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SBOS099C − SEPTEMBER 2000 − REVISED JANUARY 2005
series provides an effective means of buffering the
A/D’s input capacitance and resulting charge injection
while providing signal gain.
FEEDBACK CAPACITOR IMPROVES
RESPONSE
For optimum settling time and stability with
high-impedance feedback networks, it may be
necessary to add a feedback capacitor across the
feedback resistor, RF, as shown in Figure 4. This
capacitor compensates for the zero created by the
feedback network impedance and the OPA350’s input
capacitance (and any parasitic layout capacitance).
The effect becomes more significant with higher
impedance networks.
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 information, consult the ADS7861 data sheet
(SBAS110A).
OUTPUT IMPEDANCE
CF
The low frequency open-loop output impedance of the
OPA350’s
common-source
output
stage
is
RIN
RF
V+
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
characteristic, Output Impedance vs Frequency).
VIN
CIN
•
•
RIN CIN = RF CF
VOUT
OPA350
CL
CIN
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 from becoming too high, which can cause
stability problems when driving capacitive loads. As
mentioned previously, the OPA350 has excellent
capacitive load drive capability for an op amp with its
bandwidth.
Where CIN is equal to the OPA350’s input
capacitance (approximately 9pF) plus any
parasitic layout capacitance.
Figure 4. Feedback Capacitor Improves Dynamic
Performance
It is suggested that a variable capacitor be used for the
feedback capacitor since input capacitance may vary
between op amps and layout capacitance is difficult to
determine. For the circuit shown in Figure 4, the value
of the variable feedback capacitor should be chosen so
that the input resistance times the input capacitance of
the OPA350 (typically 9pF) plus the estimated parasitic
layout capacitance equals the feedback capacitor times
the feedback resistor:
VIDEO LINE DRIVER
Figure 6 shows a circuit for a single supply, G = 2
composite 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
configuration.
RIN @ CIN + RF @ CF
where CIN is equal to the OPA350’s input capacitance
(sum of differential and common-mode) plus the layout
capacitance. The capacitor can be varied until optimum
performance is obtained.
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.
DRIVING A/D CONVERTERS
OPA350 series op amps are optimized for driving
medium speed (up to 500kHz) sampling A/D
converters. However, they also offer excellent
performance for higher speed converters. The OPA350
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