result. However, operation at low gains may be desirable to
take advantage of the higher slew rate and exceptional DC
precision of the OPA846. Numerous external compensation
techniques are suggested for operating a high-gain op amp
at low gains. Most of these give zero/pole pairs in the closed-
loop response that cause long term settling tails in the pulse
response and/or phase nonlinearity in the frequency re-
sponse. Figure 9 shows an external compensation method
for a noninverting configuration that does not suffer from
these drawbacks.
DIFFERENTIAL OPERATION
Operating two OPA846 amplifiers in a differential inverting
configuration can further suppress even-order harmonic terms.
The Typical Characteristic curves show measured perfor-
mance for this condition. For the distortion data, the output
swing is increased to 4VPP into 400Ω to allow direct compari-
son to the 2VPP into 200Ω data for single-channel operation.
Figure 11 shows the swept frequency 2nd- and 3rd-harmonic
distortion for an inverting differential configuration, where
each channel is set up for a gain of 20.
Comparing this to the single-channel distortion (at 10MHz for
instance), about the same 3rd-harmonic and about a 5dB
improvement in the 2nd-harmonic is shown.
+5V
Power-supply
decoupling not shown.
50Ω Source
VI
RS
50Ω
VO
RT
50Ω
R1
65Ω
OPA846
–65
50Ω Load
GD = 20
VO = 4VPP
RL = 400Ω
RF
402Ω
–5V
–75
–85
RG
402Ω
2nd-Harmonic
3rd-Harmonic
–95
FIGURE 9. Noninverting Low-Gain Compensation.
–105
–115
The R1 resistor across the two inputs increases the noise
gain (i.e., decreases the loop gain) without changing the
signal gain. This approach retains the full slew rate to the
output but gives up some of the low-noise benefit of the
OPA846. Assuming a low source impedance is used, set R1
so that 1 + RF /(RG || R1) is > 7. This approach may also be
used to tune the flatness by adjusting R1. The Typical
Characteristic curves show a signal gain of +8 with the
noise gain adjusted for flatness using different values for
R1. Figure 10 shows the measured frequency response for
the circuit of Figure 9 showing the flat frequency response
possible with this compensation.
1
10
Frequency (MHz)
100
FIGURE 11. Differential Distortion vs Frequency.
SINGLE-SUPPLY OPERATION
The OPA846 may be operated from a single power supply if
system constraints require it. Operation from a single +5V to
+12V supply is possible with minimal change in AC perfor-
mance. The Typical Characteristics show the input and
output voltage ranges for a bipolar supply range from ±2.5V
to ±6V. The Common-Mode Input Range and Output Swing
vs Supply Voltage plot shows that the required headroom on
both the input and output nodes remains at approximately
1.5V over this entire range. On a single +5V supply for
instance, this means the noninverting input should remain
centered at 2.5V ±1V, as should the output pin. See Figure
12 for an example application biasing the noninverting input
at mid-supply and running an AC-coupled input to the invert-
ing gain path. Since the gain resistor is blocked off for DC,
the bias point on the noninverting input appears at the
output, centering up that node, as well on the power supply.
The OPA846 can support this mode of operation down to a
single +5V supply and up to a single +12V supply.
10
5
0
–5
–10
–15
–20
–25
105
106
107
108
109
Frequency (Hz)
FIGURE 10. Noninverting Gain of +2 Response Using External
Compensation.
OPA846
14
SBOS250C
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