OPA342, 2342, 4342

10

SBOS106A

Overshoot vs Capacitive Load.” In unity-gain configura-

tions, capacitive load drive can be improved by inserting a

small (10

Ω to 20Ω) resistor, R

S, in series with the output, as

shown in Figure 5. This significantly reduces ringing while

maintaining dc performance 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.

FIGURE 6. OPA342 in Noninverting Configuration Driving ADS7822.

FIGURE 7. Speech Bandpass Filtered Data Acquisition System.

DRIVING A/D CONVERTERS

The OPA342 series op amps are optimized for driving

medium-speed sampling ADCs. The OPA342 op amps buffer

the ADC’s input capacitance and resulting charge injection

while providing signal gain.

Figures 6 shows the OPA342 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

OPA342, the combination is ideal for space-limited, low-

power applications. In this configuration, an RC network at

the ADC’s input can be used to filter charge injection.

Figure 7 shows the OPA2342 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 500

µA quiescent current.

FIGURE 5. Series Resistor in Unity-Gain Configuration

Improves Capacitive Load Drive.

10

Ω to

20

Ω

OPA342

V+

V

IN

V

OUT

R

S

R

L

C

L

C

3

33pF

V+

GND

3

1

8

4

5

6

7

–IN

+IN

2

C

2

DCLOCK

Serial

Interface

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

C1

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.

ADS7822

12-Bit A/D

1/2

OPA2342

1/2

OPA2342

ADS7822

12-Bit A/D

DCLOCK

D

OUT

CS/SHDN

OPA342

+5V

V

IN

V+

2

+In

3

–In

V

REF

8

4

GND

Serial

Interface

1

0.1

µF

0.1

µF

7

6

5

NOTE: A/D Input = 0 to V

REF

V

IN = 0V to 5V for

0V to 5V output.

RC network filters high frequency noise.

500

Ω

3300pF