Datasheet

6
3
0
−
3
−
6
−
9
Frequency (Hz)
Gain (dB)
1M 10M 100M 1G
5.6dB
600
Ω
600
Ω
V
IN1
V
CM
V
IN2
R
E
600
Ω
ADS5272
OPA861
OPA861
3
0
-3
-6
-9
-12
-15
-18
-21
-24
-27
-30
-33
-36
-39
-42
-45
-48
Frequency(Hz)
Gain(dB)
10k 100k 1M 10M 100M
1G
50MHzFilter
20MHzFilter
1MHzFilter
ForAllFilters:
R =R =
2
¥
3
R =R =R
1 1S 2S 3S
=1/2R =R
R =R =R
1M 2M 0
C =C =C
1 2 0
OPA861
SBOS338G –AUGUST 2005–REVISED MAY 2013 www.ti.com
A few designs for a low-pass filter are shown in High-CMRR, Moderate Precision, Differential
Figure 47 and Table 2. I/O ADC Driver
The circuit shown in Figure 48 depicts an ADC driver
Table 2. Component Values for Filters Shown In
implemented with two OPA861s. Since the gain is set
Figure 47
here by the ratio of the internal 600Ω resistors and
f
O
R R
O
C
O
R
E
, its accuracy will only be as good as the input
1MHz 150 100 2nF
resistor of the ADS5272. The small-signal frequency
response for this circuit has 150MHz at –3dB
20MHz 150 100 112.5pF
bandwidth for a gain of approximately 5.6dB, as
50MHz 150 100 25pF
shown in Figure 49. The advantage of this circuit lies
in its high CMRR to 100kHz; see Figure 50. This
circuit also has more than 10 bits of linearity.
Figure 47. Butterworth Low-Pass Filter with the
Universal Active Filter
Figure 48. High CMRR, Moderate Precision,
Differential I/O ADC Driver
The advantages of building active filters using a
Current Conveyor structure are:
• The increase in output resistance of operational
amplifiers at high frequencies makes it difficult to
construct feedback filter structures (decrease in
stop-band attenuation).
• All filter coefficients are represented by
resistances, making it possible to adjust the filter
frequency response without affecting the filter
coefficients.
• The capacitors which determine the frequency are
located between the ground and the current
source outputs and are thus grounded on one
side. Therefore, all parasitic capacitances can be
viewed as part of these capacitors, making them
easier to comprehend.
• The features which determine the frequency
Figure 49. ADC Driver, Small-Signal Frequency
characteristics are currents, which charge the
Response
integration capacitors. This situation is similar to
the transfer characteristic of the Diamond
Transistor.
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