AD8510/AD8512/AD8513

Rev. H | Page 18 of 20

TIME (2ms/DIV)

The AD8510 single has two additional active terminals that are

not present on the AD8512 dual or AD8513 quad parts. These

pins are labeled “null” and are used for fine adjustment of the

input offset voltage. Although the guaranteed maximum offset

voltage at room temperature is 400 μV and over the −40°C to

+125°C range is 800 mV maximum, this offset voltage can be

reduced by adding a potentiometer to the null pins as shown in

Figure 58. With the 20 kΩ potentiometer shown, the adjustment

range is approximately ±3.5 mV. The potentiometer parallels

low value resistors in the drain circuit of the JFET differential

input pair and allows unbalancing of the drain currents to

change the offset voltage. If offset adjustment is not required,

these pins should be left unconnected.

Caution should be used when adding adjusting potentiometers to

any op amp with this capability for several reasons. First, there is

gain from these nodes to the output; therefore, capacitive coupling

from noisy traces to these nodes will inject noise into the signal

path. Second, the temperature coefficient of the potentiometer

will not match the temperature coefficient of the internal resistors,

so the offset voltage drift with temperature will be slightly affected.

Third, this provision is for adjusting the offset voltage of the

op amp, not for adjusting the offset of the overall system. Although

it is tempting to decrease the value of the potentiometer to attain

more range, this will adversely affect the dc and ac parameters.

Instead, increase the potentiometer to 50 kΩ to decrease the

range if needed.

Figure 55. Pulse-Width Modulation

Crosstalk

Crosstalk, also known as channel separation, is a measure of

signal feedthrough from one channel to another on the same

IC. The AD8512/AD8513 have a channel separation of better

than −90 dB for frequencies up to 10 kHz and of better than

−50 dB for frequencies up to 10 MHz. Figure 57 shows the

typical channel separation behavior between Amplifier A

(driving amplifier) and each of the following: Amplifier B,

Amplifier C, and Amplifier D.

1

2

7

6

5

4

3

8

20kΩ

2.2kΩ

5kΩ

5kΩ

18V p-p

CROSSTALK = 20 log

1

5

4

7

3

6

2

AD8510

INPUT

OUTPUT

V+

TYPICALLY ±3.5mV

20kΩ

V–

–

+

Figure 56. Crosstalk Test Circuit

FREQUENCY (Hz)

100

–160

10k

–140

–120

–80

1k

–60

–20

–40

100k

1M

10M

–100

0

CH D

CH C

CH B

Figure 58. Optional Offset Nulling Circuit

Figure 57. Channel Separation