REV. C

AD8519/AD8529

–9–

500mV

50 s

AVCC = 1

TPC 19. Large Signal Transient Response

APPLICATIONS INFORMATION

Maximum Power Dissipation

The maximum power that can be safely dissipated by the

AD8519/AD8529 is limited by the associated rise in junction

temperature. The maximum safe junction temperature is 150

∞C

for these plastic packages. If this maximum is momentarily

exceeded, proper circuit operation will be restored as soon as

the die temperature is reduced. Operating the product in the

“overheated” condition for an extended period can result in

permanent damage to the device.

Precision Full-Wave Rectifier

Slew rate is probably the most underestimated parameter

when designing a precision rectifier. Yet without a good slew

rate large glitches will be generated during the period when

both diodes are off.

Let’s examine the operation of the basic circuit (shown in

Figure 1) before considering slew rate further. U1 is set up to

have two states of operation. D1 and D2 diodes switch the

output between the two states. State one is an inverter with a

gain of +1, and state two is a simple unity gain buffer where the

output is equal to the value of the virtual ground. The virtual

ground is the potential present at the noninverting node of

ground, D2 turns off and D1 turns on. This causes the output

of U1 to simply buffer the virtual ground and this configuration

is state two. So, the function of U1, which results from these

two states of operation, is a half wave inverter. The U2 function

takes the inverted half wave at a gain of two and sums it into the

VV

V

OUT

IN

IN

=-

<

-

20

1

This type of rectifier can be very precise if the following electrical

parameters are adhered to:

1. All passive components should be of tight tolerance, 1% resistors

and 5% capacitors.

2. If the application circuit requires high impedance (i.e., direct

sensor interface), then a FET amplifier is probably a better

choice than the AD8519.

3. An amp such as the AD8519, which has a great slew rate

specification, will yield the best result because the circuit involves

switching.

AD8519

D1

1N914

R6

5k

R1

10k

R4

10k

R2

10k

D2

1N914

R3

4.99k

R5

10k

R7

3.32k

AD8519

U2

U1

VIRTUAL GROUND =

2

NODE A

Figure 1. Precision Full-Wave Rectifier

Switching glitches are caused when D1 and D2 are both momen-

tarily off. This condition occurs every time the input signal is

equal to the virtual ground potential. When this condition occurs,

¥ R5 ¥ (R4 R1 + R2 + R3). Note that Node A should be

virtual ground, glitches are generated at the sharp negative peaks

of the rectified sine wave. If the glitches are hard to notice on an

oscilloscope, raise the frequency of the sine wave until they

become apparent. The size of the glitches is proportional to the

input frequency, the diode turn-on potential (0.2 V or 0.65 V),

and the slew rate of the op amp.

R6 and R7 are both necessary to limit the amount of bias current

related voltage offset. Unfortunately, there is no “perfect” value

for R6 because the impedance at the inverting node is altered as

D1 and D2 switch. Therefore, there will also be some unresolved

bias current related offset. To minimize this offset, use lower

value resistors or choose a FET amplifier if the optimized offset

is still intolerable.

The AD8519 offers a unique combination of speed versus power

ratio at 2.7 V single supply, small size (SC70 and SOT-23), and

low noise that make it an ideal choice for most high volume and

high precision rectifier circuits.

10

Microphone Preamp Meets PC99 Specifications

This circuit, while lacking a unique topology, is anything but

featureless when an AD8519 is used as the op amp. This preamp

gives 20 dB gain over a frequency range of 20 Hz to 20 kHz and

is fully PC99 compliant in all parameters including THD+N,

dynamic range, frequency range, amplitude range, crosstalk, and

so on. Not only does this preamp comply with the PC99 specifi-

noise of around 100 dB, which is suitable for most professional

20-bit audio systems. Referred to input noise is 120 dB. At 120 dB

THD+N in unity gain, the AD8519 is suitable for 24-bit profes-

sional audio systems. In other words, the AD8519 will not be

the limiting performance factor in audio systems despite its small

size and low cost.