Data Sheet

AD8671/AD8672/AD8674

Rev. F | Page 13 of 20

TOTAL HARMONIC DISTORTION (THD) AND NOISE

The AD8671/AD8672/AD8674 exhibit low total harmonic

distortion (THD) over the entire audio frequency range. This

makes them suitable for applications with high closed-loop

gains, including audio applications. Figure 33 shows

approximately 0.0006% of THD + N in a positive unity gain, the

worst-case configuration for distortion.

Hz

100

1k

10k

LT1007

0.0001

0.0002

0.0005

0.0010

0.0020

0.0050

0.0100

0.0200

0.0500

0.1000

50

20

500

200

5k

2k

AD8671

20k

VS = ±5V

VIN = 2.5V

RL = 600Ω

Figure 33. Total Harmonic Distortion and Noise

DRIVING CAPACITIVE LOADS

The AD8671/AD8672/AD8674 can drive large capacitive loads

without causing instability. However, when configured in unity

gain, driving very large loads can cause unwanted ringing or

instability.

Figure 34 shows the output of the AD8671 with a capacitive

load of 1 nF. If heavier loads are used in low closed-loop gain or

unity-gain configurations, it is recommended to use external

compensation as shown in the circuit in Figure 35. This

technique reduces the overshoot and prevents the op amp from

oscillation. The trade-off of this circuit is a reduction in output

swing. However, a great added benefit stems from the fact that

the input signal and the op amp’s noise are filtered, and thus the

overall output noise is kept to a minimum.

The output response of the circuit is shown in Figure 36.

VSY = ±15V

RL = 2kΩ

CL = 1nF

VIN = 100mV

AV = +1

CH2 +OVER

39.80%

CH2 –OVER

39.80%

TIME (10

µs/DIV)

Figure 34. AD8671 Capacitive Load Drive

500

Ω

RF

VCC

220pF

CF

VIN

VEE

RG

500

Ω

10

Ω

RS

1nF

CL

2k

Ω

RL

Figure 35. Recommended Capacitive Load Circuit

VSY = ±15V

RL = 2kΩ

CL = 1nF

CF = 220pF

VIN = 100mV

AV = +2

CH2 +OVER

5.051%

CH2 –OVER

6.061%

TIME (10

µs/DIV)

Figure 36. Compensated Load Drive