OPA650 Burr-Brown, OPA650 Datasheet - Page 10

• Manufacturer Part Number: OPA650
• Description: Wideband / Low Power Voltage Feedback OPERATIONAL AMPLIFIER
• Manufacturer: Burr-Brown

the phase margin and avoid peaking by keeping the break
frequency of this zero sufficiently high. When high closed-
loop gains are required, a three-resistor attenuator (tee-
network) is recommended to avoid using large value resis-
tors with large time constants.
PULSE SETTLING TIME
High speed amplifiers like the OPA650 are capable of
extremely fast settling time with a pulse input. Excellent
frequency response flatness and phase linearity are required
to get the best settling times. As shown in the specifications
table, settling time for a 1V step at a gain of +1 for the
OPA650 is extremely fast. The specification is defined as
the time required, after the input transition, for the output to
settle within a specified error band around its final value. For
a 2V step, 1% settling corresponds to an error band of
error band of 0.2mV. For the best settling times, particu-
larly into an ADC capacitive load, little or no peaking in the
frequency response can be allowed. Using the recommended
R
the settling times. Fast, extremely fine scale settling (0.01%)
requires close attention to ground return currents in the
supply decoupling capacitors. For highest performance, con-
sider the OPA642 which isolates the output stage decoupling
from the rest of the amplifier.
DIFFERENTIAL GAIN AND PHASE
Differential Gain (DG) and Differential Phase (DP) are
among the more important specifications for video applica-
tions. The percentage change in closed-loop gain over a
specified change in output voltage level is defined as DG.
DP is defined as the change in degrees of the closed-loop
phase over the same output voltage change. DG and DP are
both specified at the NTSC sub-carrier frequency of 3.58MHz.
DG and DP increase closed-loop gain and output voltage
transition. All measurements were performed using a
Tektronix model VM700 Video Measurement Set.
DISTORTION
The OPA650’s harmonic distortion characteristics into a
100 load are shown versus frequency and power output in
the typical performance curves. Distortion can be signifi-
cantly improved by increasing the load resistance as illus-
trated in Figure 5. Remember to include the contribution of
the feedback network when calculating the effective load
resistance seen by the amplifier.
NOISE FIGURE
The OPA650 voltage noise spectral density is specified in
the Typical Performance Curves. For RF applications, how-
ever, Noise Figure (NF) is often the preferred noise specifi-
cation since it allows system noise performance to be more
easily calculated. The OPA650’s Noise Figure vs Source
Resistance is shown in Figure 6.
20mV, 0.1% to an error band of 2mV, and 0.01% to an
ISO
for capacitive loads will limit this peaking and reduce
®
OPA650
10
FIGURE 5. 5MHz Harmonic Distortion vs Load Resistance.
FIGURE 6. Noise Figure vs Source Resistance.
SPICE MODELS AND EVALUATION BOARD
Computer simulation of circuit performance using SPICE is
often useful when analyzing the performance of analog
circuits and systems. This is particularly true for Video and
RF amplifier circuits where parasitic capacitance and induc-
tance can have a major effect on circuit performance. SPICE
models are available on a disk from the Burr-Brown Appli-
cations Department.
Demonstration boards are available for each OPA650 pack-
age style. These boards implement a very low parasitic
layout that will produce the excellent frequency and pulse
responses shown in the Typical Performance Curves. For
each package style, the recommended demonstration board
is:
Contact your local Burr-Brown sales office or distributor to
order demonstration boards.
30
25
20
15
10
–60
–70
–80
–90
5
0
10
10
DEM-OPA65xP
DEM-OPA65xU
DEM-OPA6xxN
NOISE FIGURE vs SOURCE RESISTANCE
100
Source Resistance ( )
Load Resistance ( )
8-Pin DIP for the OPA650P
SO-8 for the OPA650U
SOT23 for the OPA650N
NF = 10 LOG 1 +
1k
100
2f
3f
O
O
10k
e
n
2
4KTR
+ (I
n
R
S
S
)
2
100k
1k