DRIVING A/D CONVERTERS
performance curve “Small-Signal Overshoot vs Capacitive
Load.” In unity-gain configurations, capacitive load drive
can be improved by inserting a small (10Ω to 20Ω) resistor,
RS, in series with the output, as shown in Figure 5. This
significantly reduces ringing while maintaining dc perfor-
mance for purely capacitive loads. However, if there is a
resistive load in parallel with the capacitive load, a voltage
divider is created, introducing a dc error at the output and
slightly reducing the output swing. The error introduced is
proportional to the ratio RS/RL, and is generally negligible.
The OPA344 and OPA345 series op amps are optimized for
driving medium-speed sampling A/D converters. The
OPA344 and OPA345 op amps buffer the A/D’s input
capacitance and resulting charge injection while providing
signal gain.
Figures 6 shows the OPA344 in a basic noninverting con-
figuration driving the ADS7822. The ADS7822 is a 12-bit,
micro-power sampling converter in the MSOP-8 package.
When used with the low-power, miniature packages of the
OPA344, the combination is ideal for space-limited, low-
power applications. In this configuration, an RC network at
the A/D’s input can be used to filter charge injection.
V+
Figure 7 shows the OPA2344 driving an ADS7822 in a
speech bandpass filtered data acquisition system. This small,
low-cost solution provides the necessary amplification and
signal conditioning to interface directly with an electret
microphone. This circuit will operate with VS = +2.7V to
+5V with less than 500µA quiescent current.
RS
VOUT
OPA344
10Ω to
20Ω
VIN
CL
RL
FIGURE 5. Series Resistor in Unity-Gain Configuration
Improves Capacitive Load Drive.
+5V
0.1µF
0.1µF
1
VREF
8
V+
7
6
5
DCLOCK
DOUT
500Ω
+In
2
Serial
Interface
ADS7822
12-Bit A/D
OPA344
VIN
–In
CS/SHDN
3
3300pF
GND
4
VIN = 0V to 5V for
0V to 5V output.
NOTE: A/D Input = 0 to VREF
RC network filters high frequency noise.
FIGURE 6. OPA344 in Noninverting Configuration Driving ADS7822.
V+ = +2.7V to 5V
Passband 300Hz to 3kHz
R9
510kΩ
R1
R4
R2
1.5kΩ
20kΩ
1MΩ
C3
C
1
33pF
1000pF
R7
51kΩ
R8
150kΩ
V
8
+
1
VREF
1/2
7
6
DCLOCK
DOUT
OPA2344
+IN
2
–IN
1/2
OPA2344
R3
1MΩ
ADS7822
12-Bit A/D
Electret
Microphone(1)
Serial
Interface
R6
100kΩ
1000pF
C2
5
CS/SHDN
3
4
G = 100
NOTE: (1) Electret microphone
powered by R1.
R5
20kΩ
GND
FIGURE 7. Speech Bandpass Filtered Data Acquisition System.
11
®
OPA344, 2344, 4344
OPA345, 2345, 4345
BB [ BURR-BROWN CORPORATION ]
