OPA861 Datasheet, PDF (18/28 Pages) Texas Instruments – Wide Bandwidth OPERATIONAL TRANSCONDUCTANCE AMPLIFIER (OTA)

OPA861
SBOS338 – AUGUST 2005
The input impedance is shown in Equation 6:
Z _IN
+
1
2sC
)
R
1 ) sRC
1 ) 2sRC
(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
C₁
+
1
2
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.
0
−10
−20
−30
−40
−50
−60
−70
−80
1k
10k 100k
1M
10M 100M 1G
Frequency (Hz)
Figure 37. Small-Signal Frequency Response for a
Low-Pass Negative Impedance Converter Filter
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Differential Line Driver/Receiver
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.
The OPA861 used as a differential receiver exhibits
excellent common-mode rejection ratio, as can be
seen in Figure 38.
0
−10
−20
−30
−40
−50
−60
−70
−80
−90
−100
0.001
0.01
0.1
1
10
100
Frequency (MHz)
Figure 38. Differential Driver Common-Mode
Rejection Ratio for 2V_PP Input Signals
V_IN
50Ω
50Ω
10Ω
To 50Ω Load
50Ω
100Ω 10Ω
50Ω
50Ω
Figure 39. Twisted-Pair Differential Driver and Receiver with the OPA861
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