Datasheet

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SBOS233E − MARCH 2002− REVISED MAY 2009
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14
ADS7816, ADS7861,
or ADS7864
12−Bit A/D Converter
OPA354
+5V
V
IN
V+
+In
−
In
V
REF
GND
NOTE: A/D Converter Input = 0V to V
REF
+2.5V
5k
Ω
5k
Ω
330pF
V
IN
=0Vto
−
5V for 0V to 5V output.
Figure 6. The OPA354 in Inverting Configuration Driving the ADS7816
CAPACITIVE LOAD AND STABILITY
The OPA354 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 OPA354’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 curve 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 7. 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.
OPA354
V+
V
IN
V
OUT
C
L
R
L
R
S
Figure 7. Series Resistor in Unity-Gain
Configuration Improves Capacitive Load Drive
WIDEBAND TRANSIMPEDANCE AMPLIFIER
Wide bandwidth, low input bias current, and low input
voltage and current noise make the OPA354 an ideal
wideband photodiode transimpedance amplifier for
low-voltage single-supply applications. Low-voltage noise
is important because photodiode capacitance causes the
effective noise gain of the circuit to increase at high
frequency.