OPA861
www.ti.com
SBOS338–AUGUST 2005
•
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.
6
3
0
3
6
9
5.6dB
•
The features which determine the frequency
characteristics are currents, which charge the
integration capacitors. This situation is similar to
the transfer characteristic of the Diamond Tran-
sistor.
−
−
−
High-CMRR, Moderate Precision, Differential
I/O ADC Driver
1M
10M
100M
1G
Frequency (Hz)
The circuit shown in Figure 45 depicts an ADC driver
implemented with two OPA861s. Since the gain is set
here by the ratio of the internal 600Ω resistors and
RE, its accuracy will only be as good as the input
resistor of the ADS5272. The small-signal frequency
response for this circuit has 150MHz at –3dB
bandwidth for a gain of approximately 5.6dB, as
shown in Figure 46. The advantage of this circuit lies
in its high CMRR to 100kHz, as shown in Figure 47.
This circuit also has more than 10 bits of linearity.
Figure 46. ADC Driver, Small-Signal Frequency
Response
75
Input−Referred
70
65
60
55
50
45
40
35
30
25
20
ADS5272
OPA861
Ω
600
VIN1
RE
VCM
Ω
600
1k
10k
100k
1M
10M
100M
1G
Frequency (Hz)
OPA861
VIN2
Ω
600
Figure 47. CMRR of the ADC Driver
Figure 45. High CMRR, Moderate Precision, Differ-
ential I/O ADC Driver
22
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