Datasheet

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SBOS279E − SEPTEMBER 2003 − REVISED MAY 2008
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11
DRIVING ADCs
The OPA373 and OPA374 series op amps are optimized
for driving medium-speed sampling ADCs. The OPA373
and OPA374 op amps buffer the ADC input capacitance
and resulting charge injection, while providing signal gain.
The OPA373 is shown driving the ADS7816 in a basic
noninverting configuration, as shown in Figure 7. The
ADS7816 is a 12-bit, MicroPower sampling converter in
the MSOP-8 package. When used with the low-power,
miniature packages of the OPA373, the combination is
ideal for space-limited, low-power applications. In this
configuration, an RC network at the ADC input can be used
to provide anti-aliasing filtering.
Figure 8 shows the OPA373 driving the ADS7816 in a
speech band-pass 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
V
S
= 2.7V to 5V.
The OPA373 is shown in the inverting configuration
described in Figure 9. In this configuration, filtering may be
accomplished with the capacitor across the feedback
resistor.
ENABLE/SHUTDOWN
OPA373 and OPA374 series op amps typically require
585µA quiescent current. The enable/shutdown feature of
the OPA373 allows the op amp to be shut off in order to
reduce this current to less than 1µA.
ADS7816
12−Bit ADC
DCLOCK
D
OUT
CS/SHDN
OPA373
+5V
V
IN
V+
2
+In
3
−
In
V
REF
8
4GND
Serial
Interface
1
0.1
µ
F 0.1
µ
F
7
6
5
NOTE: ADC Input = 0 to V
REF
V
IN
=0Vto5Vfor
0V to 5V output.
RC network filters high frequency noise.
500
Ω
3300pF
f
SAMPLE
=100kHz
Figure 7. The OPA373 in Noninverting Configuration Driving the ADS7816
C
3
33pF
V+
3
1
8
4GND
5
6
7
−
IN
+IN
2
DCLOCK
Serial
Interface
C
2
1000pF
R
1
1.5k
Ω
R
4
20k
Ω
R
5
20k
Ω
R
6
100k
Ω
R
8
150k
Ω
R
9
510k
Ω
R
7
51k
Ω
D
OUT
V
REF
V+ = +2.7V to +5V
CS/SHDN
C
1
1000pF
Electret
Microphone
(1)
G = 100
Passband 300Hz to 3kHz
R
3
1M
Ω
R
2
1M
Ω
NOTE: (1) Electret microphone
powered by R
1
.
ADS7816
12−Bit ADC
1/2
OPA2373
1/2
OPA2373
Figure 8. The OPA2373 as a Speech Bypass Filtered Data Acquisition System