mV over the
full temperature range.
A fine-scale output offset null, or DC operating point
adjustment, is often required. Numerous techniques are
available for introducing a DC offset control into an op amp
circuit. Most of these techniques eventually reduce to
setting up a DC current through the feedback resistor. One
key consideration to selecting a technique is to insure that
it has a minimal impact on the desired signal path
frequency response. If the signal path is intended to be
noninverting, the offset control is best applied as an
inverting summing signal to avoid interaction with the
signal source. If the signal path is intended to be inverting,
applying the offset control to the noninverting input can be
considered. For a DC-coupled inverting input signal, this
DC offset signal will set up a DC current back into the
source that must be considered. An offset adjustment
placed on the inverting op amp input can also change the
noise gain and frequency response flatness.