AD8519/AD8529

Rev. D | Page 12 of 16

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 is restored as soon as the

die temperature is reduced. Operating the product in an

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 are generated during the period when both diodes

are off.

The operation of the basic circuit (shown in Figure 23) should

be examined before considering the 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 the

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. Therefore, 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

0

2

1

<

−

=

−

IN

IN

OUT

V

V

V

This type of rectifier can be very precise if the following

electrical parameters are adhered to:

•

All passive components should be of tight tolerance, 1% for

resistors and 5% for capacitors.

•

If the application circuit requires high impedance (that is,

direct sensor interface), then an FET amplifier is a better

choice than the AD8519.

•

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

specification, yields the best result because the circuit

involves switching.

AD8519

AD8519

U2

U1

VIRTUAL GROUND =

2

NODE A

R1

10kΩ

R4

10kΩ

R2

10kΩ

R3

4.99kΩ

R5

10kΩ

D2

1N914

D1

1N914

R7

3.32kΩ

R6

5kΩ

Figure 23. Precision Full-Wave Rectifier

Switching glitches are caused when D1 and D2 are both

momentarily off. This condition occurs every time the input

signal is equal to the virtual ground potential. When this

input centered around 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 fre-

quency 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 is also some

unresolved bias current related offset. To minimize this offset,

use lower value resistors or choose an FET amplifier if the

optimized offset is still intolerable.

The AD8519 offers a unique combination of speed vs. power

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

noise that makes it an ideal choice for most high volume and

high precision rectifier circuits.