OPA4658 Burr-Brown, OPA4658 Datasheet - Page 11

• Manufacturer Part Number: OPA4658
• Description: Quad Wideband / Low Power Current Feedback OPERATIONAL AMPLIFIER
• Manufacturer: Burr-Brown

FIGURE 5. 5MHz Harmonic Distortion vs Load Resistance.
The third-order intercept is an important parameter for many
RF amplifier applications. Figure 6 shows the OPA4658’s
two tone, third-order intercept vs frequency. This curve is
particularly useful for determining the magnitude of the
third harmonic as a function of frequency, load resistance,
and gain. For example, assume that the application requires
the OPA4658 to operate in a gain of +2V/V and drive
2Vp-p into 100
Figure 6 we find that the intercept point is +38dBm. The
magnitude of the third harmonic can now be easily calcu-
lated from the expression:
For this case OPI
Harmonic = 2(38 – 7) = 62dB below the fundamental. The
OPA4658’s low distortion makes the device an excellent
choice for a variety of RF signal processing applications.
CROSSTALK
Crosstalk is the undesired result of the signal of one channel
mixing with and reproducing itself in the output of another
FIGURE 6. Third Order Intercept Point vs Frequency.
–100
where OPI
–50
–60
–70
–80
–90
70
60
50
40
30
20
100k
10
Third Harmonic (dBc) = 2(OPI
G = +2
G = +2
P
3
P = third-order output intercept, dBm
O
3
(G = +2, R
at a frequency of 10MHz. Referring to
P = 38dBm, P
= output level, dBm
1M
Load Resistance ( )
L
Frequency (Hz)
= 100 , R
100
O
FB
2f
= 7dBm, and the third
O
= 402 )
10M
3f
3
O
P – P
O
)
100M
1k
11
channel or channels. Crosstalk is inclined to occur in most
multichannel integrated circuits. In quad devices, the effect of
crosstalk is measured by driving three channels and observing
the output of the undriven channel over various frequencies.
The magnitude of this effect is referenced in terms of channel-
to-channel isolation and expressed in decibels. Input referred
points to the fact that there is a direct correlation between gain
and crosstalk, therefore at increased gain, crosstalk also in-
creases by a factor equal to that of the gain. Figure 7 illustrates
the measured effect of crosstalk in the OPA4658U.
DIFFERENTIAL GAIN AND PHASE
Differential Gain (DG) and Differential Phase (DP) are criti-
cal 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.
DG and DP of the OPA4658 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 8
delivered a 100IRE modulated ramp to the 75 input of the
video analyzer. The signal averaging feature of the analyzer
FIGURE 8. Configuration for Testing Differential Gain/Phase.
FIGURE 7. Channel-to-Channel Isolation (three active channels).
TEK TSG 130A
–10
–20
–30
–40
–50
–60
–70
–80
–90
10
0
1M
75
75
G = +2
402
10M
OPA4658
Frequency (Hz)
1/4
402
OPA4658
75
100M
TEK VM700A
75
®