OPA861
www.ti.com
SBOS338–AUGUST 2005
Differential Line Driver/Receiver
The input impedance is shown in Equation 6:
1 ) sRC
1 ) 2sRC
1
2sC
The wide bandwidth and high slew rate of the
OPA861 current-mode amplifier make it an ideal line
driver. The circuit in Figure 39 makes use of two
OPA861s to realize a single-ended to differential
conversion. The high-impedance current source out-
put of the OPA861 allows it to drive low-impedance or
capacitive loads without series resistances and
avoids any attenuation that would have otherwise
occured in the resistive network.
ZIN
+
) R
(6)
Figure 37 shows the frequency responses for
low-pass, Butterworth filters set at 20kHz and 10MHz.
For the 20kHz filter, set
R
to 1KΩ and
1
2
C1 + C2 + 5.6mF
. For the 10MHz filter, the
parasitic capacitance at the output pin needs to be
taken into consideration. In the example of Figure 37,
the parasitic is 3pF, which gives us the settings of R
= 1.13kΩ, C1 = 10pF, and C2 = 17pF.
The OPA861 used as a differential receiver exhibits
excellent common-mode rejection ratio, as can be
seen in Figure 38.
0
0
−
−
−
−
−
−
−
−
−
10
20
30
40
50
60
70
80
90
−
−
−
−
−
−
−
−
10
20
30
40
50
60
70
80
−
100
0.001
1k
10k
100k
1M
10M
100M
1G
0.01
0.1
1
10
100
Frequency (Hz)
Frequency (MHz)
Figure 38. Differential Driver Common-Mode
Rejection Ratio for 2VPP Input Signals
Figure 37. Small-Signal Frequency Response for a
Low-Pass Negative Impedance Converter Filter
Ω
To 50 Load
Ω
Ω
50
50
VIN
Ω
50
Ω
10
Ω
100
Ω
Ω
10
50
Ω
50
Figure 39. Twisted-Pair Differential Driver and Receiver with the OPA861
18
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