OPA644U BURR-BROWN [Burr-Brown Corporation], OPA644U Datasheet - Page 11

• Manufacturer Part Number: OPA644U
• Description: Low Distortion Current Feedback OPERATIONAL AMPLIFIER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

In general, capacitive loads should be minimized for opti-
mum high frequency performance. Coax lines can be driven
if the cable is properly terminated. The capacitance of coax
cable (29pF/foot for RG-58) will not load the amplifier
when the coaxial cable or transmission line is terminated in
its characteristic impedance.
COMPENSATION
The OPA644 is internally compensated and is stable in unity
gain with a phase margin of approximately 70 . (Note that,
from a stability standpoint, an inverting gain of –1V/V is
equivalent to a noise gain of 2.) Gain and phase response for
other gains are shown in the Typical Performance Curves.
The high-frequency response of the OPA644 in a good
layout is very flat with frequency.
DISTORTION
The OPA644’s harmonic distortion characteristics into a
100
Typical Performance Curves. Distortion can be further im-
proved by increasing the load resistance as illustrated in
Figure 6. Remember to include the contribution of the
feedback resistance when calculating the effective load re-
sistance seen by the amplifier.
FIGURE 6. 5MHz Harmonic Distortion vs Load Resistance.
DIFFERENTIAL GAIN AND PHASE
Differential Gain (DG) and Differential Phase (DP) are among
the more important specifications for video applications. 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 and the PAL
subcarrier of 4.43MHz. All NTSC measurements were
performed using a Tektronix model VM700A Video
Measurement Set.
–100
–50
–60
–70
–80
–90
load are shown vs frequency and power output in the
10
20
50
Load Resistance ( )
100
200
500
1k
11
DG and DP of the OPA644 were measured with the amplifier
in a gain of +2V/V with 75 input impedance and the output
back-terminated in 75 . The input signal selected from the
generator was a 0V to 1.4V modulated ramp with sync pulse.
With these conditions the test circuit shown in Figure 7
delivered a 100IRE modulated ramp to the 75 input of the
video analyzer. The signal averaging feature of the analyzer
was used to establish a reference against which the performance
of the amplifier was measured. Signal averaging was also used
to measure the DG and DP of the test signal in order to
eliminate the generator’s contribution to measured amplifier
performance. Typical performance of the OPA644 is 0.008%
differential gain and 0.009 differential phase to both NTSC
and PAL standards.
FIGURE 7. Configuration for Testing Differential
NOISE FIGURE
The OPA644’s voltage and current noise spectral densities
are specified in the Typical Performance Curves. For RF
applications, however, Noise Figure (NF) is often the pre-
ferred noise specification since it allows system noise per-
formance to be more easily calculated. The OPA644’s Noise
Figure vs Source Resistance is shown in Figure 8.
FIGURE 8. Noise Figure vs Source Resistance.
TEK TSG 130A
25
20
15
10
5
0
10
75
Gain/Phase.
100
75
Source Resistance ( )
402
OPA644
NF = 10LOG 1 +
402
1K
OPA644
75
10K
e
n
2
4KTR
+ (InR
TEK VM700A
S
s
)
2
100K
75
®