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參數(shù)資料
| 型號： | OPA621KP |
| 文件頁數(shù)： | 14/16頁 |
| 文件大?。?/td> | 193K |
| 代理商： | OPA621KP |
OPA621
14
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 as shown in the Typical Performance
Curves. All measurements were performed using a Tektronix
model VM700 Video Measurement Set.
DISTORTION
The OPA621’s Harmonic Distortion characteristics into a
50
load are shown vs frequency and power output in the
Typical Performance Curves. Distortion can be further
improved by increasing the load resistance as illustrated in
Figure 8. Remember to include the contribution of the
feedback resistance when calculating the effective load
resistance seen by the amplifier.
FIGURE 9. Two-Tone Third-Order Intermodulation Inter-
cept vs Frequency.
FIGURE 8. 10MHz Harmonic Distortion vs Load Resistance.
Two-tone, third-order intermodulation distortion (IM) is an
important parameter for many RF amplifier applications.
Figure 9 shows the OPA621’s two-tone, third-order IM
intercept vs frequency. For these measurements, tones were
spaced 1MHz apart. This curve is particularly useful for
determining the magnitude of the third-order IM products as
a function of frequency, load resistance, and gain. For
example, assume that the application requires the OPA621
to operate in a gain of +2V/V and drive 2Vp-p (4dBm for
each tone) into 50
at a frequency of 10MHz. Referring to
Figure 9 we find that the intercept point is +47dBm. The
magnitude of the third-order IM products can now be easily
calculated from the expression:
Third IMD = 2(OPI
3
P – P
O
)
where OPI
3
P = third-order output intercept, dBm
P
O
= output level/tone, dBm/tone
Third IMD = third-order intermodulation ratio
below each output tone, dB
For this case OPI
3
P = 47dBm, P
O
= 4dBm, and the third-
order IMD = 2(47 – 4) = 86dB below either 4dBm tone. The
OPA621’s low IMD makes the device an excellent choice
for a variety of RF signal processing applications.
NOISE FIGURE
The OPA621’s voltage and current noise spectral densities
are specified in the Typical Performance Curves. For RF
applications, however, Noise Figure (NF) is often the
preferred noise specification since it allows system noise
performance to be more easily calculated. The OPA621’s
Noise Figure vs Source Resistance is shown in Figure 10.
SPICE MODELS
Computer simulation 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 inductance can have a major
effect on circuit performance. A SPICE model using
MicroSim Corporation’s PSpice is available for the OPA621.
This simulation model is available through the Burr-Brown
web site at www.burr-brown.com or by calling the Burr-
Brown Applications Department.
NOISE FIGURE vs SOURCE RESISTANCE
25
20
15
10
5
0
10
100
1k
10k
100k
N
Source Resistance ( )
NF
dB
= 10log 1 +
e
n2
+ (i
n
R
S
)
2
4kTR
S
FIGURE 10. Noise Figure vs Source Resistance.
V = 2Vp-p
10MHz HARMONIC DISTORTION
vs LOAD RESISTANCE
–40
–50
–60
–70
–80
–90
0
100
200
300
400
500
H
Load Resistance ( )
3f
2f
G = +2V/V
G = +5V/V
P
OUT
250
250
R
L
–
+
G = +2V/V
0
10
20
30
40
50
60
70
80
90
100
10
15
20
25
30
35
40
45
50
55
60
Frequency (MHz)
I
R = 50
R = 100
R = 400
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