HOS-050_15 Datasheet, PDF (6/7 Pages) Analog Devices – FAST SETTLING VIDEO OPERATIONAL AMPLIFIERS

being driven by the op amp. This makes it necessary to set the
The Analog Devices HOS-050 has different performance charac-
gain of the circuit to provide tWice the desired voltage.
Three different values of resistors and cables are "phantomed"
into the figure as examples of possible characteristic impedances
which might be used. Figure 8 is nor meant to imply the HOS-050
teristics when operating as a noninverting amplifier, but the
care used in the design makes the differences less pronounced
than they are in many competing units.
The HOS-050 can be considered a true differential video op
can drive three cables simultaneously.
amp. It requires little or no external compensation because its
c..
rolloff characteristics approach a 6dB/octave slope. This helps
the user determine summing errors and loop response; and
...
v.
v...,
lOW l OUTPUT
helps assure the stability of the system.
The performance parameters for both inverting and noninverting
I
I
operation are shown elsewhere in this data sheet (see SPECIFI-
I 00..
~S1 ^~
~5i=- ^I
I
I_~
:-.
~
I
I
~
~~ ::-.
CATIONS section and figures). A comparison of the characteristics
will highlight the similarities in performance, with the exceptions
noted above.
= = = (RFF RFB soon; A -1)
-40
- -- = ~ 2V pop; Rl 1SOU
.. - -.. = 4V pop; Rl 150U
= 2V pop; Rl 1k
= 4V pop; Rl
1k
OBSOLE. --TE Figure 8. HOS-050 Impedance Matching
NONINVERTING OPERATION
The vast majority of video operational amplifiers display marked
differences in settling times and bandwidths when operated in a
noninverting mode instead of the inverting mode. There are a
number of valid reasons for this characteristic.
Most high-speed op amps use feed-forward compensation for
optimizing performance in the inverting mode. This is necessary
to obtain wide gaiQ-bandwidth products while maintaining dc
performance in these types of devices. In effect, the op amp has
a wideband ac channel which is not perfectly matched to the dc
channel.
Feed-forward techniques enhance the performance of the op
amp in the inverting mode by incresing the slew rate and small-
signal bandwidth. These techniques, however, also decrease the
amplifier's tolerance to stray capacitances, so must be employed
ct ^-45
:j -so
:.J..
g -55
IIIII)
II)
'CO -60
-65
0.5 1
2
4
8
- OUTPUT FREQUENCY MHz
Figure 9. Harmonic Distortion - Inverting
= = = (RFF RFB 1k; A 2)
.. - - -4 2VpopR; l = 150U
1
= ~ 4V pop; Rl 150U
-40
= 2V pop; Rl 1k
judiciously.
III
..
....
4V pop; Rl = 1k
The overall input capacitance of the op amp is kept as low as
possible in the design; and any mismatch in the capacitance of
the two channels appears as an error in the output. Because of
ct
Ii: -45
........
:J _so
~
the inherently low total input capacitance of the op amp, even a
small capacitive mismatch between channels shows up as a large
effective error signal.
0....
~ -55
II)
'CO
-60
Decreasing the channel mismatch can be achieved only by com-
plicating the design of the op amp with additional components,
and rigorous selection of those components in the manufacturing
process.
0.5 1
2
4
8
OUTPUT FREQUENCY - MHz
As a consequence, the mismatch is reduced to the smallest practical
value consistent with the economics of producing and using the
op amp. But it remains a mismatch, and manifests itself as a
difference in performance in the inverting versus noninverting
modes.
There are video op amps available at low cost which use a 74 I-type
amplifier for high dc open loop gain in the noninverting channel.
The user of these kinds of designs may sometimes gain an economic
advantage, but at a high cost in performance. Bandwidths for
noninverting applications are often measured in kHz, not MHz,
for this approach.
A video op amp is acting as a voltage mode device at both inputs
when operating in the noninverting mode. This contrasts with
the inverting mode, where it is operating as a current mode
device.
Figure 10. Harmonic Distortion - Noninverting
IN SUMMARY. . . A CAVEAT
Settling time specifications, bandwidth capabilities, harmonic
distortion performance, and other parameters for video op amps
cannot possibly include all possible situations and applications.
A multitude of seemingly insignificant conditions can have a
major impact on the unit and its ability to operate in any given
circuit.
The potential user is strongly urged to evaluate the effectiveness
of the HOS-O50 in the actual circuit in which it will be used.
In many instances, the application conditions are different from
the conditions used in specifying; there is no substitute for a
trial in the proposed circuit to determine if t;le op amp will
provide the desired results.
2-250 OPERATIONALAMPLIFIERS