Datasheet

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SBOS235E − MARCH 2002− REVISED MAY 2009
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15
DRIVING ANALOG−TO−DIGITAL
CONVERTERS
The OPA357 series op amps offer 60ns of settling time to
0.01%, making them a good choice for driving high- and
medium-speed sampling A/D converters and reference
circuits. The OPA357 series provide an effective means of
buffering the A/D converter’s input capacitance and
resulting charge injection while providing signal gain.
See Figure 7 for the OPA357 driving an A/D converter.
With the OPA357 in an inverting configuration, a capacitor
across the feedback resistor can be used to filter
high-frequency noise in the signal; see Figure 7.
CAPACITIVE LOAD AND STABILITY
The OPA357 series op amps can drive a wide range of
capacitive loads. However, all op amps under certain
conditions may become unstable. Op amp configuration,
gain, and load value are just a few of the factors to consider
when determining stability. An op amp in unity-gain
configuration is most susceptible to the effects of
capacitive loading. The capacitive load reacts with the op
amp’s output resistance, along with any additional load
resistance, to create a pole in the small-signal response
that degrades the phase margin. Refer to the typical
characteristic curve Frequency Response for Various C
L
for details.
The OPA357’s topology enhances its ability to drive
capacitive loads. In unity gain, these op amps perform well
with large capacitive loads. Refer to the typical
characteristic curves Recommended R
S
vs Capacitive
Load and Frequency Response vs Capacitive Load for
details.
One method of improving capacitive load drive in the
unity-gain configuration is to insert a 10Ω to 20Ω resistor
in series with the output, as shown in Figure 8. This
significantly reduces ringing with large capacitive
loadssee the typical characteristic curve Frequency
Response vs Capacitive Load. However, if there is a
resistive load in parallel with the capacitive load, R
S
creates a voltage divider. This introduces a DC error at the
output and slightly reduces output swing. This error may
be insignificant. For instance, with R
L
= 10kΩ and R
S
=
20Ω, there is only about a 0.2% error at the output.
ADS7818, ADS7861,
or ADS7864
12−Bit A/D Converter
OPA357
+5V
V
IN
+2.5V
V+
+In
−
In
V
REF
GND
0.1
µ
F
NOTE: A/D Converter Input = 0V to V
REF
5k
Ω
5k
Ω
330pF
V
IN
=0Vto
−
5V for 0V to 5V output.
5kΩ
Figure 7. The OPA357 in Inverting Configuration Driving an A/D Converter
OPA357
V+
V
IN
V
OUT
C
L
R
L
R
S
To enable,
connect to V+
or drive with logic.
Figure 8. Series Resistor in Unity-Gain Configuration Improves Capacitive Load Drive