OPA4658 Datasheet, PDF (11/15 Pages) Burr-Brown (TI) – Quad Wideband, Low Power Current Feedback OPERATIONAL AMPLIFIER

–50
G = +2
–60
–70
–80
–90
2f_O
3f_O
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.
–100
10
100
1k
Load Resistance (Ω)
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 100Ω at a frequency of 10MHz. Referring to
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:
Third Harmonic (dBc) = 2(OPI³P – P_O)
where OPI³P = third-order output intercept, dBm
P_O = output level, dBm
For this case OPI³P = 38dBm, P_O = 7dBm, and the third
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
(G = +2, R_L = 100Ω, R_FB = 402Ω)
70
G = +2
60
50
40
30
20
100k
1M
10M
Frequency (Hz)
100M
FIGURE 6. Third Order Intercept Point vs Frequency.
10
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
1M
G = +2
10M
Frequency (Hz)
100M
FIGURE 7. Channel-to-Channel Isolation (three active channels).
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 75Ω input impedance and the output
back-terminated in 75Ω. 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 75Ω input of the
video analyzer. The signal averaging feature of the analyzer
75Ω
1/4
OPA4658
75Ω
402Ω
TEK TSG 130A
402Ω
75Ω
75Ω
TEK VM700A
FIGURE 8. Configuration for Testing Differential Gain/Phase.
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OPA4658