Datasheet

50
Ω
50
Ω
50
Ω
50
Ω
V
IN
50
Ω
10
Ω
100
Ω
10
Ω
To 50
Ω
Load
0
−
10
−
20
−
30
−
40
−
50
−
60
−
70
−
80
Frequency (Hz)
Gain (dB)
1k 10k 100k 1M 10M 100M 1G
0
−
10
−
20
−
30
−
40
−
50
−
60
−
70
−
80
−
90
−
100
Frequency (MHz)
Common−Mode Rejection Ratio (dBc)
0.001 0.01 0.1 1 10 100
C
1
+
1
2
C
2
+ 5.6mF
Z
IN
+
1
2sC
) R
1 ) sRC
1 ) 2sRC
OPA861
SBOS338G –AUGUST 2005–REVISED MAY 2013 www.ti.com
Differential Line Driver/Receiver
The input impedance is shown in Equation 6:
The wide bandwidth and high slew rate of the
(6)
OPA861 current-mode amplifier make it an ideal line
driver. The circuit in Figure 42 makes use of two
Figure 40 shows the frequency responses for low-
OPA861s to realize a single-ended to differential
pass, Butterworth filters set at 20kHz and 10MHz.
conversion. The high-impedance current source
For the 20kHz filter, set R to 1kΩ and
output of the OPA861 allows it to drive low-
impedance or capacitive loads without series
. For the 10MHz filter, the
resistances and avoids any attenuation that would
parasitic capacitance at the output pin needs to be
have otherwise occured in the resistive network.
taken into consideration. In the example of Figure 40,
The OPA861 used as a differential receiver exhibits
the parasitic is 3pF, which gives us the settings of R
excellent common-mode rejection ratio, as can be
= 1.13kΩ, C
1
= 10pF, and C
2
= 17pF.
seen in Figure 41.
Figure 41. Differential Driver Common-Mode
Figure 40. Small-Signal Frequency Response for
Rejection Ratio for 2V
PP
Input Signals
a Low-Pass Negative Impedance Converter Filter
Figure 42. Twisted-Pair Differential Driver and Receiver with the OPA861
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