Figure 5 shows the OPA350 driving an ADS7861. The
ADS7861 is a dual, 500kHz 12-bit sampling converter in
the tiny SSOP-24 package. When used with the miniature
package options of the OPA350 series, the combination is
ideal for space-limited applications. For further informa-
tion, consult the ADS7861 data sheet.
OUTPUT IMPEDANCE
The low frequency open-loop output impedance of the
OPA350’s common-source output stage is approximately
1kΩ. When the op amp is connected with feedback, this
value is reduced significantly by the loop gain of the op
amp. For example, with 122dB of open-loop gain, the
output impedance is reduced in unity-gain to less than
0.001Ω. For each decade rise in the closed-loop gain, the
loop gain is reduced by the same amount which results in
a ten-fold increase in effective output impedance (see the
typical performance curve, “Output Impedance vs Fre-
quency”).
At higher frequencies, the output impedance will rise as
the open-loop gain of the op amp drops. However, at these
frequencies the output also becomes capacitive due to
parasitic capacitance. This prevents the output impedance
C
B1
2kΩ
from becoming too high, which can cause stability prob-
lems when driving capacitive loads. As mentioned previ-
ously, the OPA350 has excellent capacitive load drive
capability for an op amp with its bandwidth.
VIDEO LINE DRIVER
Figure 6 shows a circuit for a single supply, G = 2 com-
posite video line driver. The synchronized outputs of a
composite video line driver extend below ground. As
shown, the input to the op amp should be ac-coupled and
shifted positively to provide adequate signal swing to
account for these negative signals in a single-supply con-
figuration.
The input is terminated with a 75Ω resistor and ac-coupled
with a 47µF capacitor to a voltage divider that provides the
dc bias point to the input. In Figure 6, this point is
approximately (V–) + 1.7V. Setting the optimal bias point
requires some understanding of the nature of composite
video signals. For best performance, one should be careful
to avoid the distortion caused by the transition region of
the OPA350’s complementary input stage. Refer to the
discussion of rail-to-rail input.
2kΩ
+5V
2
V
IN
B1
4
1/4
3
OPA4350
1
0.1µF
0.1µF
C
B0
24
2kΩ
2kΩ
2
6
1/4
5
OPA4350
7
3
4
5
6
7
2kΩ
2kΩ
8
9
9
1/4
10
OPA4350
8
10
11
CH B1+
CH B1–
CH B0+
CH B0–
CH A1+
CH A1–
CH A0+
CH A0–
REF
IN
REF
OUT
DGND
1
2kΩ
2kΩ
AGND
12
ADS7861
+V
D
13
+V
A
SERIAL DATA A
SERIAL DATA B
BUSY
CLOCK
CS
RD
CONVST
A0
M0
M1
23
22
21
20
19
18
17
16
15
14
Serial
Interface
V
IN
B0
C
A1
V
IN
A1
C
A0
12
13
1/4
OPA4350
11
14
V
IN
A0
V
IN
= 0V to 2.45V for 0V to 4.9V output.
Choose C
B1
, C
B0
, C
A1
, C
A0
to filter high frequency noise.
FIGURE 5. OPA4350 Driving Sampling A/D Converter.
®
OPA350, 2350, 4350
10
BURR-BROWN [ BURR-BROWN CORPORATION ]
