OPA641H BURR-BROWN [Burr-Brown Corporation], OPA641H Datasheet - Page 10

• Manufacturer Part Number: OPA641H
• Description: Wideband Voltage Feedback OPERATIONAL AMPLIFIER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

Settling time, specified in an inverting gain of one, occurs in
only 18ns to 0.01% for a 2V step, making the OPA641 one
of the fastest settling monolithic amplifiers commercially
available. Settling time increases with closed-loop gain and
output voltage change as described in the Typical Perform-
ance Curves. Preserving settling time requires critical atten-
tion to the details as mentioned under “Wiring Precautions.”
The amplifier also recovers quickly from input overloads.
Overload recovery time to linear operation from a 50%
overload is typically only 30ns.
In practice, settling time measurements on the OPA641
prove to be very difficult to perform. Accurate measurement
is next to impossible in all but the very best equipped labs.
Among other things, a fast flat-top generator and high speed
oscilloscope are needed. Unfortunately, fast flat-top genera-
tors, which settle to 0.01% in sufficient time, are scarce and
expensive. Fast oscilloscopes, however, are more commonly
available. For best results, a sampling oscilloscope is recom-
mended. Sampling scopes typically have bandwidths that
are greater than 1GHz and very low capacitance inputs.
They also exhibit faster settling times in response to signals
that would tend to overload a real-time oscilloscope.
Figure 6 shows the test circuit used to measure settling time
for the OPA641. This approach uses a 16-bit sampling
oscilloscope to monitor the input and output pulses. These
waveforms are captured by the sampling scope, averaged,
and then subtracted from each other in software to produce
the error signal. This technique eliminates the need for the
traditional “false-summing junction,” which adds extra para-
sitic capacitance. Note that instead of an additional flat-top
generator, this technique uses the scope’s built-in calibration
source as the input signal.
DIFFERENTIAL GAIN AND PHASE
Differential Gain (DG) and Differential Phase (DP) are
among the more important specifications for video applica-
tions. DG is defined as the percent change in closed-loop
gain over a specified change in output voltage level. DP is
defined as the change in degrees of the closed-loop phase
over the same output voltage change. Both DG and DP are
specified at the NTSC sub-carrier frequency of 3.58MHz.
DG and DP increase with closed-loop gain and output
voltage transition. All measurements were performed using
a Tektronix model VM700 Video Measurement Set.
DISTORTION AND NOISE
The OPA641’s harmonic distortion characteristics vs fre-
quency and power output in the Typical Performance Curves.
Distortion can be further improved by increasing the load
resistance (refer to Figure 5). Remember to include the
contribution of the feedback resistance when calculating the
effective load resistance seen by the amplifier.
Although harmonic distortion may decrease with higher
load resistances (i.e., higher feedback resistors), the effec-
tive output noise will increase due to the higher resistance.
Therefore, noise or harmonic distortion may be optimized
by picking the appropriate feedback resistor.
®
OPA641
10
FIGURE 5. 5MHz Harmonic Distortion vs Load Resistance.
The third-order intercept point is an important parameter for
many RF amplifier applications. Figure 6 shows the
OPA641’s single-tone third-order intercept versus frequency.
This curve is particularly useful for determining the magni-
tude of the third harmonic as a function of frequency, load
resistance, and gain. For example, assume that the applica-
tion requires the OPA641 to operate in a gain of +2V/V and
drive 2Vp-p into 100 at a frequency of 5MHz. Referring to
Figure 6 we find that the intercept point is +38dBm. The
magnitude of the third harmonic can now be easily calcu-
lated from the expression:
For this case OPI
harmonic = 2(38 – 7) = 62dB below the fundamental tone.
The OPA641’s low IMD makes the device an excellent
choice for a variety of RF signal processing applications.
The value for the two-tone third-order intercept is typically
6dB lower than the single-tone value.
FIGURE 6. Single-Tone Third-Order Intercept Point vs Fre-
–100
where OPI
–70
–80
–90
60
50
40
30
20
10
1M
10
Third Harmonic (dBc) = 2(OPI
quency.
P
3
P = third-order output intercept, dBm
O
3
P = 38dBm, P
= output level/tone, dBm/tone
100
Load Resistance ( )
Frequency (Hz)
G = +2, V
10M
O
= 7dBm, and the third
O
= 2Vp-p, f
1k
3
2f
3f
P – P
O
O
G = +2V/V
O
O
= 5MHz
)
100M
10k