HOS-050_15 Datasheet, PDF (4/7 Pages) Analog Devices – FAST SETTLING VIDEO OPERATIONAL AMPLIFIERS

(continued from Features page)
The HOS-OSO Series stands up under close scrutiny of these
characteristics because of its IOOMHz gain bandwidth product.
The use of these amplifiers in a wide variety of applications has
confirmed their suitability for video circuits.
VOL T AGE AMPLIFIERS/CURRENT BOOSTERS
Video op amps such as the HOS-OSOare generally characterized
by high gain bandwidth products, fast settling times, and high
where: Eo max = peak voltage needed
10 max = maximum continuous current HOS-OSO can
produce
IRFB = current in feedback resistor at peak voltage
Assume the feedback resistor value is s,ooon. Minimum load
would be:
Eo max
IOV
IOV
10 max - IRFB ^{lOOmA - 2mA = 98mA = 1O2n minimum load}
output drive.
Designs which strive for driving a minimum load (by increasing
One of the most common uses of video op amps is for D/ A
converter output voltage amplification or current boosting.
Figure 3 is one example of this type of application. In this circuit,
the internal resistance of the D/A is the feed forward resistor for
the op amp.
the feedback resistor) can create settling problems because of a
fundamental characteristic of op amp circuits. . . the higher the
feedback resistance, the slower the system response.
This phenomenon is the result of increased impedance for driving
stray capacitances in the circuit employing the op amp, and
- ISV + ISV
fixed capacitances in the summing node.
Impedances need to be kept as low as possible consistent with
low distortion; and stray capacitances need to be eliminated to
OBSOLETE 'UNIPOLAR"R,. -'024mA'
'BIPOlAR" R, . - 5 12mA'
--Figure 3. Inverting Unipolar or Bipolar Voltage Output
The HDS Series D/A converters are fast-settling, current output
D/As available in 8-, 10-, and 12-bit resolUtions. Both TTL and
ECL versions are avai4ible, and settling times range from IOns
for 8-bit units through 40ns for 12-bit units.
The cIrcuit which is shown will provide a negative unipolar
oUtpUt with binary coding on the inpUt, and bipolar offset ground-
ed. It will provide a bipolar output with complementary offset
binary coding on the input, and bipolar offset connected to 10'
the maximum possible extent. A large ground plane structure is
recommended to help assure low ground impedances. In addition,
0.1f-LFceramic capacitors and 3-IOf-LFtantalum capacitors con-
nected as close as possible to power supply inputs will decrease
the potential for parasitic oscillations and other noise signals.
Another argument for limiting the size of the feedback resistor
is because of its effect on bandwidth. Bandwidth of the HOS-OSO
op amp and the value of the feedback resistor are inversely
related.
At any given gain of the op amp, the gain setting with the widest
bandwidth will be the one which employs the lower value of
feedback. As an example, a gain of I can be achieved with RFF
= RFB = soon; or RFF = RFB = 1,ooon. Small-signal bandwidth
for the first combination is typically 4SMHz; bandwidth for the
second is typically 3SMHz.
OFFSET AND GAIN ADJUSTMENT
An approximation of the total settling time for the D/A op amp
combination is calculated by:
Figure 4 shows a method of using the HOS-OSOop amp which
allows adjusting the offset and gain of the oUtput voltage.
Ts = YTO2+ Ta2
where T D is D/A settling time and To is HOS-OSO settling time.
This approximation is valid because both the D/A and the HOS-
OSOexhibit 6dB/octave roll-off charateristics (single pole response);
and the combination of low D/A oUtpUt capacitance and op amp
inpUt capacitance does not materially affect the formula.
The user of the HOS-OSOshould remember the current flowing
in the feedback resistor (Rl) must be subtracted from the output
available from the HOS-OSO.
There is a tendency, because of this fact, to use a high value of
OFFSET
AOJU51
20k
v"
. v"
01,,'
1~ R"",..
Coo
KEEP "ADS
TOSUMMI-NG
. . GAIN
ADJUST
~^{~~ NODSEHOR'-T}
AS POSS'B"
R
Roo
R"
v,.
voo'
~ GA'N = -I I
feedback resistor to assure maximum current drive being available
for driving low impedances; but this approach may create unde-
sirable side effects.
Figure 4. HOS-O50 Offset and Gain Adjust
As shown, the gain of the circuit is established by the equation:
Calculating the minimum load that can be driven under two
conditions of feedback resistor values will serve to illustrate the
difference.
Assume the feedback resistor value is soon. If output voltage of
the HOS-OSOis 10 volts, and output current is lOOmA, minimum
load would be:
Eo max
IOV
IOV
10 nux - IRFB = lOOmA- 20mA = 80mA = 12511minimum load
G = - (R_RFF_FB)
where RFB is the total of RGAINand Gain Adjust.
Once the user has established the desired gain for the illustrated
circuit, the value of RFB can be used to determine the correct
value of RoFFSET with the equation:
( ^{) Vcc x RfB}
RoFFSET = -
.:lEa'
where .:lEa is the desired amount of offset on the oUtput.
2-248 OPERA TIONAL AMPLIFIERS